Heart rate

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https://en.wikipedia.org/wiki/Heart_rate 

Heart rate (or pulse rate) is the frequency of the heartbeat measured by the number of contractions of the heart per minute (beats per minute, or bpm). The heart rate can vary according to the body's physical needs, including the need to absorb oxygen and excrete carbon dioxide, but is also modulated by numerous factors, including (but not limited to) genetics, physical fitness, stress or psychological status, diet, drugs, hormonal status, environment, and disease/illness as well as the interaction between and among these factors. It is usually equal or close to the pulse measured at any peripheral point.

The American Heart Association states the normal resting adult human heart rate is 60-100 bpm. Tachycardia is a high heart rate, defined as above 100 bpm at rest. Bradycardia is a low heart rate, defined as below 60 bpm at rest. When a human sleeps, a heartbeat with rates around 40–50 bpm is common and is considered normal. When the heart is not beating in a regular pattern, this is referred to as an arrhythmia. Abnormalities of heart rate sometimes indicate disease.

Physiology

The human heart

While heart rhythm is regulated entirely by the sinoatrial node under normal conditions, heart rate is regulated by sympathetic and parasympathetic input to the sinoatrial node. The accelerans nerve provides sympathetic input to the heart by releasing norepinephrine onto the cells of the sinoatrial node (SA node), and the vagus nerve provides parasympathetic input to the heart by releasing acetylcholine onto sinoatrial node cells. Therefore, stimulation of the accelerans nerve increases heart rate, while stimulation of the vagus nerve decreases it.

As water and blood are incompressible fluids, one of the physiological ways to deliver more blood to an organ is to increase heart rate. Normal resting heart rates range from 60 to 100 bpm. Bradycardia is defined as a resting heart rate below 60 bpm. However, heart rates from 50 to 60 bpm are common among healthy people and do not necessarily require special attention. Tachycardia is defined as a resting heart rate above 100 bpm, though persistent rest rates between 80 and 100 bpm, mainly if they are present during sleep, may be signs of hyperthyroidism or anemia (see below).

• Central nervous system stimulants such as substituted amphetamines increase heart rate.
• Central nervous system depressants or sedatives decrease the heart rate (apart from some particularly strange ones with equally strange effects, such as ketamine which can cause – amongst many other things – stimulant-like effects such as tachycardia).

There are many ways in which the heart rate speeds up or slows down. Most involve stimulant-like endorphins and hormones being released in the brain, some of which are those that are 'forced'/'enticed' out by the ingestion and processing of drugs such as cocaine or atropine.

This section discusses target heart rates for healthy persons, which would be inappropriately high for most persons with coronary artery disease.

Influences from the central nervous system

Cardiovascular centres

The heart rate is rhythmically generated by the sinoatrial node. It is also influenced by central factors through sympathetic and parasympathetic nerves. Nervous influence over the heart rate is centralized within the two paired cardiovascular centres of the medulla oblongata. The cardioaccelerator regions stimulate activity via sympathetic stimulation of the cardioaccelerator nerves, and the cardioinhibitory centers decrease heart activity via parasympathetic stimulation as one component of the vagus nerve. During rest, both centers provide slight stimulation to the heart, contributing to autonomic tone. This is a similar concept to tone in skeletal muscles. Normally, vagal stimulation predominates as, left unregulated, the SA node would initiate a sinus rhythm of approximately 100 bpm.

Both sympathetic and parasympathetic stimuli flow through the paired cardiac plexus near the base of the heart. The cardioaccelerator center also sends additional fibers, forming the cardiac nerves via sympathetic ganglia (the cervical ganglia plus superior thoracic ganglia T1–T4) to both the SA and AV nodes, plus additional fibers to the atria and ventricles. The ventricles are more richly innervated by sympathetic fibers than parasympathetic fibers. Sympathetic stimulation causes the release of the neurotransmitter norepinephrine (also known as noradrenaline) at the neuromuscular junction of the cardiac nerves. This shortens the repolarization period, thus speeding the rate of depolarization and contraction, which results in an increased heartrate. It opens chemical or ligand-gated sodium and calcium ion channels, allowing an influx of positively charged ions.

Norepinephrine binds to the beta–1 receptor. High blood pressure medications are used to block these receptors and so reduce the heart rate.

Autonomic Innervation of the Heart – Cardioaccelerator and cardioinhibitory areas are components of the paired cardiac centers located in the medulla oblongata of the brain. They innervate the heart via sympathetic cardiac nerves that increase cardiac activity and vagus (parasympathetic) nerves that slow cardiac activity.

Parasympathetic stimulation originates from the cardioinhibitory region of the brain with impulses traveling via the vagus nerve (cranial nerve X). The vagus nerve sends branches to both the SA and AV nodes, and to portions of both the atria and ventricles. Parasympathetic stimulation releases the neurotransmitter acetylcholine (ACh) at the neuromuscular junction. ACh slows HR by opening chemical- or ligand-gated potassium ion channels to slow the rate of spontaneous depolarization, which extends repolarization and increases the time before the next spontaneous depolarization occurs. Without any nervous stimulation, the SA node would establish a sinus rhythm of approximately 100 bpm. Since resting rates are considerably less than this, it becomes evident that parasympathetic stimulation normally slows HR. This is similar to an individual driving a car with one foot on the brake pedal. To speed up, one need merely remove one's foot from the brake and let the engine increase speed. In the case of the heart, decreasing parasympathetic stimulation decreases the release of ACh, which allows HR to increase up to approximately 100 bpm. Any increases beyond this rate would require sympathetic stimulation.

Effects of Parasympathetic and Sympathetic Stimulation on Normal Sinus Rhythm – The wave of depolarization in a normal sinus rhythm shows a stable resting HR. Following parasympathetic stimulation, HR slows. Following sympathetic stimulation, HR increases.

Input to the cardiovascular centres

The cardiovascular centre receive input from a series of visceral receptors with impulses traveling through visceral sensory fibers within the vagus and sympathetic nerves via the cardiac plexus. Among these receptors are various proprioreceptors, baroreceptors, and chemoreceptors, plus stimuli from the limbic system which normally enable the precise regulation of heart function, via cardiac reflexes. Increased physical activity results in increased rates of firing by various proprioreceptors located in muscles, joint capsules, and tendons. The cardiovascular centres monitor these increased rates of firing, suppressing parasympathetic stimulation or increasing sympathetic stimulation as needed in order to increase blood flow.

Similarly, baroreceptors are stretch receptors located in the aortic sinus, carotid bodies, the venae cavae, and other locations, including pulmonary vessels and the right side of the heart itself. Rates of firing from the baroreceptors represent blood pressure, level of physical activity, and the relative distribution of blood. The cardiac centers monitor baroreceptor firing to maintain cardiac homeostasis, a mechanism called the baroreceptor reflex. With increased pressure and stretch, the rate of baroreceptor firing increases, and the cardiac centers decrease sympathetic stimulation and increase parasympathetic stimulation. As pressure and stretch decrease, the rate of baroreceptor firing decreases, and the cardiac centers increase sympathetic stimulation and decrease parasympathetic stimulation.

There is a similar reflex, called the atrial reflex or Bainbridge reflex, associated with varying rates of blood flow to the atria. Increased venous return stretches the walls of the atria where specialized baroreceptors are located. However, as the atrial baroreceptors increase their rate of firing and as they stretch due to the increased blood pressure, the cardiac center responds by increasing sympathetic stimulation and inhibiting parasympathetic stimulation to increase HR. The opposite is also true.

Increased metabolic byproducts associated with increased activity, such as carbon dioxide, hydrogen ions, and lactic acid, plus falling oxygen levels, are detected by a suite of chemoreceptors innervated by the glossopharyngeal and vagus nerves. These chemoreceptors provide feedback to the cardiovascular centers about the need for increased or decreased blood flow, based on the relative levels of these substances.

The limbic system can also significantly impact HR related to emotional state. During periods of stress, it is not unusual to identify higher than normal HRs, often accompanied by a surge in the stress hormone cortisol. Individuals experiencing extreme anxiety may manifest panic attacks with symptoms that resemble those of heart attacks. These events are typically transient and treatable. Meditation techniques have been developed to ease anxiety and have been shown to lower HR effectively. Doing simple deep and slow breathing exercises with one's eyes closed can also significantly reduce this anxiety and HR.

Factors influencing heart rate

Table 1: Major factors increasing heart rate and force of contraction Factor Effect Cardioaccelerator nerves Release of norepinephrine Proprioreceptors Increased rates of firing during exercise Chemoreceptors Decreased levels of O2; increased levels of H+, CO2, and lactic acid Baroreceptors Decreased rates of firing, indicating falling blood volume/pressure Limbic system Anticipation of physical exercise or strong emotions Catecholamines Increased epinephrine and norepinephrine Thyroid hormones Increased T3 and T4 Calcium Increased Ca2+ Potassium Decreased K+ Sodium Decreased Na+ Body temperature Increased body temperature Nicotine and caffeine Stimulants, increasing heart rate

Table 2: Factors decreasing heart rate and force of contraction Factor Effect Cardioinhibitor nerves (vagus) Release of acetylcholine Proprioreceptors Decreased rates of firing following exercise Chemoreceptors Increased levels of O2; decreased levels of H+ and CO2 Baroreceptors Increased rates of firing, indicating higher blood volume/pressure Limbic system Anticipation of relaxation Catecholamines Decreased epinephrine and norepinephrine Thyroid hormones Decreased T3 and T4 Calcium Decreased Ca2+ Potassium Increased K+ Sodium Increased Na+ Body temperature Decrease in body temperature

Using a combination of autorhythmicity and innervation, the cardiovascular center is able to provide relatively precise control over the heart rate, but other factors can impact on this. These include hormones, notably epinephrine, norepinephrine, and thyroid hormones; levels of various ions including calcium, potassium, and sodium; body temperature; hypoxia; and pH balance.

Epinephrine and norepinephrine

The catecholamines, epinephrine and norepinephrine, secreted by the adrenal medulla form one component of the extended fight-or-flight mechanism. The other component is sympathetic stimulation. Epinephrine and norepinephrine have similar effects: binding to the beta-1 adrenergic receptors, and opening sodium and calcium ion chemical- or ligand-gated channels. The rate of depolarization is increased by this additional influx of positively charged ions, so the threshold is reached more quickly and the period of repolarization is shortened. However, massive releases of these hormones coupled with sympathetic stimulation may actually lead to arrhythmias. There is no parasympathetic stimulation to the adrenal medulla.

Thyroid hormones

In general, increased levels of the thyroid hormones (thyroxine(T4) and triiodothyronine (T3)), increase the heart rate; excessive levels can trigger tachycardia. The impact of thyroid hormones is typically of a much longer duration than that of the catecholamines. The physiologically active form of triiodothyronine, has been shown to directly enter cardiomyocytes and alter activity at the level of the genome. It also impacts the beta-adrenergic response similar to epinephrine and norepinephrine.

Calcium

Calcium ion levels have a great impact on heart rate and myocardial contractility: increased calcium levels cause an increase in both. High levels of calcium ions result in hypercalcemia and excessive levels can induce cardiac arrest. Drugs known as calcium channel blockers slow HR by binding to these channels and blocking or slowing the inward movement of calcium ions.

Caffeine and nicotine

Caffeine and nicotine are both stimulants of the nervous system and of the cardiac centres causing an increased heart rate. Caffeine works by increasing the rates of depolarization at the SA node, whereas nicotine stimulates the activity of the sympathetic neurons that deliver impulses to the heart.

Effects of stress

Both surprise and stress induce physiological response: elevate heart rate substantially. In a study conducted on 8 female and male student actors ages 18 to 25, their reaction to an unforeseen occurrence (the cause of stress) during a performance was observed in terms of heart rate. In the data collected, there was a noticeable trend between the location of actors (onstage and offstage) and their elevation in heart rate in response to stress; the actors present offstage reacted to the stressor immediately, demonstrated by their immediate elevation in heart rate the minute the unexpected event occurred, but the actors present onstage at the time of the stressor reacted in the following 5 minute period (demonstrated by their increasingly elevated heart rate). This trend regarding stress and heart rate is supported by previous studies; negative emotion/stimulus has a prolonged effect on heart rate in individuals who are directly impacted. In regard to the characters present onstage, a reduced startle response has been associated with a passive defense, and the diminished initial heart rate response has been predicted to have a greater tendency to dissociation. Current evidence suggests that heart rate variability can be used as an accurate measure of psychological stress and may be used for an objective measurement of psychological stress.

Factors decreasing heart rate

The heart rate can be slowed by altered sodium and potassium levels, hypoxia, acidosis, alkalosis, and hypothermia. The relationship between electrolytes and HR is complex, but maintaining electrolyte balance is critical to the normal wave of depolarization. Of the two ions, potassium has the greater clinical significance. Initially, both hyponatremia (low sodium levels) and hypernatremia (high sodium levels) may lead to tachycardia. Severely high hypernatremia may lead to fibrillation, which may cause cardiac output to cease. Severe hyponatremia leads to both bradycardia and other arrhythmias. Hypokalemia (low potassium levels) also leads to arrhythmias, whereas hyperkalemia (high potassium levels) causes the heart to become weak and flaccid, and ultimately to fail.

Heart muscle relies exclusively on aerobic metabolism for energy. Severe myocardial infarction (commonly called a heart attack) can lead to a decreasing heart rate, since metabolic reactions fueling heart contraction are restricted.

Acidosis is a condition in which excess hydrogen ions are present, and the patient's blood expresses a low pH value. Alkalosis is a condition in which there are too few hydrogen ions, and the patient's blood has an elevated pH. Normal blood pH falls in the range of 7.35–7.45, so a number lower than this range represents acidosis and a higher number represents alkalosis. Enzymes, being the regulators or catalysts of virtually all biochemical reactions – are sensitive to pH and will change shape slightly with values outside their normal range. These variations in pH and accompanying slight physical changes to the active site on the enzyme decrease the rate of formation of the enzyme-substrate complex, subsequently decreasing the rate of many enzymatic reactions, which can have complex effects on HR. Severe changes in pH will lead to denaturation of the enzyme.

The last variable is body temperature. Elevated body temperature is called hyperthermia, and suppressed body temperature is called hypothermia. Slight hyperthermia results in increasing HR and strength of contraction. Hypothermia slows the rate and strength of heart contractions. This distinct slowing of the heart is one component of the larger diving reflex that diverts blood to essential organs while submerged. If sufficiently chilled, the heart will stop beating, a technique that may be employed during open heart surgery. In this case, the patient's blood is normally diverted to an artificial heart-lung machine to maintain the body's blood supply and gas exchange until the surgery is complete, and sinus rhythm can be restored. Excessive hyperthermia and hypothermia will both result in death, as enzymes drive the body systems to cease normal function, beginning with the central nervous system.

Physiological control over heart rate

Dolphin heart rate graph

A study shows that bottlenose dolphins can learn – apparently via instrumental conditioning – to rapidly and selectively slow down their heart rate during diving for conserving oxygen depending on external signals. In humans regulating heart rate by methods such as listening to music, meditation or a vagal maneuver takes longer and only lowers the rate to a much smaller extent. 

In different circumstances

Heart rate (HR) (top trace) and tidal volume (Vt) (lung volume, second trace) plotted on the same chart, showing how heart rate increases with inspiration and decreases with expiration.

Heart rate is not a stable value and it increases or decreases in response to the body's need in a way to maintain an equilibrium (basal metabolic rate) between requirement and delivery of oxygen and nutrients. The normal SA node firing rate is affected by autonomic nervous system activity: sympathetic stimulation increases and parasympathetic stimulation decreases the firing rate.

Resting heart rate

Normal pulse rates at rest, in beats per minute (BPM):

newborn (0–1 months old)	infants (1 – 11 months)	children (1 – 2 years old)	children (3 – 4 years)	children (5 – 6 years)	children (7 – 9 years)	children over 10 years & adults, including seniors	well-trained adult athletes
70-190	80–160	80-130	80-120	75–115	70–110	60–100	40–60

The basal or resting heart rate (HR_rest) is defined as the heart rate when a person is awake, in a neutrally temperate environment, and has not been subject to any recent exertion or stimulation, such as stress or surprise. The available evidence indicates that the normal range for resting heart rate is 50-90 beats per minute. This resting heart rate is often correlated with mortality. For example, all-cause mortality is increased by 1.22 (hazard ratio) when heart rate exceeds 90 beats per minute. The mortality rate of patients with myocardial infarction increased from 15% to 41% if their admission heart rate was greater than 90 beats per minute. ECG of 46,129 individuals with low risk for cardiovascular disease revealed that 96% had resting heart rates ranging from 48 to 98 beats per minute. Finally, in one study 98% of cardiologists suggested that as a desirable target range, 50 to 90 beats per minute is more appropriate than 60 to 100. The normal resting heart rate is based on the at-rest firing rate of the heart's sinoatrial node, where the faster pacemaker cells driving the self-generated rhythmic firing and responsible for the heart's autorhythmicity are located. For endurance athletes at the elite level, it is not unusual to have a resting heart rate between 33 and 50 bpm.

Maximum heart rate

The maximum heart rate (HR_max) is the age-related highest number of beats per minute of the heart when reaching a point of exhaustion without severe problems through exercise stress. In general it is loosely estimated as 220 minus one's age. It generally decreases with age. Since HR_max varies by individual, the most accurate way of measuring any single person's HR_max is via a cardiac stress test. In this test, a person is subjected to controlled physiologic stress (generally by treadmill or bicycle ergometer) while being monitored by an electrocardiogram (ECG). The intensity of exercise is periodically increased until certain changes in heart function are detected on the ECG monitor, at which point the subject is directed to stop. Typical duration of the test ranges ten to twenty minutes.^{[citation needed]} Adults who are beginning a new exercise regimen are often advised to perform this test only in the presence of medical staff due to risks associated with high heart rates.

The theoretical maximum heart rate of a human is 300 bpm, however there have been multiple cases where this theoretical upper limit has been exceeded. The fastest human ventricular conduction rate recorded to this day is a conducted tachyarrhythmia with ventricular rate of 480 beats per minute, which is comparable to the heart rate of a mouse.

For general purposes, a number of formulas are used to estimate HR_max. However, these predictive formulas have been criticized as inaccurate because they only produce generalized population-averages and may deviate significantly from the actual value. (See § Limitations.)

Formulas for estimating HR_max

Name	Data	HRmax Formula	Error
Haskell & Fox (1971)	35 data points	220 − age	SD=12-15 bpm
Inbar, et al. (1994)	1424 men	205.8 − (0.685 × age)	SD = 6.4 bpm
Tanaka, Monahan, & Seals (2001)	315 studies, 514 individuals	208 − (0.7 × age)	SD ~10 bpm
Wohlfart, B. and Farazdaghi, G.R.	81 men, 87 women	Men: 203.7 / ( 1 + exp( 0.033 × (age − 104.3) ) ) Women: 190.2 / ( 1 + exp( 0.0453 × (age − 107.5) ) )	SD=6.5% men, 5.5% women
Oakland University (2007)	100 men, 32 women, 908 longitudinal observations	Linear: 207 − (0.7 × age) Nonlinear: 192 − (0.007 × age2)	1 SD confidence interval: ±5–8 bpm (linear), ±2–5 bpm (nonlinear)
Gulati (2010)	5437 women	Women: 206 − (0.88 × age)	SD=11.8 bpm
Nes, et al. (2013)	1726 men, 1594 women	211 − (0.64 × age)	SEE=10.8 bpm
Wingate (2015)	20,691 males, 7446 females	Men: 208.609-0.716 × age Women: 209.273-0.804 × age	SD = 10.81 (male), 12.15 (female)

Haskell & Fox (1970)

Fox and Haskell formula; widely used.

Notwithstanding later research, the most widely cited formula for HR_max is still:

HR_max = 220 − age

Although attributed to various sources, it is widely thought to have been devised in 1970 by Dr. William Haskell and Dr. Samuel Fox. They did not develop this formula from original research, but rather by plotting data from approximately 11 references consisting of published research or unpublished scientific compilations. It gained widespread use through being used by Polar Electro in its heart rate monitors, which Dr. Haskell has "laughed about", as the formula "was never supposed to be an absolute guide to rule people's training."

While this formula is the most common (and easy to remember and calculate), research has consistently found that it is subject to bias. Compared to the age-specific average HR_max, the Haskell and Fox formula overestimates HR_max in young adults, agrees with it at age 40, and underestimates HR_max in older adults. For example, in one study, the average HR_max at age 76 was about 10bpm higher than the Haskell and Fox equation. Consequently, the formula cannot be recommended for use in exercise physiology and related fields.

Yet, it is common practice to use 85% of the predicted HR_max (Haskell & Fox) to evaluate chronotropic response to exercise. An ongoing study has found that the 5th percentile of the study cohort maximal heart rate correlates much better with the 85% of predicted HR_max (Haskell & Fox) than the 5th percentile of the study cohort heart rate reserve to the 80% of the predicted heart rate reserve.

Other formulas

The various formulae provide slightly different numbers for the maximum heart rates by age.

HR_max is strongly correlated to age, and most formulas are solely based on this. Studies have been mixed on the effect of gender, with some finding that gender is statistically significant, although small when considering overall equation error, while others finding negligible effect. The inclusion of physical activity status, maximal oxygen uptake, smoking, body mass index, body weight, or resting heart rate did not significantly improve accuracy. Nonlinear models are slightly more accurate predictors of average age-specific HR_max, particularly above 60 years of age, but are harder to apply, and provide statistically negligible improvement over linear models. The Wingate formula is the most recent, had the largest data set, and performed best on a fresh data set when compared with other formulas, although it had only a small amount of data for ages 60 and older so those estimates should be viewed with caution. In addition, most formulas are developed for adults and are not applicable to children and adolescents.

Limitations

Maximum heart rates vary significantly between individuals. Age explains only about half of HR_max variance. For a given age, the standard deviation of HR_max from the age-specific population mean is about 12bpm, and a 95% interval for the prediction error is about 24bpm. For example, Dr. Fritz Hagerman observed that the maximum heart rates of men in their 20s on Olympic rowing teams vary from 160 to 220. Such a variation would equate to an age range of -16 to 68 using the Wingate formula. The formulas are quite accurate at predicting the average heart rate of a group of similarly-aged individuals, but relatively poor for a given individual.

Robergs and Landwehr opine that for VO2 max, prediction errors in HR_max need to be less than ±3 bpm. No current formula meets this accuracy. For prescribing exercise training heart rate ranges, the errors in the more accurate formulas may be acceptable, but again it is likely that, for a significant fraction of the population, current equations used to estimate HR_max are not accurate enough. Froelicher and Myers describe maximum heart formulas as "largely useless". Measurement via a maximal test is preferable whenever possible, which can be as accurate as ±2bpm.

Heart rate reserve

Heart rate reserve (HR_reserve) is the difference between a person's measured or predicted maximum heart rate and resting heart rate. Some methods of measurement of exercise intensity measure percentage of heart rate reserve. Additionally, as a person increases their cardiovascular fitness, their HR_rest will drop, and the heart rate reserve will increase. Percentage of HR_reserve is statistically indistinguishable from percentage of VO₂ reserve.

HR_reserve = HR_max − HR_rest

This is often used to gauge exercise intensity (first used in 1957 by Karvonen).

Karvonen's study findings have been questioned, due to the following:

• The study did not use VO₂ data to develop the equation.
• Only six subjects were used.
• Karvonen incorrectly reported that the percentages of HR_reserve and VO₂ max correspond to each other, but newer evidence shows that it correlated much better with VO₂ reserve as described above.

Target heart rate

For healthy people, the Target Heart Rate (THR) or Training Heart Rate Range (THRR) is a desired range of heart rate reached during aerobic exercise which enables one's heart and lungs to receive the most benefit from a workout. This theoretical range varies based mostly on age; however, a person's physical condition, sex, and previous training also are used in the calculation.

By percent, Fox–Haskell-based

The THR can be calculated as a range of 65–85% intensity, with intensity defined simply as percentage of HR_max. However, it is crucial to derive an accurate HR_max to ensure these calculations are meaningful.

Example for someone with a HR_max of 180 (age 40, estimating HR_max As 220 − age):

65% Intensity: (220 − (age = 40)) × 0.65 → 117 bpm

85% Intensity: (220 − (age = 40)) × 0.85 → 154 bpm

Karvonen method

The Karvonen method factors in resting heart rate (HR_rest) to calculate target heart rate (THR), using a range of 50–85% intensity:

THR = ((HR_max − HR_rest) × % intensity) + HR_rest

Equivalently,

THR = (HR_reserve × % intensity) + HR_rest

Example for someone with a HR_max of 180 and a HR_rest of 70 (and therefore a HR_reserve of 110):

50% Intensity: ((180 − 70) × 0.50) + 70 = 125 bpm

85% Intensity: ((180 − 70) × 0.85) + 70 = 163 bpm

Zoladz method

An alternative to the Karvonen method is the Zoladz method, which is used to test an athlete's capabilities at specific heart rates. These are not intended to be used as exercise zones, although they are often used as such. The Zoladz test zones are derived by subtracting values from HR_max:

THR = HR_max − Adjuster ± 5 bpm

Zone 1 Adjuster = 50 bpm

Zone 2 Adjuster = 40 bpm

Zone 3 Adjuster = 30 bpm

Zone 4 Adjuster = 20 bpm

Zone 5 Adjuster = 10 bpm

Example for someone with a HR_max of 180:

Zone 1(easy exercise): 180 − 50 ± 5 → 125 − 135 bpm

Zone 4(tough exercise): 180 − 20 ± 5 → 155 − 165 bpm

Heart rate recovery

Heart rate recovery (HRR) is the reduction in heart rate at peak exercise and the rate as measured after a cool-down period of fixed duration. A greater reduction in heart rate after exercise during the reference period is associated with a higher level of cardiac fitness.

Heart rates assessed during treadmill stress test that do not drop by more than 12 bpm one minute after stopping exercise (if cool-down period after exercise) or by more than 18 bpm one minute after stopping exercise (if no cool-down period and supine position as soon as possible) are associated with an increased risk of death. People with an abnormal HRR defined as a decrease of 42 beats per minutes or less at two minutes post-exercise had a mortality rate 2.5 times greater than patients with a normal recovery. Another study reported a four-fold increase in mortality in subjects with an abnormal HRR defined as ≤12 bpm reduction one minute after the cessation of exercise. A study reported that a HRR of ≤22 bpm after two minutes "best identified high-risk patients". They also found that while HRR had significant prognostic value it had no diagnostic value.

Development

See also: Heart development

At 21 days after conception, the human heart begins beating at 70 to 80 beats per minute and accelerates linearly for the first month of beating.

Fetal heart rate monitoring. 30 weeks pregnancy.

The human heart beats more than 2.8 billion times in an average lifetime. The heartbeat of a human embryo begins at approximately 21 days after conception, or five weeks after the last normal menstrual period (LMP), which is the date normally used to date pregnancy in the medical community. The electrical depolarizations that trigger cardiac myocytes to contract arise spontaneously within the myocyte itself. The heartbeat is initiated in the pacemaker regions and spreads to the rest of the heart through a conduction pathway. Pacemaker cells develop in the primitive atrium and the sinus venosus to form the sinoatrial node and the atrioventricular node respectively. Conductive cells develop the bundle of His and carry the depolarization into the lower heart.

The human heart begins beating at a rate near the mother's, about 75–80 beats per minute (bpm). The embryonic heart rate then accelerates linearly for the first month of beating, peaking at 165–185 bpm during the early 7th week, (early 9th week after the LMP). This acceleration is approximately 3.3 bpm per day, or about 10 bpm every three days, an increase of 100 bpm in the first month.

After peaking at about 9.2 weeks after the LMP, it decelerates to about 150 bpm (+/-25 bpm) during the 15th week after the LMP. After the 15th week the deceleration slows reaching an average rate of about 145 (+/-25 bpm) bpm at term. The regression formula which describes this acceleration before the embryo reaches 25 mm in crown-rump length or 9.2 LMP weeks is:

${\displaystyle \mathrm{A}\mathrm{g}\mathrm{e}\mathrm{i}\mathrm{n}\mathrm{d}\mathrm{a}\mathrm{y}\mathrm{s}=\mathrm{E}\mathrm{H}\mathrm{R}(0.3)+6}$

Clinical significance

Manual measurement

Wrist heart rate monitor (2009)

Heart rate monitor with a wrist receiver

Heart rate is measured by finding the pulse of the heart. This pulse rate can be found at any point on the body where the artery's pulsation is transmitted to the surface by pressuring it with the index and middle fingers; often it is compressed against an underlying structure like bone. The thumb should not be used for measuring another person's heart rate, as its strong pulse may interfere with the correct perception of the target pulse.

The radial artery is the easiest to use to check the heart rate. However, in emergency situations the most reliable arteries to measure heart rate are carotid arteries. This is important mainly in patients with atrial fibrillation, in whom heart beats are irregular and stroke volume is largely different from one beat to another. In those beats following a shorter diastolic interval left ventricle does not fill properly, stroke volume is lower and pulse wave is not strong enough to be detected by palpation on a distal artery like the radial artery. It can be detected, however, by doppler.

Possible points for measuring the heart rate are:

1. The ventral aspect of the wrist on the side of the thumb (radial artery).
2. The ulnar artery.
3. The inside of the elbow, or under the biceps muscle (brachial artery).
4. The groin (femoral artery).
5. Behind the medial malleolus on the feet (posterior tibial artery).
6. Middle of dorsum of the foot (dorsalis pedis).
7. Behind the knee (popliteal artery).
8. Over the abdomen (abdominal aorta).
9. The chest (apex of the heart), which can be felt with one's hand or fingers. It is also possible to auscultate the heart using a stethoscope.
10. In the neck, lateral of the larynx (carotid artery)
11. The temple (superficial temporal artery).
12. The lateral edge of the mandible (facial artery).
13. The side of the head near the ear (posterior auricular artery).

ECG-RRinterval

Electronic measurement

In obstetrics, heart rate can be measured by ultrasonography, such as in this embryo (at bottom left in the sac) of 6 weeks with a heart rate of approximately 90 per minute.

A more precise method of determining heart rate involves the use of an electrocardiograph, or ECG (also abbreviated EKG). An ECG generates a pattern based on electrical activity of the heart, which closely follows heart function. Continuous ECG monitoring is routinely done in many clinical settings, especially in critical care medicine. On the ECG, instantaneous heart rate is calculated using the R wave-to-R wave (RR) interval and multiplying/dividing in order to derive heart rate in heartbeats/min. Multiple methods exist:

• HR = 1000*60/(RR interval in milliseconds)
• HR = 60/(RR interval in seconds)
• HR = 300/number of "large" squares between successive R waves.
• HR= 1,500 number of large blocks

Heart rate monitors allow measurements to be taken continuously and can be used during exercise when manual measurement would be difficult or impossible (such as when the hands are being used). Various commercial heart rate monitors are also available. Some monitors, used during sport, consist of a chest strap with electrodes. The signal is transmitted to a wrist receiver for display.

Alternative methods of measurement include seismocardiography.

Optical measurements

Pulsatile retinal blood flow in the optic nerve head region revealed by laser Doppler imaging

Pulse oximetry of the finger and laser Doppler imaging of the eye fundus are often used in the clinics. Those techniques can assess the heart rate by measuring the delay between pulses.

Tachycardia

Main article: Tachycardia

Tachycardia is a resting heart rate more than 100 beats per minute. This number can vary as smaller people and children have faster heart rates than average adults.

Physiological conditions where tachycardia occurs:

1. Pregnancy
2. Emotional conditions such as anxiety or stress.
3. Exercise

Pathological conditions where tachycardia occurs:

1. Sepsis
2. Fever
3. Anemia
4. Hypoxia
5. Hyperthyroidism
6. Hypersecretion of catecholamines
7. Cardiomyopathy
8. Valvular heart diseases
9. Acute Radiation Syndrome

Bradycardia

Main articles: Bradycardia and Athletic heart syndrome

Bradycardia was defined as a heart rate less than 60 beats per minute when textbooks asserted that the normal range for heart rates was 60–100 bpm. The normal range has since been revised in textbooks to 50–90 bpm for a human at total rest. Setting a lower threshold for bradycardia prevents misclassification of fit individuals as having a pathologic heart rate. The normal heart rate number can vary as children and adolescents tend to have faster heart rates than average adults. Bradycardia may be associated with medical conditions such as hypothyroidism.

Trained athletes tend to have slow resting heart rates, and resting bradycardia in athletes should not be considered abnormal if the individual has no symptoms associated with it. For example, Miguel Indurain, a Spanish cyclist and five time Tour de France winner, had a resting heart rate of 28 beats per minute, one of the lowest ever recorded in a healthy human. Daniel Green achieved the world record for the slowest heartbeat in a healthy human with a heart rate of just 26 bpm in 2014.

Arrhythmia

Main article: Cardiac dysrhythmia

Arrhythmias are abnormalities of the heart rate and rhythm (sometimes felt as palpitations). They can be divided into two broad categories: fast and slow heart rates. Some cause few or minimal symptoms. Others produce more serious symptoms of lightheadedness, dizziness and fainting.

Correlation with cardiovascular mortality risk

A number of investigations indicate that faster resting heart rate has emerged as a new risk factor for mortality in homeothermic mammals, particularly cardiovascular mortality in human beings. Faster heart rate may accompany increased production of inflammation molecules and increased production of reactive oxygen species in cardiovascular system, in addition to increased mechanical stress to the heart. There is a correlation between increased resting rate and cardiovascular risk. This is not seen to be "using an allotment of heart beats" but rather an increased risk to the system from the increased rate.

An Australian-led international study of patients with cardiovascular disease has shown that heart beat rate is a key indicator for the risk of heart attack. The study, published in The Lancet (September 2008) studied 11,000 people, across 33 countries, who were being treated for heart problems. Those patients whose heart rate was above 70 beats per minute had significantly higher incidence of heart attacks, hospital admissions and the need for surgery. Higher heart rate is thought to be correlated with an increase in heart attack and about a 46 percent increase in hospitalizations for non-fatal or fatal heart attack.

Other studies have shown that a high resting heart rate is associated with an increase in cardiovascular and all-cause mortality in the general population and in patients with chronic diseases. A faster resting heart rate is associated with shorter life expectancy  and is considered a strong risk factor for heart disease and heart failure, independent of level of physical fitness. Specifically, a resting heart rate above 65 beats per minute has been shown to have a strong independent effect on premature mortality; every 10 beats per minute increase in resting heart rate has been shown to be associated with a 10–20% increase in risk of death. In one study, men with no evidence of heart disease and a resting heart rate of more than 90 beats per minute had a five times higher risk of sudden cardiac death. Similarly, another study found that men with resting heart rates of over 90 beats per minute had an almost two-fold increase in risk for cardiovascular disease mortality; in women it was associated with a three-fold increase.

Given these data, heart rate should be considered in the assessment of cardiovascular risk, even in apparently healthy individuals. Heart rate has many advantages as a clinical parameter: It is inexpensive and quick to measure and is easily understandable. Although the accepted limits of heart rate are between 60 and 100 beats per minute, this was based for convenience on the scale of the squares on electrocardiogram paper; a better definition of normal sinus heart rate may be between 50 and 90 beats per minute.

Standard textbooks of physiology and medicine mention that heart rate (HR) is readily calculated from the ECG as follows: HR = 1000*60/RR interval in milliseconds, HR = 60/RR interval in seconds, or HR = 300/number of large squares between successive R waves. In each case, the authors are actually referring to instantaneous HR, which is the number of times the heart would beat if successive RR intervals were constant.

Lifestyle and pharmacological regimens may be beneficial to those with high resting heart rates. Exercise is one possible measure to take when an individual's heart rate is higher than 80 beats per minute. Diet has also been found to be beneficial in lowering resting heart rate: In studies of resting heart rate and risk of death and cardiac complications on patients with type 2 diabetes, legumes were found to lower resting heart rate. This is thought to occur because in addition to the direct beneficial effects of legumes, they also displace animal proteins in the diet, which are higher in saturated fat and cholesterol. Another nutrient is omega-3 long chain polyunsaturated fatty acids (omega-3 fatty acid or LC-PUFA). In a meta-analysis with a total of 51 randomized controlled trials (RCTs) involving 3,000 participants, the supplement mildly but significantly reduced heart rate (-2.23 bpm; 95% CI: -3.07, -1.40 bpm). When docosahexaenoic acid (DHA) and eicosapentaenoic acid (EPA) were compared, modest heart rate reduction was observed in trials that supplemented with DHA (-2.47 bpm; 95% CI: -3.47, -1.46 bpm), but not in those received EPA.

A very slow heart rate (bradycardia) may be associated with heart block. It may also arise from autonomous nervous system impairment.

Landlocked developing countries

From Wikipedia, the free encyclopedia
https://en.wikipedia.org/wiki/Landlocked_developing_countries

The landlocked developing countries (LLDC) are developing countries that are landlocked. The economic and other disadvantages experienced by such countries makes the majority of landlocked countries the least developed countries (LDCs), with inhabitants of these countries occupying the bottom billion tier of the world's population in terms of poverty. Outside of Europe, there is not a single highly developed landlocked country as measured by the Human Development Index (HDI), and nine of the twelve countries with the lowest HDI scores are landlocked. Landlocked European countries are exceptions in terms of development outcomes due to their close integration with the regional European market. Landlocked countries that rely on transoceanic trade usually suffer a cost of trade that is double that of their maritime neighbours. Landlocked countries experience economic growth 6% less than non-landlocked countries, holding other variables constant.

32 out of the world's 44 landlocked countries, including all the landlocked countries in Africa, Asia, and South America, have been classified as the Landlocked Developing Countries (LLDCs) by the United Nations. As of 2012, about 442.8 million people lived in these LLDCs.

UN-OHRLLS

Main article: UN-OHRLLS

The United Nations has an Office of the High Representative for the Least Developed Countries, Landlocked Developing Countries and Small Island Developing States (UN-OHRLLS). It mainly holds the view that high transport costs due to distance and terrain result in the erosion of competitive edge for exports from landlocked countries. In addition, it recognizes the constraints on landlocked countries to be mainly physical, including lack of direct access to the sea, isolation from world markets and high transit costs due to physical distance. It also attributes geographic remoteness as one of the most significant reasons why developing landlocked nations cannot alleviate themselves, while European landlocked cases are mostly developed because of short distances to the sea through well-developed countries. One other commonly cited factor is the administrative burdens associated with border crossings as there is a heavy load of bureaucratic procedures, paperwork, custom charges, and most importantly, traffic delay due to border wait times, which affect delivery contracts. Delays and inefficiency compound geographically, where a 2 to 3 week wait due to border customs between Uganda and Kenya makes it impossible to book ships ahead of time in Mombasa, furthering delivery contract delays. Despite these explanations, it is also important to consider the transit countries that neighbour LLDCs, from whose ports the goods of LLDCs are exported.

Dependency problems

Although Adam Smith and traditional thought hold that geography and transportation are the culprits for keeping LLDCs from realizing development gains, Faye, Sachs and Snow hold the argument that no matter the advancement of infrastructure or lack of geographic distance to a port, landlocked nations are still dependent on their neighbouring transit nations. Outlying this specific relationship of dependency, Faye et al. insist that though LLDCs vary across the board in terms of HDI index scores, LLDCs almost uniformly straddle at the bottom of HDI rankings in terms of region, suggesting a correlated dependency relationship of development for landlocked countries with their respective regions. In fact, HDI levels decrease as one moves inland along the major transit route that runs from the coast of Kenya, across the country before going through Uganda, Rwanda and then finally Burundi. Just recently, it has been economically modeled that if the economic size of a transit country is increased by just 1%, a subsequent increase of at least 2% is experienced by the landlocked country, which shows that there is hope for LLDCs if the conditions of their transit neighbours are addressed. In fact, some LLDCs are seeing the brighter side of such a relationship, with the Central Asian nations geographic location between three BRIC nations (China, Russia and India) hungry for the region's oil and mineral wealth serving to boost economic development. The three major factors that LLDCs are dependent on their transit neighbours are dependence on transit infrastructure, dependence on political relations with neighbours, and dependence on internal peace and stability within transit neighbours.

Burundi

Burundi is shown in blue. Its possible export routes were: *dependent on infrastructure of transit neighbour Tanzania (yellow), or *dependent on political relations with transit neighbour Kenya (orange), or *dependent on internal stability of transit neighbour Mozambique (red). When all three routes were unavailable, Burundi had to rely on the port of Durban in South Africa (brown).

Burundi has relatively good internal road networks, but it cannot export its goods using the most direct route to the sea since the inland infrastructure of Tanzania is poorly connected to the port of Dar es Salaam. Thus Burundi relies on Kenya's port of Mombasa for export; but this route was severed briefly in the 1990s when political relations with Kenya deteriorated. Further, Burundi's exports could not pass through Mozambique around the same time due to the Mozambican civil war (1977-1992). Thus, Burundi had to export its goods using a 4500 km route, crossing several borders and changing transport modes, to reach the port of Durban in South Africa.

Other African countries

• Mali had problems exporting goods in the 1990s as nearly all its transit neighbours (Algeria, Togo, Sierra Leone, Liberia, Guinea and Côte d'Ivoire) were engaged in civil war around the same time: the Algerian civil war (1991-2002), the Sierra Leone civil war (1991-2002), the Guinea-Bissau civil war involving Guinea (1999), the First Liberian Civil War (1989-1997), the Second Liberian Civil War (1999) and the First Ivorian Civil War (2002-2007). The lone exception was Ghana, which was under military rule but did not have an active civil war at the time.
• The Central African Republic's export routes are seasonal: in the rainy season Cameroon's roads are too poor to use; and during the dry season the Democratic Republic of Congo's Oubangui River water levels are too low for river travel.

Central Asia

The mineral resource-rich countries of Central Asia and Mongolia offer a unique set of landlocked cases to explore in more depth, as these are nations where economic growth has grown exceptionally in recent years. In Central Asia, oil and coal deposits have influenced development: Kazakhstan’s GDI per capita in purchasing power parity was five times greater than Kyrgyzstan's in 2009. Despite substantial development growth, these nations are not on a stable and destined path to being well developed, as the exploitation of their natural resources translates into an overall low average income and disparity of income, and because their limited deposits of resources allow growth only in the short term, and most importantly because dependence on unprocessed materials increases the risk of shocks due to variations in market prices. And though it is widely conceived that free trade can permit faster economic growth, Mongolia is now subjected to a new geopolitical game about the traffic on its railway lines between China and Russia. Russian Railways now effectively owns 50% of Mongolia's rail infrastructure, which could mean more efficient modernization and the laying of new rail lines, but in reality also translates into powerful leverage to pressure the government of Mongolia to concede unfair terms for license grants of coal, copper, and gold mines. Thus, it can be argued that these nations with extraordinary mineral wealth should pursue economic diversification. All of these nations possess education qualifications, as they are inheritors of the Soviet Union's social education system. This implies that it is due to poor economic policies that more than 40% of the labour force is bogged down in the agricultural sector instead of being diverted into secondary or tertiary economic activity. Yet, it cannot be ignored that Mongolia benefits exceptionally from its proximity to two giant BRIC nations, resulting in a rapid development of railway ports along its borders, especially along the Chinese border, as the Chinese seek to direct coking coal from Mongolia to China's northwestern industrial core, and, as well as for transportation southeast towards Japan and South Korea, resulting in revenue generation through the seaport of Tianjin.

Armenia

The Republic of Armenia is a landlocked country having geographic disadvantages and faces limitations on foreign policy options. It needs to transport its goods via coastal neighbors to access ports to participate in international trade, to which Azerbaijan and Turkey are hostile and deny its access. Therefore, Armenia mainly depends on the Georgian ports of Batumi and Poti and the Georgian train system to participate in international trade. Armenia also shares a small border with neighboring Iran, through which it trades despite American sanctions. Armenia remains heavily dependent on imports from and export of moderately unsophisticated goods to Russia. While Russia stayed Armenia's dominant trade partner, in 2020, trade with the EU accounted for around 18% of Armenia's total trade. As of 2020, European Union is Armenia's third biggest export market, with a 17% share in total Armenian exports, and the second largest source of Armenian imports, with an 18.6% share in total Armenian imports. 

Nepal

Nepal is another landlocked country with extreme dependency on its transit neighbour India. India does not have poor relations with Nepal, nor does it lack relevant transport infrastructure or internal stability. However, there have been two cases of economic blockades imposed by the government of India on Nepal – the official 1989 blockade and the unofficial 2015 blockade – both of which left the nation in severe economic crisis. In the 1970s, Nepal suffered from large commodity concentration and a high geographic centralization in its export trade: over 98% of its exports were to India, and 90% of its imports came from India. As a result of all this, Nepal had a poor trade bargaining position. In the 1950s, Nepal was forced to comply with India's external tariffs as well as the prices of India's exports. This was problematic since the two countries have different levels of development, resulting in greater gains for India which was larger, more advanced and with more resources. It was feared that a parasitic relationship might emerge, since India had a head start in industrialization, and dominated Nepal in manufacturing, which could reduce Nepal to being just a supplier of raw materials. Because of these problems, and Nepal's inability to develop its own infant industries (as it could not compete with Indian manufactures) treaties were drafted in 1960 and 1971, with amendments to the equal tariffs conditions, and terms of trade have since progressed.

Almaty Ministerial Conference

In August, 2003, the International Ministerial Conference of Landlocked and Transit Developing Countries and Donor Countries on Transit Transport Cooperation (Almaty Ministerial Conference) was held in Almaty, Kazakhstan, setting the necessities of LLDCs in a universal document whereas there were no coordinated efforts on the global scale to serve the unique needs of LLDCs in the past. Other than acknowledging the main forms of dependency that must be addressed, it also acknowledged the additional dependency issue where neighbouring transit countries are often observed to export the same products as their landlocked neighbours. One result of the conference was a direct call for donor countries to step in to direct aid into setting up suitable infrastructure of transit countries to alleviate the burden of supporting LLDCs in regions of poor development in general. The general objectives of the Almaty Program of Action is as follows:

• Reduce customs processes and fees to minimize costs and transport delays
• Improve infrastructure with respect to existing preferences of local transport modes, where road should be focused in Africa and rail in South Asia
• Implement preferences for landlocked countries’ commodities to boost their competitiveness in the international market
• To establish relationships between donor countries with landlocked and transit countries for technical, financial and policy improvements

Current LLDCs

Map of current landlocked developing countries

Africa (16 countries)

•  Botswana
•  Burkina Faso
•  Burundi
•  Central African Republic
•  Chad
•  Eswatini
•  Ethiopia
•  Lesotho
•  Malawi
•  Mali
•  Niger
•  Rwanda
•  South Sudan
•  Uganda
•  Zambia
•  Zimbabwe

Asia (12 countries)

•  Afghanistan
•  Armenia
•  Azerbaijan
•  Bhutan
•  Kazakhstan
•  Kyrgyzstan
•  Laos
•  Mongolia
•    Nepal
•  Tajikistan
•  Turkmenistan
•  Uzbekistan

Europe (2 countries)

•  Moldova
•  North Macedonia

South America (2 countries)

•  Bolivia
•  Paraguay

Cumulonimbus and aviation

From Wikipedia, the free encyclopedia
https://en.wikipedia.org/wiki/Cumulonimbus_and_aviation

Numerous accidents have occurred in the vicinity of thunderstorms due to the density of clouds. It is often said that the turbulence can be extreme enough inside a cumulonimbus to tear an aircraft into pieces, and even strong enough to hold a skydiver. However, this kind of accident is relatively rare. Moreover, the turbulence under a thunderstorm can be non-existent and is usually no more than moderate. Most thunderstorm-related crashes occur due to a stall close to the ground when the pilot gets caught by surprise by a thunderstorm-induced wind shift. Moreover, aircraft damage caused by thunderstorms is rarely in the form of structural failure due to turbulence but is typically less severe and the consequence of secondary effects of thunderstorms (e.g., denting by hail or paint removal by high-speed flight in torrential rain).

Thus, cumulonimbus are known to be extremely dangerous to air traffic, and it is recommended to avoid them as much as possible. Cumulonimbus can be extremely insidious, and an inattentive pilot can end up in a very dangerous situation while flying in apparently very calm air.

While there is a gradation with respect to thunderstorm severity, there is little quantitative difference between a significant shower generated by a cumulus congestus and a small thunderstorm with a few thunderclaps associated with a small cumulonimbus. For this reason, a glider pilot could exploit the rising air under a thunderstorm without recognising the situation – thinking instead that the rising air was due to a more benign variety of cumulus. However, forecasting thunderstorm severity is an inexact science; in numerous occasions, pilots got trapped by underestimating the severity of a thunderstorm that suddenly strengthened.

General hazards to aircraft

Even large airliners avoid crossing the path of a cumulonimbus. Two dangerous effects of cumulonimbus have been put forward to explain the crash of flight AF447 that sank into the sea on 31 May 2009 about 600 kilometres (370 mi) northeast of Brazil. It encountered a mesoscale convective system in the Intertropical Convergence Zone (known by sailors as the "doldrums"), where cumulonimbus rise to more than 15 kilometres (49,000 ft) in altitude. However, the aircraft did not disintegrate in flight. A different hypothesis was put forward and later confirmed: accumulation of ice on the aircraft's pitot tubes.

The inconsistency between the airspeeds measured by the different sensors is one of the causes of the accident according to the final report.

The US FAA recommends that aircraft (including gliders) stay at least 20 nautical miles away from a severe thunderstorm, while a glider pilot could be tempted to use the updraughts below and inside the cloud. There are two sorts of danger for this type of aircraft. One is related to the shear effects between updraughts and downdraughts inside the cloud – effects that can smash the glider. This shear creates a Kelvin-Helmholtz instability that can generate extremely violent sub-vortices. The second danger is more insidious: the strong updraughts below a supercell cumulonimbus can cover a large area and contain little or no turbulence as explained below. In this case, the glider can be sucked into the cloud, where the pilot can quickly lose visual reference to the ground, causing conditions to quickly become Instrument meteorological conditions (IMC), meaning that pilots are forced to fly by instruments alone, without visual reference of the ground or sky. In these conditions, the aircraft (if not equipped for IMC flight and flown by a pilot experienced in IMC flight) is likely to enter a graveyard spiral and eventually break up by exceeding the wing load limit. In this situation, the cause of the disintegration of the aircraft is not atmospheric turbulence but is the inability of the pilot to control the aircraft following the loss of visual reference to the ground. In the case of an instrument flight, cumulonimbus can catch a pilot by surprise when embedded in a more benign cloud mass. For example, nimbostratus can originate from the spreading of a cumulonimbus (nimbostratus cumulonimbogenitus), making the presence of active convective cells likely. Small private airplanes are generally not equipped with on-board weather radars; and during an IFR approach, they can be sent accidentally by air traffic control to non-obvious active cells.

Updraft characteristics

Figure 1 : Forward area of a severe cumulonimbus moving to the west. This area is precipitation-free and the site of widespread updrafts.

The updrafts under a cumulonimbus can be extremely laminar, extensive, and uniform, this is particularly true during the buildup of the thunderstorm. They can last more than one hour and correspond to a steady state of the cumulonimbus.

The updraft under the cloud is mostly due to buoyancy, but there is also a large pressure difference between the base and the top of the cumulonimbus (larger than would be found in this height range outside the cloud) and local low-level mechanical lifting such as the lifting generated by a downburst. The two last phenomena can overcome a stable air zone close to the surface by lifting cooler air parcels to a level where they are eventually warmer than the surrounding air. This can happen if these mechanical phenomena lift the parcel above the lifted condensation level (LCL), above which height the parcel's temperature T_p(z) decreases less with height (due to the release of latent heat and at approximately 6.5 K/km) than the surrounding air temperature T_s(z) decreases with height in the case of a conditionally unstable lapse rate aloft. In other words, the parcel can be lifted to a height where ${\displaystyle \left|\frac{d{T}_p(z)}{dz}\right|<\left|\frac{d{T}_s(z)}{dz}\right|}$, where the former is the cooling rate of the parcel and the latter is the ambient lapse rate. In these conditions, the rising parcel may eventually become warmer than the surrounding air; in other words, there may exist a level above which ${\displaystyle T_p(z)>T_s(z)}$. This scenario's conditionally unstable lapse rate aloft is relatively common when thunderstorms exist. In effect, at low level, such air parcels are sucked into the cloud as if by a vacuum cleaner. Soaring pilots refer to this near-base sucking as "cloud suck", a phenomenon known to generally be more intense the taller the cumulus cloud – and to thus be at maximum intensity with a cumulonimbus. Since the dynamic updraft is wide, the updraft velocity varies little laterally and thus the turbulence is minimised. So, it is said:

The observations reported by Marwitz (1973), Grandia and Marwitz (1975), and Ellrod and Marwitz (1976) indicate that the updraft air entering the base of cumulonimbi is smooth and relatively free of turbulence and remains so through a significant depth of the WER.

In fact, Ellrod and Marwitz's paper is more general. These authors state that in general, the buoyancy beneath the cumulonimbus cloud base is often negative. This explains why updrafts underneath the base of a cumulonimbus are often laminar. This phenomenon is well known by glider pilots. (see below). The phenomenon is enhanced under the weak echo region of a supercell thunderstorm that is extremely dangerous. At approximately 4 kilometres (13,000 ft) these smooth updrafts become suddenly very turbulent.

In general, updrafts reach their maximum intensity at 6 kilometres (20,000 ft) above the ground. At this altitude, a phase change occurs where water droplets become ice crystals and therefore release energy in the form of latent heat and thus the updraft strength increases. Supercell thunderstorms or derechos can have gigantic updrafts at this altitude, updrafts with speeds that can exceed 40 metres per second (78 kn). Such an updraft speed corresponds to the wind speed of a small hurricane. The speed can even exceed 50 metres per second (97 kn). The maximum number in the Beaufort scale is 12 ("hurricane force" wind) and is assigned to wind speeds of 64 knots or greater. If the Beaufort scale were extended, these updrafts would have a Beaufort number of 14 in the vertical direction. The turbulence is then extreme at this altitude.

Moreover, the diameters of the updraft columns vary between 2 km (air mass thunderstorm) and 10 km (supercell thunderstorm). The height of the cumulonimbus base is extremely variable. It varies from a few tens of meters above the ground to 4000 m above the ground. In the latter case, the updrafts can originate either from the ground (if the air is very dry – typical of deserts) or from aloft (when altocumulus castellanus degenerates into cumulonimbus). When the updraft originates from aloft, this is considered elevated convection.

Dangers pertaining to downbursts

Main article: Downburst

Figure 2: Spreading of a downburst close to the ground.

Downbursts are dangerous for many reasons. First, downdraughts under cumulonimbus can be severe and extensive. A sailplane flying at 50 knots in a downdraught of 15 knots has an approximate glide ratio of 3, meaning that it covers only about three metres of ground for every metre it descends. Assuming that the glider is at cloud base height at 2,000 metres (6,600 ft), if it remains in the downdraught the entire time, it will only be able to glide 6 kilometres (3.7 mi) before being forced to land – likely under difficult and dangerous conditions. Even if the glider lands safely, it could be destroyed later by a wind gust. So when a rain curtain shows a downburst, it is of paramount importance to not land in this area.

Downdraughts of 50 knots are possible and can generate wind gusts of 60 knots or more. Safely landing a light aircraft in these conditions can be virtually impossible. Moreover, close to the ground, a glider or airplane pilot can be caught by surprise by a sudden reversal of the wind direction and transition from an upwind to a downwind situation. If the airspeed becomes too low, the aircraft will stall and may crash into the ground due to the altitude lost recovering from the stall. As a consequence of famous instances of crashes of this nature in the United States, a network of wind profilers and Terminal Doppler Weather Radars was developed in the vicinity of airports to monitor this wind shear. Based on FAA rules, every pilot must inquire about the wind speed and direction before landing.

Compared to airliners, sailplanes fly at low airspeeds. The usual approach speed of a sailplane is around 50 knots, but let's assume that the pilot is extra "careful" and flies his approach at 65 knots. William Cotton claims that the wind shear can be as high as 50 knots. In such a case, if the shear direction is such that the airspeed is reduced by the shear amount, this pilot's airspeed will drop to 15 knots, which is well below his glider's stall speed (typically 35–40 knots). If this airspeed drop occurs during the transition from the base leg to the final approach, the aircraft may enter into a spin from which there isn't enough altitude to recover. The exact quotation is the following:

Upon encountering a downburst with say a 50 kt tailwind component, airspeed can drop from say 65 kts to more like 15 kts. If the sailplane is making a turn from baseleg to final, the pilot finds himself (herself) in one of the deadliest situations a pilot can encounter, a "stall-spin" situation with no chance to recover since the aircraft is close to the ground on final approach.

So when the pilot encounters benign cumulonimbus, it may be a better choice to stay aloft and use the updraughts under the cumulus in front of the thunderstorm along the flanking line (or even under the cumulonimbus itself in its laminar region) and wait for the thunderstorm to dissipate instead of attempting a landing in the presence of possible downbursts.

Flight inside cumulonimbus

Soaring

In some countries, sailplanes are permitted to fly inside clouds. For example, during the 1972 World Soaring Championship at Vršac, Yugoslavia, Helmut Reichmann attempted to use the violent updraughts associated with cumulonimbus. Initially, he found an updraught of +8 m/s. After half a circle, he was in a downdraught of −15 m/s. He had to land very shortly afterward. The thunderstorm was in its mature stage. In another example, Terry Delore got trapped in a severe thunderstorm. He entered a seemingly innocuous cumulus at 2,000 feet (610 m). This cumulus evolved into a large cumulonimbus. At first, the flight inside the cloud was turbulence-free. Then his glider suddenly became uncontrollable. He was either inverted, in a nosedive, or in a chandelle. The airbrakes became stuck open due to hailstones blocking the orifices. When he landed, the airfield was still covered by hailstones. The wind gusts were between 30 and 40 knots. Everyone on the ground feared for the pilot's life. In the same book, the author narrates that an Italian instructor at Rieti had his students climb 10,000 metres (33,000 ft) inside cumulonimbus so that they get accustomed to them.

As mentioned above, a climb inside a cumulonimbus can be initially very smooth (due to the negative buoyancy of the air parcel) and suddenly become horribly turbulent. As an example, a glider pilot found initially very laminar updraughts and got sucked into the cloud where he encountered accelerations of 18 g and became unconscious.

Due to the phase change of water droplets (to ice), the cumulonimbus top is almost always turbulent. The glider can become covered with ice, and the controls can freeze and remain stuck. Many accidents of this kind have occurred. If the pilot bails out and opens their parachute, they may be sucked upward (or at least held aloft) as happened to William Rankin after ejecting from an F-8 fighter jet and falling into a cumulonimbus (within which his parachute opened).

A skydiver or paraglider pilot under a cumulonimbus is exposed to a potentially deadly risk of being rapidly sucked up to the top the cloud and being suffocated, struck by lightning, or frozen. If they survive, they may suffer irreversible brain damage due to lack of oxygen or require amputation as a consequence of frostbite. German paraglider pilot Ewa Wiśnierska barely survived a climb of more than 9,000 metres (30,000 ft) inside a cumulonimbus.

Commercial aviation

Heavy transportation airplanes may occasionally have to cross a thunderstorm line associated with a cold front or a squall. They may not be able to overfly the cumulonimbus, because at 36,000 feet, the aircraft may be in or near what is known as the coffin corner (stall speed is close to speed of sound), thus making it structurally dangerous to climb higher. However, some cells can rise to 70,000 feet. Another option would be to navigate around the cells. This is strongly discouraged, however, because in the opening, new cells can grow very rapidly and engulf the aircraft. Whenever an aircraft moves to the west and crosses a thunderstorm line, the pilot will first encounter a line of powerful and laminar updraughts (that are not thermal but dynamic). The pilot should refrain from pushing the stick to try to maintain a constant altitude (similar to mountain waves), because pushing the stick can cause the airspeed to increase to the point of hitting the yellow arc (on the airspeed indicator). An airspeed this high is not permissible in turbulent conditions and may lead to break-up of the aircraft. When the pilot exits the updraught zone, he will encounter very strong turbulence due to the shear between rising and sinking air. If the airspeed is too high at this point, the airplane will break apart. The crash of Flight AF 447 is indirectly related to this situation: the pilot opted for the shortest path while crossing the thunderstorm line associated with the Intertropical Convergence Zone, and the pitot tubes iced over. What followed is known.

On-board radars can be deceiving. Hail shafts generate weak radar echoes, which means radar would guide the pilot there—but, they're significantly more dangerous than cloudbursts. Close to the ground, heavy rain (or snow at altitude) tends to dampen turbulence (it is said that when rain comes, most of the danger is gone). So another counter-intuitive recommendation is to fly toward the zone of heavy precipitation or toward the darkest area of the thunderstorm line. This recommendation contradicts the usual use of on-board radars to avoid areas of strong precipitation, which is usually the best course of action. There is no "miracle" solution, and the best option is to avoid these thunderstorm systems by having enough fuel on board, thus reducing the temptation to take a more dangerous route in the interest of fuel savings.

Also, St. Elmo's fires while flying inside cumulonimbus can burn out the on-board electronic equipment and even pierce a wing by melting the metal skin.

Dangers pertaining to supercell thunderstorms

Main article: Supercell thunderstorm

Figure 3 : Picture of a supercell with its characteristics

Figure 4 : Picture of the forward area of a supercell that seems usable by a glider. It is made of small cumulonimbus and an arcus. This area is treacherous because the updraughts will be laminar.

The updraughts inside a cumulonimbus associated with a supercell thunderstorm can reach 45 metres per second (87 kn). This corresponds to the wind speed of a weak hurricane. Moreover, the turbulence inside a cloud can become extreme and break apart an aircraft. Thus, it is extremely dangerous to fly inside such a system.

The thunderstorm system can be divided into two zones in the figure to the left: the precipitation-free zone, located on the left where the airmass has a widespread up motion, and the precipitation zone, on the right where the airmass is sinking. At the point where the two zones meet, there is a wall cloud that could initiate tornadoes. Moreover, even the cumulus congestus associated with a supercell thunderstorm can be very dangerous. Tornadoes can be produced up to 36 kilometres (22 mi) from the main cell.

In the updraught area, the air has a negative buoyancy and is sucked up by a low pressure zone at altitude. Turbulence is annihilated. In particular, in the forward area of the supercell, one can find a flanking line made of cumulus congestus or small cumulonimbus. The cloud base of the flanking line is higher than the base of the main cumulonimbus.

Since the updraught under these clouds (in the flanking line) is mainly dynamic, the airmass being smooth and the cloud base higher, a glider pilot could be tempted to fly in this zone. However, conditions can rapidly become dangerous, since the wall cloud can generate a tornado that will pulverise any aircraft. Moreover, since the rising air is widespread, the glider pilot (especially if flying a low-speed, low-performance glider like a paraglider) may be unable to escape and may be sucked into the cloud up to its top. Thus, the FAA recommends that aircraft should never be closer than 20 miles from severe thunderstorms.

Other dangers pertaining to cumulonimbus

Lightning

Although it rarely happens, a glider can be struck by lightning. Metal sailplanes are Faraday cages and thus should not be destroyed by a lightning strike. However, gliders made of wood or fibreglass can be destroyed. Moreover, modern sailplanes are filled with electronic devices that can be damaged by lightning. Also, any winch launch is discouraged when a thunderstorm is less than 20 kilometres (12 mi) away, because the air is electrified, and the cable will act as a lightning rod.

Hail

Hail can shred a sailplane canopy and seriously damage the wings and fuselage. Hail is barely visible and can be encountered in the updraught zone under the cloud. On 5 August 1977, an airplane pilot was taken by surprise in the vicinity of Colorado Springs by a supercell thunderstorm that produced 20 tornadoes. The pilot was flying in eerily calm air (the updraught zone can be laminar) when he saw the sky transitioning from pale grey to inky black. The pilot heard a loud sound that reoccurred more and more frequently. Then a hailstone pierced the windshield, rendering the pilot semi-unconscious. Eventually, the pilot landed his shredded airplane in a field.

Tornadoes

An EF5 tornado can generate ground winds of unbelievable speed; common sense dictates that an aircraft should never be close to such a meteorological phenomenon. Indeed the wind speed can reach 130 metres per second (250 kn), and one can easily guess that the aircraft can be torn into pieces in such conditions. However, airline transportation aircraft have overflown tornadoes by more than 8,000 feet (2,400 m) without damage. The fact that an airliner does not get destroyed can be explained as follows: tornadoes are violent phenomena only close to the ground and become weaker at height. A glider dared to cross a weak tornado during a soaring contest in Texas in 1967. The cumulonimbus base was at 12,000 feet (3,700 m). The glider crossed an extremely turbulent zone and ended up in a turbulence-free zone inverted. The controls were not responding, and the pilot contemplated abandoning the aircraft. After some time and a big fright, the controls started to respond again, and the pilot was able to continue his flight. Pilots in the vicinity did not notice anything.

On 6 October 1981 a Fokker aircraft hit a tornado which occurred in a supercell near the town of Moerdijk in the Netherlands, all 17 occupants of the aircraft were killed.

An empirical criterion for tornado formation has been developed by Dan Sowa from Northwest Orient Airlines as follows: the cumulonimbus overshooting top must enter into the stratosphere by at least 10000 feet. This criterion is, however, incorrect and the Sonnac tornado is a counter-example. It reached level EF2 while being generated by a small cumulonimbus that did not attain 9,000 metres (30,000 ft).

Myths and truth about cumulonimbus

Figure 5 : Picture of a tornado in South Oklahoma City, Oklahoma shortly before it entered Moore shot from a precipitation-free sunlit area. An absent-minded glider pilot would probably have found smooth and moderate updraughts in this sunlit zone.

Conventional wisdom

As a result of a faulty generalisation, it is very often incorrectly said that cumulonimbus and the updraughts under them are always turbulent. This fallacy originates from the fact that cumulonimbus are actually extremely turbulent at high altitude, and therefore, one might falsely deduce that cumulonimbus are turbulent at all altitudes. Reliable studies and glider pilots' experience have demonstrated that updraughts under cumulonimbus were generally smooth. As seen above, updraughts under a cumulonimbus are often dynamic and thus will be very smooth. The phenomenon is enhanced under the weak echo region of a supercell thunderstorm that is extremely dangerous. However, this phenomenon is little known in the aviation world. Thus, a widespread view in the aeronautical community is that cumulonimbus are always associated with very strong turbulence (at all altitudes) and severe thunderstorms. For example, Gil Roy, in a book endorsed by the fr:Fédération française de vol à voile, claims that:

Les cumulo-nimbus [sic] sont le siège de très violents orages. La partie avant, baptisée " front d'orage " est le théâtre de très fortes turbulences mais aussi de puissantes ascendances. (Translation: The cumulo-nimbus [sic] are always the seat of very violent thunderstorms. The forward area called thunderstorm front is the site of very strong turbulence but also of powerful updraughts.)

Also, the author talks about cumulo-nimbus [sic] of gigantic size that can reach a height of several thousand metres. While the word "several" isn't very precise, a thickness of 8000 metres is fairly typical for a cumulonimbus, with some as thick as 20000 metres or more. Moreover, the majority of cumulonimbus are associated with weak pulse thunderstorms or even simple showers without electric phenomena.

The reference to the thunderstorm front corresponds to the outflow boundary associated with downbursts that are indeed very dangerous and are the site of vortices associated with the Kelvin-Helmholtz instability at the junction between updraughts and downdraughts. However, in front of the thunderstorm, updraughts are generally laminar due to the negative buoyancy of air parcels (see above).

Also, the LUXORION web site states:

Les cumulonimbus provoquent toujours une turbulence sévère [...] Elle peut être rencontrée dans les basses couches et devancer le cumulonimbus de 10 à 25 km. (Translation: The cumulonimbus always generate a severe turbulence [...]. It can be encountered in the lower layers and get ahead of the cumulonimbus by 10 to 25 km.)

Such a claim is too broad and again contradicts the fact that updraughts in front of a thunderstorm are often laminar. However, it is true that the upper layers are almost always turbulent. However, in most cases, the aforesaid turbulence is not extreme. Along the same lines, Didier Morieux states:

Le cumulonimbus [...] est aussi le siège d'ascendances et de descendances pouvant atteindre des vitesses de 15 à 20 m/s donnant lieu à une turbulence considérable, susceptible de mettre en péril la structure des avions les plus solides.(Translation: The cumulonimbus is also the site of updraughts and downdraughts of speeds of 15 to 20 m/s generating considerable turbulence, likely to imperil the structure of most robust airplanes.)

Dennis Pagen is even more explicit. He states:

All the updrafts and downdrafts in a thunderstorm create considerable turbulence due to shear. All we have to do is think of the velocities involved and you can imagine the severity of the turbulence. Thunderstorm turbulence can (and has) tear apart airplanes.

The International cloud atlas soothes these claims: it simply states that "la turbulence is often very strong " below the cloud.

Serious hazard to glider pilots

A glider pilot convinced that cumulonimbus are always violent risks getting a nasty surprise. If he flies under the flanking line of a supercell thunderstorm and finds that the air is very smooth and updraughts are moderate, he may falsely infer that he is safe and not under a cumulonimbus; since he believes cumulonimbus are always turbulent. He may thus not realise when he is under a secondary cumulonimbus that can suck him inside the cloud, and he may encounter a wall cloud that could generate a tornado that could disintegrate his fragile skiff as shown in Figure 5. Dominique Musto cautions paraglider pilots (that might otherwise be swayed by the above myth) against the false sensation of safety in a region of extended updraughts that are rather weak as follows:

Pourtant malgré un ciel sombre et l'absence de soleil, les ascendances sont douces et généralisées dans tout le secteur. Quelque chose cloche ! Si nous ne réagissons pas très vite pour descendre, une main invisible risque de nous happer et de nous jeter en enfer! (Translation: However, notwithstanding a dark sky and lack of sunlight, the updraughts are smooth and extended in the entire area. Something is wrong. If we do not react quickly and descend, an invisible hand is likely to grab us and throw us into hell!)

This quotation summarises in three sentences the often-insidious dangers associated with cumulonimbus, dangers that are exacerbated for paraglider pilots, as German paraglider pilot Ewa Wiśnierska experienced. She survived climbing above 9,000 metres (30,000 ft) inside a cumulonimbus. A nearby fellow pilot caught in the same weather event wasn't so fortunate.

As well, in 2014, the 66 years old general Paolo Antoniazzi died after its paraglider got sucked into a cumulonimbus up to the altitude of 9,300 metres (30,500 ft).

Forerunners of a thunderstorm

The above quotation puts informally the harbingers of a thunderstorm. So a cumulonimbus acts as an enormous thermal machine that sucks up the air in the front (left side of Figure 3) and violently throws it out in the back through downbursts (right side of Figure 3). Consequently, a broad area of updraughts will be located in front of the thunderstorm. Typically, in a humid airmass, the updraughts will be on the order of 1 m/s; and in a dry air mass, they will be on the order of 2 to 3 m/s. Therefore, when a glider pilot is in an area where "updraughts are everywhere" and he is close to large clouds (that can be cumulus congestus), he is likely in the vicinity of a building thunderstorm.

Associated gravity waves

The downbursts associated with cumulonimbus can generate gravity waves far way from thunderstorms. These gravity waves can be felt up to 50 kilometres (31 mi) away and in some conditions several hundreds of kilometres away. A severe thunderstorm generating these gravity waves located at more than 40 kilometres (25 mi) away (according to Federal Aviation Administration recommendations) should not affect the safety of aircraft this far from the thunderstorm. These gravity waves can be modelled in the same manner as mountain waves and can be usable by a glider pilot.

Utilising cumulonimbus in cross-country flight or other

Exploitation of "small" cumulonimbus

Small cumulonimbus can relatively safely be exploited by experienced glider pilots. They generate moderate updraughts that are generally laminar. Thus, pulse-like summer thunderstorms can be used during cross-country flights, since the glider will move away from the cumulonimbus after having (in theory) climbed up to 500 feet below the cloud base (the maximum permissible height in the United States) and the passage of the glider in the proximity of the thunderstorm will be short. For example, during an official contest of the Soaring Society of America, pilots openly played with the cumulonimbus (and even with the updraughts contiguous to downbursts) and boasted about it. However, a rule of thumb says that the distance between two thermals is equal to three times the height of the cloud. Consequently, a cumulonimbus that is 13 km thick will eliminate any convective activity over a radius of approximately 40 km. Most gliders cannot perform such long glides, and therefore, an encounter with a pulse-like thunderstorm in a glider will often be followed soon by the end of the flight.

Shear exploitation in the vicinity of a downburst

Figure 3.22 from this reference shows the presence of a rotor outside a downburst. A more-than-foolhardy pilot could easily locate this updraught and exploit it. However, this photograph will dissuade any sensible pilot from using such monstrosities. Downbursts are the most significant hazard pertaining to thunderstorms. Moreover, if for any reason the pilot must land (hail storm or other), he will have to cross the downburst immediately above him and there will be a greatly increased chance of crashing – due to the unpredictable decrease of the airspeed. Moreover, if the glider transitions from the updraught to the downdraught, severe turbulence will occur due to the Kelvin-Helmholtz instability in the shear area. However, pilots have nonetheless exploited such updraughts.

Exploitation of flanking lines

Reckless pilots have exploited squalls by flying in front of thunderstorm systems as if flying along a ridge. The pilot really must land at an airport and put the glider in a hangar; the squall line will catch him again soon and imperil the glider if it is not protected. Dennis Pagen performed a similar flight in front of a supercell cumulonimbus during the Preliminaries of the hang glider 1990 World championship in Brazil where he was able to fly 35 km at high speed without a turn. Pagen acknowledges that his achievement was very risky, since hang gliders (and even more so paragliders) are significantly slower than sailplanes and can much more easily be sucked inside the cloud.

Conclusion

The only cumulonimbus clouds that could be usable by a glider pilot, subject to all necessary reservations, might be isolated small cumulonimbus or at a pinch the flanking lines associated with strong thunderstorms. However, examples above show that a seemingly innocuous cloud can rapidly become very dangerous. Squalls and supercell thunderstorms are definitely deadly hazards to uninformed pilots. Based on visual flight rules, flights in pre-storm areas must be visual; the pilots must be able to watch the evolution of a thundercloud and take the necessary actions of avoidance or to quickly land when appropriate.

The above examples demonstrate that the different phenomena associated with cumulonimbus can jeopardise any type of aircraft and its occupants when the pilot flies in the vicinity and especially inside a thundercloud. An airplane pilot should never come near a cumulonimbus.