NOx

From Wikipedia, the free encyclopedia

 

In atmospheric chemistry, NOx is a generic term for the nitrogen oxides that are most relevant for air pollution, namely nitric oxide (NO) and nitrogen dioxide (NO2 ). These gases contribute to the formation of smog and acid rain, as well as affecting tropospheric ozone.

 

NO
x gases are usually produced from the reaction among nitrogen and oxygen during combustion of fuels, such as hydrocarbons, in air; especially at high temperatures, such as occur in car engines. In areas of high motor vehicle traffic, such as in large cities, the nitrogen oxides emitted can be a significant source of air pollution. NO
x gases are also produced naturally by lightning.

The term NO
x is chemistry shorthand for molecules containing one nitrogen and one or more oxygen atom. It is generally meant to include nitrous oxide (N₂O), although nitrous oxide is a fairly inert oxide of nitrogen that has many uses as an oxidizer for rockets and car engines, an anesthetic, and a propellant for aerosol sprays and whipped cream. Nitrous oxide plays hardly any role in air pollution, although it may have a significant impact on the ozone layer, and is a significant greenhouse gas.

NO
y is defined as the sum of NO
x plus the NO
z compounds produced from the oxidation of NO
x which include nitric acid, nitrous acid (HONO), dinitrogen pentoxide (N₂O₅), peroxyacetyl nitrate (PAN), alkyl nitrates (RONO₂), peroxyalkyl nitrates (ROONO₂), the nitrate radical (NO₃), and peroxynitric acid (HNO₄).

Formation and reactions

Because of energy limitations, oxygen and nitrogen do not react at ambient temperatures. But at high temperatures, they undergo an endothermic reaction producing various oxides of nitrogen. Such temperatures arise inside an internal combustion engine or a power station boiler, during the combustion of a mixture of air and fuel, and naturally in a lightning flash. 

In atmospheric chemistry, the term NO
x denotes the total concentration of NO and NO
2 since the conversion between these two species is rapid in the stratosphere and troposphere. During daylight hours, these concentrations together with that of ozone are in steady state, also known as photostationary state(PSS); the ratio of NO to NO
2 is determined by the intensity of sunshine (which converts NO
2 to NO) and the concentration of ozone (which reacts with NO to again form NO
2 ).

In other words, the concentration of ozone in the atmosphere is determined by the ratio of these two species.

${\displaystyle {\ce {NO2 + h\nu -> NO + O\ (^3P)}},\qquad \lambda <424 annotation="" ce="" nm="">$

(1)

${\displaystyle {\ce {O\ (^3P) + O2 + M -> O3 + M}}}$

(2)

${\displaystyle {\ce {O3 + NO -> NO2 + O2}}}$

(3)

${\displaystyle {\frac {{\ce {[NO2]}}}{{\ce {[NO]}}}}={\frac {k_{3}[{\ce {O3}}]}{j_{{\ce {NO2}}}}}}$

(4)

This relationship between NO
x and ozone is also known as the Leighton relationship. 

The time τ that is needed to reach a steady state among NO
x and ozone is dominated by reaction (3), which reverses reactions (1)+(2):

${\displaystyle \tau ={\frac {1}{k_{3}[{\ce {NO}}]}}}$

(5)

for mixing ratio of NO, [NO] = 1 part per billion (ppb), the time constant is 40 minutes; for [NO] = 10 ppb, 4 minutes.

Formation of smog

When NO
x and volatile organic compounds (VOCs) react in the presence of sunlight, they form photochemical smog, a significant form of air pollution. The presence of photochemical smog increases during the summer when the incident solar radiation is higher. The emitted hydrocarbons from industrial activities and transportation react with NO_x quickly and increase the concentration of ozone and peroxide compounds, especially peroxyacetyl nitrate (PAN).

Children, people with lung diseases such as asthma, and people who work or exercise outside are particularly susceptible to adverse effects of smog such as damage to lung tissue and reduction in lung function.

Formation of nitric acid and acid rain

NO₂ is further oxidized in the gas phase during daytime by reaction with OH

NO₂ + OH (+M) → HNO₃ (+M),

where M denotes a third molecule required to stabilize the addition product. Nitric acid (HNO₃) is highly soluble in liquid water in aerosol particles or cloud drops. 

NO₂ also reacts with ozone to form nitrate radical

NO₂ + O₃ → NO₃ + O₂.

During the daytime, NO₃ is quickly photolyzed back to NO₂, but at nighttime it can react with a second NO₂ to form dinitrogen pentoxide.

NO₂ + NO₃ (+M) → N₂O₅ (+M).

N₂O₅ reacts rapidly with liquid water (in aerosol particles or cloud drops, but not in the gas phase) to form nitric acid HNO₃,

N₂O₅ + H₂O(liq) → 2 HNO₃(aq)

These are thought to be the principal pathways for formation of nitric acid in the atmosphere. This nitric acid contributes to acid rain or may deposit to soil, where it makes nitrate, which is of use to growing plants. The aqueous phase reaction

2 NO
2 + H₂O → HNO₂ + HNO₃

is too slow to be of any significance in the atmosphere.

Sources

Natural sources

Nitric oxide is produced during thunderstorms due to the extreme heating and cooling within a lightning strike. This causes stable molecules such as N₂ and O₂ to convert into significant amounts of NO similar to the process that occurs during high temperature fuel combustion. NO_x from lightning can become oxidized to produce nitric acid (HNO₃), this can be precipitated out as acid rain or deposited onto particles in the air. Elevated production of NO_xfrom lightning depends on the season and geographic location. The occurrence of lightning is more common over land near the equator in the inter-tropical convergence zone (ITCZ) during summer months. This area migrates slightly as seasons change. NO_x production from lightning can be observed through satellite observations. 

Scientists Ott et al. estimated that each flash of lightning on average in the several mid-latitude and subtropical thunderstorms studied turned 7 kg (15 lb) of nitrogen into chemically reactive NO
x . With 1.4 billion lightning flashes per year, multiplied by 7 kilograms per lightning strike, they estimated the total amount of NO
x produced by lightning per year is 8.6 million tonnes. However, NO
x emissions resulting from fossil fuel combustion are estimated at 28.5 million tonnes.

A recent discovery indicated that cosmic ray and solar flares can significantly influence the number of lightning strikes occurring on Earth. Therefore, space weather can be a major driving force of lightning-produced atmospheric NO
x . It should also be noted that atmospheric constituents such as nitrogen oxides can be stratified vertically in the atmosphere. Ott noted that the lightning-produced NO
x is typically found at altitudes greater than 5 km, while combustion and biogenic (soil) NO
x are typically found near the sources at near surface elevation (where it can cause the most significant health effects).

Biogenic sources

Agricultural fertilization and the use of nitrogen fixing plants also contribute to atmospheric NO
x , by promoting nitrogen fixation by microorganisms. The nitrification process transforms ammonia into nitrate. And the denitrification is basically the reverse process of nitrification. During the denitrification, nitrate is reduced to nitrite then NO then N₂O and finally nitrogen. Through these processes, NO_x is emitted to the atmosphere.

A recent study conducted by the University of California Davis, found that adding nitrogen fertilizer to soil in California is contributing 25 percent or more to state-wide NO_x pollution levels. When nitrogen fertilizer is added to the soil, excess ammonium and nitrate not used by plants can be converted to NO by microorganism in the soil, which escapes into the air. NO_x is a precursor for smog formation which is already a known issue for the state of California. In addition to contributing to smog, when nitrogen fertilizer is added to the soil and the excess is released in the form of NO, or leached as nitrate this can be a costly process for the farming industry.

A 2018 study by the Indiana University determined that forests in the eastern United States can expect to see increases in NO_x, as a result to changes in the types of trees which predominate. Due to human activity and climate change, the maples, sassafrass, and tulip poplar are pushing out the beneficial oak, beech, and hickory. The team determined that the first three tree species, maples, sassafrass, and tulip poplar, are associated with ammonia-oxidizing bacteria known to "emit reactive nitrogen from soil." By contrast, the second three tree species, oak, beech and hickory, are associated with microbes that "absorb reactive nitrogen oxides," and thus can have a positive impact on the nitrogen oxide component of air quality. Nitrogen oxide release from forest soils is expected to be highest in Indiana, Illinois, Michigan, Kentucky and Ohio.

Industrial sources (anthropogenic sources)

The three primary sources of NO
x in combustion processes:

• thermal NO
  x
• fuel NO
  x
• prompt NO
  x

Thermal NO
x formation, which is highly temperature dependent, is recognized as the most relevant source when combusting natural gas. Fuel NO
x tends to dominate during the combustion of fuels, such as coal, which have a significant nitrogen content, particularly when burned in combustors designed to minimise thermal NO
x . The contribution of prompt NO
x is normally considered negligible. A fourth source, called feed NO
x is associated with the combustion of nitrogen present in the feed material of cement rotary kilns, at between 300 °C and 800 °C, where it is considered a minor contributor.

Thermal

Thermal NO
x refers to NO
x formed through high temperature oxidation of the diatomic nitrogen found in combustion air. The formation rate is primarily a function of temperature and the residence time of nitrogen at that temperature. At high temperatures, usually above 1600 °C (2900 °F), molecular nitrogen (N₂) and oxygen (O₂) in the combustion air disassociate into their atomic states and participate in a series of reactions. 

The three principal reactions (the extended Zel'dovich mechanism) producing thermal NO
x are:

N₂+ O ⇌ NO + N

N + O₂ ⇌ NO + O

N + OH${\displaystyle {\ce {{.}}}}$ ⇌ NO + H${\displaystyle {\ce {{.}}}}$

All three reactions are reversible. Zeldovich was the first to suggest the importance of the first two reactions. The last reaction of atomic nitrogen with the hydroxyl radical, ^•HO, was added by Lavoie, Heywood and Keck to the mechanism and makes a significant contribution to the formation of thermal NO
x .

Fuel

It is estimated that transportation fuels cause 54% of the anthropogenic (i.e. human-caused) NO
x . The major source of NO
x production from nitrogen-bearing fuels such as certain coals and oil, is the conversion of fuel bound nitrogen to NO
x during combustion. During combustion, the nitrogen bound in the fuel is released as a free radical and ultimately forms free N₂, or NO. Fuel NO
x can contribute as much as 50% of total emissions through the combusting oil and as much as 80% through the combusting of coal. 

Although the complete mechanism is not fully understood, there are two primary pathways of formation. The first involves the oxidation of volatile nitrogen species during the initial stages of combustion. During the release and before the oxidation of the volatiles, nitrogen reacts to form several intermediaries which are then oxidized into NO. If the volatiles evolve into a reducing atmosphere, the nitrogen evolved can readily be made to form nitrogen gas, rather than NO
x . The second pathway involves the combustion of nitrogen contained in the char matrix during the combustion of the char portion of the fuels. This reaction occurs much more slowly than the volatile phase. Only around 20% of the char nitrogen is ultimately emitted as NO
x , since much of the NO
x that forms during this process is reduced to nitrogen by the char, which is nearly pure carbon.

Prompt

This third source is attributed to the reaction of atmospheric nitrogen, N₂, with radicals such as C, CH, and CH₂ fragments derived from fuel, rather than thermal or fuel processes. Occurring in the earliest stage of combustion, this results in the formation of fixed species of nitrogen such as NH (nitrogen monohydride), NCN(cyanonitrene), HCN (hydrogen cyanide), H₂CN (dihydrogen cyanide) and ^•CN (cyano radical) which can oxidize to NO. In fuels that contain nitrogen, the incidence of prompt NO
x is comparatively small and it is generally only of interest for the most exacting emission targets.

Health and environment effects

NO
x reacts with ammonia, moisture, and other compounds to form nitric acid vapor and related particles. Small particles can penetrate deeply into sensitive lung tissue and damage it, causing premature death in extreme cases. Inhalation of such particles causes or worsens respiratory diseases, such as emphysema or bronchitis, and aggravates existing heart disease.

NO
x reacts with volatile organic compounds in the presence of sunlight to form ozone. Ozone can cause adverse effects such as damage to lung tissue and reduction in lung function mostly in susceptible populations (children, elderly, asthmatics). Ozone can be transported by wind currents and cause health impacts far from the original sources. The American Lung Association estimates that nearly 50 percent of United States inhabitants live in counties that are not in ozone compliance. In South East England, ground level ozone pollution tends to be highest in the countryside and in suburbs, while in central London and on major roads NO emissions are able to "mop up" ozone to form NO
2 and oxygen.

NO
x also readily reacts with common organic chemicals, and even ozone, to form a wide variety of toxic products: nitroarenes, nitrosamines and also the nitrate radical some of which may cause DNA mutations. Recently another pathway, via NO
x , to ozone has been found that predominantly occurs in coastal areas via formation of nitryl chloride when NO
x comes into contact with salt mist.

The direct effect of the emission of NO
x has positive contribution to the greenhouse effect. Instead of reacting with ozone in Reaction 3, NO can also react with HO₂· and organic peroxyradicals (RO₂·) and thus increase the concentration of ozone. Once the concentration of NO
x exceeds a certain level, atmospheric reactions result in net ozone formation. Since tropospheric ozone can absorb infrared radiation, this indirect effect of NO
x is intensifying global warming. 

There are also other indirect effects of NO
x that can either increase or decrease the greenhouse effect. First of all, through the reaction of NO with HO₂ radicals, ^•OH radicals are recycled, which oxidize methane molecules, meaning NO
x emissions can counter the effect of greenhouse gases. For instance, ship traffic emits a great amount of NO_x which provides a source of NO_x over the ocean. Then, photolysis of NO₂ leads to the formation of ozone and the further formation of hydroxyl radicals (·OH) through ozone photolysis. Since the major sink of methane in the atmosphere is by reaction with ^•OH radicals, the NO_x emissions from ship travel may lead to a net global cooling. However, NO
x in the atmosphere may undergo dry or wet deposition and return to land in the form of HNO₃/NO₃⁻. Through this way, the deposition leads to nitrogen fertilization and the subsequent formation of nitrous oxide (N₂O) in soil, which is another greenhouse gas. In conclusion, considering several direct and indirect effects, NO
x emissions have a negative contribution to global warming.

NO
x in the atmosphere is removed through several pathways. During daytime, NO₂ reacts with hydroxyl radicals (·OH) and forms nitric acid (HNO₃), which can easily be removed by dry and wet deposition. Organic peroxyradicals (RO₂·) can also react with NO and NO₂ and result in the formation of organic nitrates. These are ultimately broken down to inorganic nitrate, which is a useful nutrient for plants. During nighttime, NO₂ and NO can form nitrous acid (HONO) through surface-catalyzed reaction. Although the reaction is relatively slow, it is an important reaction in urban areas. In addition, the nitrate radical (NO₃) is formed by the reaction between NO₂ and ozone. At night, NO₃ further reacts with NO₂ and establishes a equilibrium reaction with dinitrogen pentoxide (N₂O₅). Via heterogeneous reaction, N₂O₅ reacts with water vapor or liquid water and forms nitric acid (HNO₃). As mentioned above, nitric acid can be removed through wet and dry deposition and this results in the removal of NO
x from the atmosphere.

Biodiesel and NO
x

Biodiesel and its blends in general are known to reduce harmful tailpipe emissions such as: carbon monoxide; particulate matter (PM), otherwise known as soot; and unburned hydrocarbon emissions. While earlier studies suggested biodiesel could sometimes decrease NOx and sometimes increase NOx emissions, subsequent investigation has shown that blends of up to 20% biodiesel in USEPA-approved diesel fuel have no significant impact on NOx emissions compared with regular diesel. The state of California uses a special formulation of diesel fuel to produce less NOx relative to diesel fuel used in the other 49 states. This has been deemed necessary by the California Air Resources Board (CARB) to offset the combination of vehicle congestion, warm temperatures, extensive sunlight, PM, and topography that all contribute to the formation of ozone and smog. CARB has established a special regulation for Alternative Diesel Fuels to ensure that any new fuels, including biodiesel, coming into the market do not substantially increase NOx emissions. The reduction of NO
x emissions is one of the most important challenges for advances in vehicle technology. While diesel vehicles sold in the US since 2010 are dramatically cleaner than previous diesel vehicles, urban areas continue to seek more ways to reduce the formation of smog and ozone. NO
x formation during combustion is associated with a number of factors such as combustion temperature. As such, it can be observed that the vehicle drive cycle, or the load on the engine have more significant impact on NOx emissions than the type of fuel used. This may be especially true for modern, clean diesel vehicles that continuously monitor engine operation electronically and actively control engine parameters and exhaust system operations to limit NOx emission to less than 0.2 g/km. Low-temperature combustion or LTC technology. may help reduce thermal formation of NO
x during combustion, however a tradeoff exists as high temperature combustion produces less PM or soot and results in greater power and fuel efficiency.

Regulation and emission control technologies

Selective catalytic reduction (SCR) and selective non-catalytic reduction (SNCR) reduce post combustion NO
x by reacting the exhaust with urea or ammonia to produce nitrogen and water. SCR is now being used in ships, diesel trucks and in some diesel cars. The use of exhaust gas recirculation and catalytic converters in motor vehicle engines have significantly reduced vehicular emissions. NO
x was the main focus of the Volkswagen emissions violations.

Other technologies such as flameless oxidation (FLOX) and staged combustion significantly reduce thermal NO
x in industrial processes. Bowin low NO
x technology is a hybrid of staged-premixed-radiant combustion technology with a major surface combustion preceded by a minor radiant combustion. In the Bowin burner, air and fuel gas are premixed at a ratio greater than or equal to the stoichiometric combustion requirement. Water Injection technology, whereby water is introduced into the combustion chamber, is also becoming an important means of NO
x reduction through increased efficiency in the overall combustion process. Alternatively, the water (e.g. 10 to 50%) is emulsified into the fuel oil before the injection and combustion. This emulsification can either be made in-line (unstabilized) just before the injection or as a drop-in fuel with chemical additives for long term emulsion stability (stabilized). Inline emulsified fuel/water mixtures show NO
x reductions between 4 and 83%.

Thunderstorm

From Wikipedia, the free encyclopedia

 

Thunderstorm
A thunderstorm near Havelsee, Germany
Area of occurrence	Primarily tropical and also temperate regions.
Season	Most common in spring and summer.
Effect	Depends on the storm, may involve rain, hail, and/or high winds. May cause flooding or fires.

A typical thunderstorm over a field

 

A thunderstorm, also known as an electrical storm or a lightning storm, is a storm characterized by the presence of lightning and its acoustic effect on the Earth's atmosphere, known as thunder. Relatively weak thunderstorms are sometimes called thundershowers. Thunderstorms occur in a type of cloud known as a cumulonimbus. They are usually accompanied by strong winds, and often produce heavy rain and sometimes snow, sleet, or hail, but some thunderstorms produce little precipitation or no precipitation at all. Thunderstorms may line up in a series or become a rainband, known as a squall line. Strong or severe thunderstorms include some of the most dangerous weather phenomena, including large hail, strong winds, and tornadoes. Some of the most persistent severe thunderstorms, known as supercells, rotate as do cyclones. While most thunderstorms move with the mean wind flow through the layer of the troposphere that they occupy, vertical wind shear sometimes causes a deviation in their course at a right angle to the wind shear direction.

Thunderstorms result from the rapid upward movement of warm, moist air, sometimes along a front. As the warm, moist air moves upward, it cools, condenses, and forms a cumulonimbus cloud that can reach heights of over 20 kilometres (12 mi). As the rising air reaches its dew point temperature, water vapor condenses into water droplets or ice, reducing pressure locally within the thunderstorm cell. Any precipitation falls the long distance through the clouds towards the Earth's surface. As the droplets fall, they collide with other droplets and become larger. The falling droplets create a downdraft as it pulls cold air with it, and this cold air spreads out at the Earth's surface, occasionally causing strong winds that are commonly associated with thunderstorms. 

Thunderstorms can form and develop in any geographic location but most frequently within the mid-latitude, where warm, moist air from tropical latitudes collides with cooler air from polar latitudes. Thunderstorms are responsible for the development and formation of many severe weather phenomena. Thunderstorms, and the phenomena that occur along with them, pose great hazards. Damage that results from thunderstorms is mainly inflicted by downburst winds, large hailstones, and flash flooding caused by heavy precipitation. Stronger thunderstorm cells are capable of producing tornadoes and waterspouts. 

There are four types of thunderstorms: single-cell, multi-cell cluster, multi-cell lines and supercells. Supercell thunderstorms are the strongest and most severe. Mesoscale convective systems formed by favorable vertical wind shear within the tropics and subtropics can be responsible for the development of hurricanes. Dry thunderstorms, with no precipitation, can cause the outbreak of wildfires from the heat generated from the cloud-to-ground lightning that accompanies them. Several means are used to study thunderstorms: weather radar, weather stations, and video photography. Past civilizations held various myths concerning thunderstorms and their development as late as the 18th century. Beyond the Earth's atmosphere, thunderstorms have also been observed on the planets of Jupiter, Saturn, Neptune, and, probably, Venus.

Life cycle

Stages of a thunderstorm's life.

Warm air has a lower density than cool air, so warmer air rises upwards and cooler air will settle at the bottom (this effect can be seen with a hot air balloon). Clouds form as relatively warmer air, carrying moisture, rises within cooler air. The moist air rises, and, as it does so, it cools and some of the water vapor in that rising air condenses. When the moisture condenses, it releases energy known as latent heat of condensation, which allows the rising packet of air to cool less than the cooler surrounding air continuing the cloud's ascension. If enough instability is present in the atmosphere, this process will continue long enough for cumulonimbus clouds to form and produce lightning and thunder. Meteorological indices such as convective available potential energy (CAPE) and the lifted index can be used to assist in determining potential upward vertical development of clouds. Generally, thunderstorms require three conditions to form:

1. Moisture
2. An unstable airmass
3. A lifting force (heat)

All thunderstorms, regardless of type, go through three stages: the developing stage, the mature stage, and the dissipation stage. The average thunderstorm has a 24 km (15 mi) diameter. Depending on the conditions present in the atmosphere, each of these three stages take an average of 30 minutes.

Developing stage

The first stage of a thunderstorm is the cumulus stage or developing stage. During this stage, masses of moisture are lifted upwards into the atmosphere. The trigger for this lift can be solar illumination, where the heating of the ground produces thermals, or where two winds converge forcing air upwards, or where winds blow over terrain of increasing elevation. The moisture carried upward cools into liquid drops of water due to lower temperatures at high altitude, which appear as cumulus clouds. As the water vapor condenses into liquid, latent heat is released, which warms the air, causing it to become less dense than the surrounding, drier air. The air tends to rise in an updraft through the process of convection (hence the term convective precipitation). This process creates a low-pressure zone within and beneath the forming thunderstorm. In a typical thunderstorm, approximately 500 million kilograms of water vapor are lifted into the Earth's atmosphere.

Mature stage

Anvil-shaped thundercloud in the mature stage

 

In the mature stage of a thunderstorm, the warmed air continues to rise until it reaches an area of warmer air and can rise no farther. Often this 'cap' is the tropopause. The air is instead forced to spread out, giving the storm a characteristic anvil shape. The resulting cloud is called cumulonimbus incus. The water droplets coalesce into larger and heavier droplets and freeze to become ice particles. As these fall, they melt to become rain. If the updraft is strong enough, the droplets are held aloft long enough to become so large that they do not melt completely but fall as hail. While updrafts are still present, the falling rain drags the surrounding air with it, creating downdrafts as well. The simultaneous presence of both an updraft and a downdraft marks the mature stage of the storm and produces cumulonimbus clouds. During this stage, considerable internal turbulence can occur, which manifests as strong winds, severe lightning, and even tornadoes.

Typically, if there is little wind shear, the storm will rapidly enter the dissipating stage and 'rain itself out', but, if there is sufficient change in wind speed or direction, the downdraft will be separated from the updraft, and the storm may become a supercell, where the mature stage can sustain itself for several hours.

Dissipating stage

A thunderstorm in an environment with no winds to shear the storm or blow the anvil in any one direction

 

In the dissipation stage, the thunderstorm is dominated by the downdraft. If atmospheric conditions do not support super cellular development, this stage occurs rather quickly, approximately 20–30 minutes into the life of the thunderstorm. The downdraft will push down out of the thunderstorm, hit the ground and spread out. This phenomenon is known as a downburst. The cool air carried to the ground by the downdraft cuts off the inflow of the thunderstorm, the updraft disappears and the thunderstorm will dissipate. Thunderstorms in an atmosphere with virtually no vertical wind shear weaken as soon as they send out an outflow boundary in all directions, which then quickly cuts off its inflow of relatively warm, moist air, and kills the thunderstorm's further growth. The downdraft hitting the ground creates an outflow boundary. This can cause downbursts, a potential hazardous condition for aircraft to fly through, as a substantial change in wind speed and direction occurs, resulting in a decrease of airspeed and the subsequent reduction in lift for the aircraft. The stronger the outflow boundary is, the stronger the resultant vertical wind shear becomes.

Classification

Conditions favorable for thunderstorm types and complexes

 

There are four main types of thunderstorms: single-cell, multi-cell, squall line (also called multi-cell line) and supercell. Which type forms depends on the instability and relative wind conditions at different layers of the atmosphere ("wind shear"). Single-cell thunderstorms form in environments of low vertical wind shear and last only 20–30 minutes.

Organized thunderstorms and thunderstorm clusters/lines can have longer life cycles as they form in environments of significant vertical wind shear, normally greater than 25 knots (13 m/s) in the lowest 6 kilometres (3.7 mi) of the troposphere, which aids the development of stronger updrafts as well as various forms of severe weather. The supercell is the strongest of the thunderstorms, most commonly associated with large hail, high winds, and tornado formation. Precipitable water values of greater than 31.8 millimetres (1.25 in) favor the development of organized thunderstorm complexes. Those with heavy rainfall normally have precipitable water values greater than 36.9 millimetres (1.45 in). Upstream values of CAPE of greater than 800 J/kg are usually required for the development of organized convection.

Single-cell

A single-cell thunderstorm over Wagga Wagga.

 

This term technically applies to a single thunderstorm with one main updraft. Also known as air-mass thunderstorms, these are the typical summer thunderstorms in many temperate locales. They also occur in the cool unstable air that often follows the passage of a cold front from the sea during winter. Within a cluster of thunderstorms, the term "cell" refers to each separate principal updraft. Thunderstorm cells occasionally form in isolation, as the occurrence of one thunderstorm can develop an outflow boundary that sets up new thunderstorm development. Such storms are rarely severe and are a result of local atmospheric instability; hence the term "air mass thunderstorm". When such storms have a brief period of severe weather associated with them, it is known as a pulse severe storm. Pulse severe storms are poorly organized and occur randomly in time and space, making them difficult to forecast. Single-cell thunderstorms normally last 20–30 minutes.

Multi-cell clusters

A group of thunderstorms over Brazil photographed by the Space Shuttle Challenger.

 

This is the most common type of thunderstorm development. Mature thunderstorms are found near the center of the cluster, while dissipating thunderstorms exist on their downwind side. Multicell storms form as clusters of storms but may then evolve into one or more squall lines. While each cell of the cluster may only last 20 minutes, the cluster itself may persist for hours at a time. They often arise from convective updrafts in or near mountain ranges and linear weather boundaries, such as strong cold fronts or troughs of low pressure. These type of storms are stronger than the single-cell storm, yet much weaker than the supercell storm. Hazards with the multicell cluster include moderate-sized hail, flash flooding, and weak tornadoes.

Multicell lines

A squall line is an elongated line of severe thunderstorms that can form along or ahead of a cold front. In the early 20th century, the term was used as a synonym for cold front. The squall line contains heavy precipitation, hail, frequent lightning, strong straight line winds, and possibly tornadoes and waterspouts. Severe weather in the form of strong straight-line winds can be expected in areas where the squall line itself is in the shape of a bow echo, within the portion of the line that bows out the most. Tornadoes can be found along waves within a line echo wave pattern, or LEWP, where mesoscale low pressure areas are present. Some bow echoes in the summer are called derechos, and move quite fast through large sections of territory. On the back edge of the rain shield associated with mature squall lines, a wake low can form, which is a mesoscale low pressure area that forms behind the mesoscale high pressure system normally present under the rain canopy, which are sometimes associated with a heat burst. This kind of storm is also known as "Wind of the Stony Lake" (Traditional Chinese:石湖風 – shi2 hu2 feng1, Simplified Chinese: 石湖风) in southern China.

Supercells

A supercell thunderstorm over Chaparral, New Mexico.

 

The setting sun illuminates the top of a classic anvil-shaped thunderstorm cloud in eastern Nebraska, United States.

 

Supercell storms are large, usually severe, quasi-steady-state storms that form in an environment where wind speed or wind direction varies with height ("wind shear"), and they have separate downdrafts and updrafts (i.e., where its associated precipitation is not falling through the updraft) with a strong, rotating updraft (a "mesocyclone"). These storms normally have such powerful updrafts that the top of the supercell storm cloud (or anvil) can break through the troposphere and reach into the lower levels of the stratosphere. Supercell storms can be 24 kilometres (15 mi) wide. Research has shown that at least 90 percent of supercells cause severe weather. These storms can produce destructive tornadoes, extremely large hailstones (10 centimetres or 4 inches diameter), straight-line winds in excess of 130 km/h (81 mph), and flash floods. In fact, research has shown that most tornadoes occur from this type of thunderstorm. Supercells are generally the strongest type of thunderstorm.

Severe thunderstorms

In the United States, a thunderstorm is classed as severe if winds reach at least 93 kilometres per hour (58 mph), hail is 25 millimetres (1 in) in diameter or larger, or if funnel clouds or tornadoes are reported. Although a funnel cloud or tornado indicates a severe thunderstorm, a tornado warning is issued in place of a severe thunderstorm warning. A severe thunderstorm warning is issued if a thunderstorm becomes severe, or will soon turn severe. In Canada, a rainfall rate greater than 50 millimetres (2 in) in one hour, or 75 millimetres (3 in) in three hours, is also used to indicate severe thunderstorms. Severe thunderstorms can occur from any type of storm cell. However, multicell, supercell, and squall lines represent the most common forms of thunderstorms that produce severe weather.

Mesoscale convective systems

MCC moving through New England: August 2, 2006 0600 UTC

 

A mesoscale convective system (MCS) is a complex of thunderstorms that becomes organized on a scale larger than the individual thunderstorms but smaller than extratropical cyclones, and normally persists for several hours or more. A mesoscale convective system's overall cloud and precipitation pattern may be round or linear in shape, and include weather systems such as tropical cyclones, squall lines, lake-effect snow events, polar lows, and mesoscale convective complexes (MCCs), and they generally form near weather fronts. Most mesoscale convective systems develop overnight and continue their lifespan through the next day. The type that forms during the warm season over land has been noted across North America, Europe, and Asia, with a maximum in activity noted during the late afternoon and evening hours.

Forms of MCS that develop in the tropics are found in use either the Intertropical Convergence Zone or monsoon troughs, generally within the warm season between spring and fall. More intense systems form over land than over water. One exception is that of lake-effect snow bands, which form due to cold air moving across relatively warm bodies of water, and occurs from fall through spring. Polar lows are a second special class of MCS. They form at high latitudes during the cold season. Once the parent MCS dies, later thunderstorm development can occur in connection with its remnant mesoscale convective vortex (MCV). Mesoscale convective systems are important to the United States rainfall climatology over the Great Plains since they bring the region about half of their annual warm season rainfall.

Motion

Thunderstorm line viewed in reflectivity (dBZ) on a plan position indicator radar display

 

The two major ways thunderstorms move are via advection of the wind and propagation along outflow boundaries towards sources of greater heat and moisture. Many thunderstorms move with the mean wind speed through the Earth's troposphere, the lowest 8 kilometres (5.0 mi) of the Earth's atmosphere. Weaker thunderstorms are steered by winds closer to the Earth's surface than stronger thunderstorms, as the weaker thunderstorms are not as tall. Organized, long-lived thunderstorm cells and complexes move at a right angle to the direction of the vertical wind shear vector. If the gust front, or leading edge of the outflow boundary, races ahead of the thunderstorm, its motion will accelerate in tandem. This is more of a factor with thunderstorms with heavy precipitation (HP) than with thunderstorms with low precipitation (LP). When thunderstorms merge, which is most likely when numerous thunderstorms exist in proximity to each other, the motion of the stronger thunderstorm normally dictates the future motion of the merged cell. The stronger the mean wind, the less likely other processes will be involved in storm motion. On weather radar, storms are tracked by using a prominent feature and tracking it from scan to scan.

Back-building thunderstorm

A back-building thunderstorm, commonly referred to as a training thunderstorm, is a thunderstorm in which new development takes place on the upwind side (usually the west or southwest side in the Northern Hemisphere), such that the storm seems to remain stationary or propagate in a backward direction. Though the storm often appears stationary on radar, or even moving upwind, this is an illusion. The storm is really a multi-cell storm with new, more vigorous cells that form on the upwind side, replacing older cells that continue to drift downwind. When this happens, catastrophic flooding is possible. In Rapid City, South Dakota, in 1972, an unusual alignment of winds at various levels of the atmosphere combined to produce a continuously training set of cells that dropped an enormous quantity of rain upon the same area, resulting in devastating flash flooding. A similar event occurred in Boscastle, England, on 16 August 2004, and over Chennai on 1 December 2015. 

Hazards

Each year, many people are killed or seriously injured by severe thunderstorms despite the advance warning. While severe thunderstorms are most common in the spring and summer, they can occur at just about any time of the year.

Cloud-to-ground lightning

A return stroke, cloud-to-ground lightning strike during a thunderstorm.

 

Cloud-to-ground lightning frequently occurs within the phenomena of thunderstorms and have numerous hazards towards landscapes and populations. One of the more significant hazards lightning can pose is the wildfires they are capable of igniting. Under a regime of low precipitation (LP) thunderstorms, where little precipitation is present, rainfall cannot prevent fires from starting when vegetation is dry as lightning produces a concentrated amount of extreme heat. Direct damage caused by lightning strikes occurs on occasion. In areas with a high frequency for cloud-to-ground lightning, like Florida, lightning causes several fatalities per year, most commonly to people working outside.

Acid rain is also a frequent risk produced by lightning. Distilled water has a neutral pH of 7. “Clean” or unpolluted rain has a slightly acidic pH of about 5.2, because carbon dioxide and water in the air react together to form carbonic acid, a weak acid (pH 5.6 in distilled water), but unpolluted rain also contains other chemicals. Nitric oxide present during thunderstorm phenomena, caused by the oxidation of atmospheric nitrogen, can result in the production of acid rain, if nitric oxide forms compounds with the water molecules in precipitation, thus creating acid rain. Acid rain can damage infrastructures containing calcite or certain other solid chemical compounds. In ecosystems, acid rain can dissolve plant tissues of vegetations and increase acidification process in bodies of water and in soil, resulting in deaths of marine and terrestrial organisms.

Hail

Hailstorm in Bogotá, Colombia.

 

Any thunderstorm that produces hail that reaches the ground is known as a hailstorm. Thunderclouds that are capable of producing hailstones are often seen obtaining green coloration. Hail is more common along mountain ranges because mountains force horizontal winds upwards (known as orographic lifting), thereby intensifying the updrafts within thunderstorms and making hail more likely. One of the more common regions for large hail is across mountainous northern India, which reported one of the highest hail-related death tolls on record in 1888. China also experiences significant hailstorms. Across Europe, Croatia experiences frequent occurrences of hail.

In North America, hail is most common in the area where Colorado, Nebraska, and Wyoming meet, known as "Hail Alley". Hail in this region occurs between the months of March and October during the afternoon and evening hours, with the bulk of the occurrences from May through September. Cheyenne, Wyoming is North America's most hail-prone city with an average of nine to ten hailstorms per season. In South America, areas prone to hail are cities like Bogotá, Colombia.

Hail can cause serious damage, notably to automobiles, aircraft, skylights, glass-roofed structures, livestock, and most commonly, farmers' crops. Hail is one of the most significant thunderstorm hazards to aircraft. When hail stones exceed 13 millimetres (0.5 in) in diameter, planes can be seriously damaged within seconds. The hailstones accumulating on the ground can also be hazardous to landing aircraft. Wheat, corn, soybeans, and tobacco are the most sensitive crops to hail damage. Hail is one of Canada's most costly hazards. Hailstorms have been the cause of costly and deadly events throughout history. One of the earliest recorded incidents occurred around the 9th century in Roopkund, Uttarakhand, India. The largest hailstone in terms of maximum circumference and length ever recorded in the United States fell in 2003 in Aurora, Nebraska, United States.

Tornadoes and waterspouts

In June 2007, the town of Elie, Manitoba was struck by an F5 tornado.

A tornado is a violent, rotating column of air in contact with both the surface of the earth and a cumulonimbus cloud (otherwise known as a thundercloud) or, in rare cases, the base of a cumulus cloud. Tornadoes come in many sizes but are typically in the form of a visible condensation funnel, whose narrow end touches the earth and is often encircled by a cloud of debris and dust. Most tornadoes have wind speeds between 40 and 110 mph (64 and 177 km/h), are approximately 75 metres (246 ft) across, and travel several kilometers (a few miles) before dissipating. Some attain wind speeds of more than 300 mph (480 km/h), stretch more than 1,600 metres (1 mi) across, and stay on the ground for more than 100 kilometres (dozens of miles).

The Fujita scale and the Enhanced Fujita Scale rate tornadoes by damage caused. An EF0 tornado, the weakest category, damages trees but not substantial structures. An EF5 tornado, the strongest category, rips buildings off their foundations and can deform large skyscrapers. The similar TORRO scale ranges from a T0 for extremely weak tornadoes to T11 for the most powerful known tornadoes. Doppler radar data, photogrammetry, and ground swirl patterns (cycloidal marks) may also be analyzed to determine intensity and award a rating.

Formation of numerous waterspouts in the Great Lakes region. (North America)

 

A flash flood caused by a severe thunderstorm

 

Waterspouts have similar characteristics as tornadoes, characterized by a spiraling funnel-shaped wind current that form over bodies of water, connecting to large cumulonimbus clouds. Waterspouts are generally classified as forms of tornadoes, or more specifically, non-supercelled tornadoes that develop over large bodies of water. These spiralling columns of air frequently develop within tropical areas close to the equator, but are less common within areas of high latitude.

Flash flood

Flash flooding is the process where a landscape, most notably an urban environment, is subjected to rapid floods. These rapid floods occur more quickly and are more localized than seasonal river flooding or areal flooding and are frequently (though not always) associated with intense rainfall. Flash flooding can frequently occur in slow-moving thunderstorms and is usually caused by the heavy liquid precipitation that accompanies it. Flash floods are most common in densely populated urban environments, where few plants and bodies of water are present to absorb and contain the extra water. Flash flooding can be hazardous to small infrastructure, such as bridges, and weakly constructed buildings. Plants and crops in agricultural areas can be destroyed and devastated by the force of raging water. Automobiles parked within affected areas can also be displaced. Soil erosion can occur as well, exposing risks of landslide phenomena.

Downburst

Trees uprooted or displaced by the force of a downburst wind in northwest Monroe County, Wisconsin.

 

Downburst winds can produce numerous hazards to landscapes experiencing thunderstorms. Downburst winds are generally very powerful, and are often mistaken for wind speeds produced by tornadoes, due to the concentrated amount of force exerted by their straight-horizontal characteristic. Downburst winds can be hazardous to unstable, incomplete, or weakly constructed infrastructures and buildings. Agricultural crops, and other plants in nearby environments can be uprooted and damaged. Aircraft engaged in takeoff or landing can crash. Automobiles can be displaced by the force exerted by downburst winds. Downburst winds are usually formed in areas when high pressure air systems of downdrafts begin to sink and displace the air masses below it, due to their higher density. When these downdrafts reach the surface, they spread out and turn into the destructive straight-horizontal winds.

Thunderstorm asthma

Thunderstorm asthma is the triggering of an asthma attack by environmental conditions directly caused by a local thunderstorm. During a thunderstorm, pollen grains can absorb moisture and then burst into much smaller fragments with these fragments being easily dispersed by wind. While larger pollen grains are usually filtered by hairs in the nose, the smaller pollen fragments are able to pass through and enter the lungs, triggering the asthma attack.

Safety precautions

Most thunderstorms come and go fairly uneventfully; however, any thunderstorm can become severe, and all thunderstorms, by definition, present the danger of lightning. Thunderstorm preparedness and safety refers to taking steps before, during, and after a thunderstorm to minimize injury and damage.

Preparedness

Preparedness refers to precautions that should be taken before a thunderstorm. Some preparedness takes the form of general readiness (as a thunderstorm can occur at any time of the day or year). Preparing a family emergency plan, for example, can save valuable time if a storm arises quickly and unexpectedly. Preparing the home by removing dead or rotting limbs and trees, which can be blown over in high winds, can also significantly reduce the risk of property damage and personal injury.

The National Weather Service (NWS) in the United States recommends several precautions that people should take if thunderstorms are likely to occur:

• Know the names of local counties, cities, and towns, as these are how warnings are described.
• Monitor forecasts and weather conditions and know whether thunderstorms are likely in the area.
• Be alert for natural signs of an approaching storm.
• Cancel or reschedule outdoor events (to avoid being caught outdoors when a storm hits).
• Take action early so you have time to get to a safe place.
• Get inside a substantial building or hard-topped metal vehicle before threatening weather arrives.
• If you hear thunder, get to the safe place immediately.
• Avoid open areas like hilltops, fields, and beaches, and don't be or be near the tallest objects in an area when thunderstorms are occurring.
• Don't shelter under tall or isolated trees during thunderstorms.
• If in the woods, put as much distance as possible between you and any trees during thunderstorms.
• If in a group, spread out to increase the chances of survivors who could come to the aid of any victims from a lightning strike.

Safety

While safety and preparedness often overlap, “thunderstorm safety” generally refers to what people should do during and after a storm. The American Red Cross recommends that people follow these precautions if a storm is imminent or in progress:

Take action immediately upon hearing thunder. Anyone close enough to the storm to hear thunder can be struck by lightning.

Avoid electrical appliances, including corded telephones. Cordless and wireless telephones are safe to use during a thunderstorm.

Close and stay away from windows and doors, as glass can become a serious hazard in high wind.

Do not bathe or shower, as plumbing conducts electricity.

If driving, safely exit the roadway, turn on hazard lights, and park. Remain in the vehicle and avoid touching metal.

The NWS stopped recommending the "lightning crouch" in 2008 as it doesn't provide a significant level of protection and will not significantly lower the risk of being killed or injured from a nearby lightning strike.

Frequent occurrences

A mild thunderstorm over Niagara Falls, Ontario.

 

Thunderstorms occur throughout the world, even in the polar regions, with the greatest frequency in tropical rainforest areas, where they may occur nearly daily. At any given time approximately 2,000 thunderstorms are occurring on Earth. Kampala and Tororo in Uganda have each been mentioned as the most thunderous places on Earth, a claim also made for Singapore and Bogor on the Indonesian island of Java. Other cities known for frequent storm activity include Darwin, Caracas, Manila and Mumbai. Thunderstorms are associated with the various monsoon seasons around the globe, and they populate the rainbands of tropical cyclones. In temperate regions, they are most frequent in spring and summer, although they can occur along or ahead of cold fronts at any time of year. They may also occur within a cooler air mass following the passage of a cold front over a relatively warmer body of water. Thunderstorms are rare in polar regions because of cold surface temperatures. 

Some of the most powerful thunderstorms over the United States occur in the Midwest and the Southern states. These storms can produce large hail and powerful tornadoes. Thunderstorms are relatively uncommon along much of the West Coast of the United States, but they occur with greater frequency in the inland areas, particularly the Sacramento and San Joaquin Valleys of California. In spring and summer, they occur nearly daily in certain areas of the Rocky Mountains as part of the North American Monsoon regime. In the Northeast, storms take on similar characteristics and patterns as the Midwest, but with less frequency and severity. During the summer, air-mass thunderstorms are an almost daily occurrence over central and southern parts of Florida.

Energy

How thunderstorms launch particle beams into space

 

If the quantity of water that is condensed in and subsequently precipitated from a cloud is known, then the total energy of a thunderstorm can be calculated. In a typical thunderstorm, approximately 5×10⁸ kg of water vapor are lifted, and the amount of energy released when this condenses is 10¹⁵ joules. This is on the same order of magnitude of energy released within a tropical cyclone, and more energy than that released during the atomic bomb blast at Hiroshima, Japan in 1945.

The Fermi Gamma-ray Burst Monitor results show that gamma rays and antimatter particles (positrons) can be generated in powerful thunderstorms. It is suggested that the antimatter positrons are formed in terrestrial gamma-ray flashes (TGF). TGFs are brief bursts occurring inside thunderstorms and associated with lightning. The streams of positrons and electrons collide higher in the atmosphere to generate more gamma rays. About 500 TGFs may occur every day worldwide, but mostly go undetected.

Studies

In more contemporary times, thunderstorms have taken on the role of a scientific curiosity. Every spring, storm chasers head to the Great Plains of the United States and the Canadian Prairies to explore the scientific aspects of storms and tornadoes through use of videotaping. Radio pulses produced by cosmic rays are being used to study how electric charges develop within thunderstorms. More organized meteorological projects such as VORTEX2 use an array of sensors, such as the Doppler on Wheels, vehicles with mounted automated weather stations, weather balloons, and unmanned aircraft to investigate thunderstorms expected to produce severe weather. Lightning is detected remotely using sensors that detect cloud-to-ground lightning strokes with 95 percent accuracy in detection and within 250 metres (820 ft) of their point of origin.

Mythology and religion

Thunderstorms strongly influenced many early civilizations. Greeks believed that they were battles waged by Zeus, who hurled lightning bolts forged by Hephaestus. Some American Indian tribes associated thunderstorms with the Thunderbird, who they believed was a servant of the Great Spirit. The Norse considered thunderstorms to occur when Thor went to fight Jötnar, with the thunder and lightning being the effect of his strikes with the hammer Mjölnir. Hinduism recognizes Indra as the god of rain and thunderstorms. Christian doctrine accepts that fierce storms are the work of God. These ideas were still within the mainstream as late as the 18th century.

Martin Luther was out walking when a thunderstorm began, causing him to pray to God for being saved and promising to become a monk.

Outside of Earth

Thunderstorms, evidenced by flashes of lightning, on Jupiter have been detected and are associated with clouds where water may exist as both a liquid and ice, suggesting a mechanism similar to that on Earth. (Water is a polar molecule that can carry a charge, so it is capable of creating the charge separation needed to produce lightning.) These electrical discharges can be up to a thousand times more powerful than lightning on the Earth. The water clouds can form thunderstorms driven by the heat rising from the interior. The clouds of Venus may also be capable of producing lightning; some observations suggest that the lightning rate is at least half of that on Earth.