Classic Maya collapse

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

In archaeology, the classic Maya collapse is the decline of the Classic Maya civilization and the abandonment of Maya cities in the southern Maya lowlands of Mesoamerica between the 7th and 9th centuries. At Ceibal, the Preclassic Maya experienced a similar collapse in the 2nd century.

The Classic Period of Mesoamerican chronology is generally defined as the period from 250 to 900 CE, the last century of which is referred to as the Terminal Classic. The Classic Maya collapse is one of the greatest unsolved mysteries in archaeology. Urban centers of the southern lowlands, among them Palenque, Copán, Tikal, and Calakmul, went into decline during the 8th and 9th centuries and were abandoned shortly thereafter. Archaeologically, this decline is indicated by the cessation of monumental inscriptions and the reduction of large-scale architectural construction at the primary urban centers of the Classic Period.

Although termed a collapse, it did not mark the end of the Maya civilization but rather a shift away from the Southern Lowlands as a power center; the Northern Yucatán in particular prospered afterwards, although with very different artistic and architectural styles, and with much less use of monumental hieroglyphic writing. In the Post-Classic Period following the collapse, the state of Chichén Itzá built an empire that briefly united much of the Maya region, and centers such as Mayapán and Uxmal flourished, as did the Highland states of the Kʼicheʼ and Kaqchikel Maya. Independent Maya civilization continued until 1697 when the Spanish conquered Nojpetén, the last independent city-state. Millions of Maya people still inhabit the Yucatán peninsula today.

Because parts of Maya civilization unambiguously continued, a number of scholars strongly dislike the term "collapse". Regarding the proposed collapse, E. Wyllys Andrews IV went as far as to say, "in my belief no such thing happened."

Progression of the decline

The Maya often recorded dates on monuments they built. Few dated monuments were being built circa 500 – around ten per year in 514, for example. The number steadily increased to twenty per year by 672 and forty by around 750. After this, the number of dated monuments begins to falter relatively quickly, collapsing back to ten by 800 and to zero by 900. Likewise, recorded lists of kings complement this analysis. Altar Q at Copán shows a reign of kings from 426 to 763. One last king not recorded on Altar Q was Ukit Took, "Patron of Flint", who was probably a usurper. The dynasty is believed to have collapsed entirely shortly thereafter. In Quirigua, 49 kilometres (30 mi) north of Copán, the last king Jade Sky began his rule between 895 and 900, and throughout the Maya area all kingdoms similarly fell around that time.

A third piece of evidence of the progression of Maya decline, gathered by Ann Corinne Freter, Nancy Gonlin, and David Webster, uses a technique called obsidian hydration dating. The technique allowed them to map the spread and growth of settlements in the Copán Valley and estimate their populations. Between 400 and 450, the population was estimated at a peak of twenty-eight thousand, between 750 and 800 – larger than London at the time. The population then began to steadily decline. By 900 the population had fallen to fifteen thousand, and by 1200 the population was again less than 1000.

Theories

Over 80 different theories or variations of theories attempting to explain the Classic Maya collapse have been identified. From climate change to deforestation to lack of action by Maya kings, there is no universally accepted collapse theory, although drought has gained momentum in the first quarter of the 21st century as the leading explanation, as more scientific studies are conducted.

Foreign invasion

The archaeological evidence of the Toltec intrusion into Seibal, Peten, suggests to some the theory of foreign invasion. The latest hypothesis states that the southern lowlands were invaded by a non-Maya group whose homelands were probably in the gulf coast lowlands. This invasion began in the 9th century and set off, within 100 years, a group of events that destroyed the Classic Maya. It is believed that this invasion was somehow influenced by the Toltec people of central Mexico. However, most Mayanists do not believe that foreign invasion was the main cause of the Classic Maya collapse; they postulate that no military defeat can explain or be the cause of the protracted and complex Classic collapse process. Teotihuacan influence across the Maya region may have involved some form of military invasion; however, it is generally noted that significant Teotihuacan-Maya interactions date from at least the Early Classic period, well before the episodes of Late Classic collapse.

The foreign invasion theory does not answer the question of where the inhabitants went. David Webster believed that the population should have increased because of the lack of elite power. Further, it is not understood why the governmental institutions were not remade following the revolts, which happened under similar circumstances in places like China. A study by anthropologist Elliot M. Abrams came to the conclusion that buildings, specifically in Copán, did not require an extensive amount of time and workers to construct. However, this theory was developed during a period when the archaeological evidence showed that there were fewer Maya people than there are now known to have been. Revolutions, peasant revolts, and social turmoil change circumstances, and are often followed by foreign wars, but they run their course. There are no documented revolutions that caused wholesale abandonment of entire regions.

Collapse of trade routes

It has been hypothesized that the decline of the Maya is related to the collapse of their intricate trade systems, especially those connected to the central Mexican city of Teotihuacán. Preceding improved knowledge of the chronology of Mesoamerica, Teotihuacan was believed to have fallen during 700–750, forcing the "restructuring of economic relations throughout highland Mesoamerica and the Gulf Coast". This remaking of relationships between civilizations would have then given the collapse of the Classic Maya a slightly later date. However, after researchers learned more about the events and the periods when they occurred, it is believed that the strongest Teotihuacan influence was during the 4th and 5th centuries. In addition, the civilization of Teotihuacan started to lose its power, and maybe abandoned the city, during 600–650. This differs greatly from the previous belief that Teotihuacán power decreased during 700–750. But since the new decline date of 600–650 has been accepted, the Maya civilizations are now thought to have lived on and prospered "for another century and more" than what was previously believed. Rather than the decline of Teotihuacan directly preceding the collapse of the Maya, their decline is now seen as contributing "to the 6th-century 'hiatus'".

Epidemic diseases

The disease theory is also a contender as a factor in the Classic Maya collapse. Widespread disease could explain some rapid depopulation, both directly through the spread of infection itself and indirectly as an inhibition to recovery over the long run. According to Dunn (1968) and Shimkin (1973), infectious diseases spread by parasites are common in tropical rainforest regions, such as the Maya lowlands. Shimkin specifically suggests that the Maya may have encountered endemic infections related to American trypanosomiasis, Ascaris, and some enteropathogens that cause acute diarrheal illness. Furthermore, some experts believe that, through development of their civilization (that is, development of agriculture and settlements), the Maya could have created a "disturbed environment", in which parasitic and pathogen-carrying insects often thrive. Among the pathogens listed above, it is thought that those that cause the acute diarrheal illnesses would have been the most devastating to the Maya population, because such illness would have struck a victim at an early age, thereby hampering nutritional health and the natural growth and development of a child. This would have made them more susceptible to other diseases later in life, and would have been exacerbated by an increasing dependence on carbohydrate-rich crops. Such ideas as this could explain the role of disease as at least a possible partial reason for the Classic Maya Collapse.

Drought theory

See also: Medieval Warm Period

The drought theory holds that rapid climate change in the form of severe drought (a megadrought) brought about the Classic Maya collapse. Paleoclimatologists have discovered abundant evidence that prolonged droughts occurred in the Yucatán Peninsula and Petén Basin areas during the Terminal Classic. Large droughts likely caused a decline in agricultural fertility due to regular seasonal drought drying up surface waters, as well as causing thin tropical soils to erode when deprived of vegetation and forest cover.

Climatic factors were first implicated in the collapse as early as 1931 by Mayanists Thomas Gann and J. E. S. Thompson. In 1995, Hodell, Curtis and Brenner published a paleoclimate record from Lake Chichancanab on the Yucatán Peninsula that showed an intense, protracted drought occurred in the 9th century AD and coincided with the Classic Maya collapse. In The Great Maya Droughts, Richardson Gill gathered and analyzed an array of climatic, historical, hydrologic, tree ring, volcanic, geologic, and archeological research, and suggested that a prolonged series of droughts likely caused the Classic Maya collapse. The drought theory provides a comprehensive explanation, because non-environmental and cultural factors (excessive warfare, foreign invasion, peasant revolt, less trade, etc.) can all be explained by the effects of prolonged drought on Classic Maya civilization.

According to Gill in The Great Maya Droughts:

[Studies of] Yucatecan lake sediment cores ... provide unambiguous evidence for a severe 200-year drought from AD 800 to 1000 ... the most severe in the last 7,000 years ... precisely at the time of the Maya Collapse.

The role of drought in the collapse of Classic Maya civilization has remained controversial, however, largely because the majority of paleoclimate records only provide qualitative data, for example whether conditions were simply "wetter" or "drier". The lack of quantitative data makes it difficult to predict how climatic changes would have affected human populations and the environment in which they lived. In 2012, a study attempted to quantify the drought using four detailed paleoclimate records of the drought event. Semi-quantitative rainfall estimates were achieved by correlating oxygen isotopes measurements in carbonate cave formations (speleothems) with modern seasonal rainfall amounts recorded in the nearby city of Mérida, northern Yucatán, which were extrapolated back to the time of the Terminal Classic Period. The authors suggest that modest rainfall reductions, amounting to only 25 to 40 percent of annual rainfall, may have been the tipping point to the Maya collapse. Although this analysis referred to the estimated decrease in rainfall as "modest", subsequent studies suggest that the same data could represent a 20 to 65 percent decrease in rainfall.

A study published in the journal Science in 2018 provides the most robust estimate of the magnitude of rainfall reduction to date. Evans and co-authors developed a method to measure the different isotopes of water trapped in the hydrate mineral, gypsum, a mineral that forms in lakes of the Yucatán Peninsula during times of drought when the water level is lowered. When gypsum forms, water molecules are incorporated directly into its crystalline structure, and this water records the different isotopes that were present in the ancient lake water at the time of its formation. The "fossil water" inside the crystals allowed Evans and his co-authors to analyze the properties of the lake water during each drought period. Based on these measurements, the researchers found that annual precipitation decreased between 41 and 54 percent during the period of the Maya civilisation's collapse, with periods of up to 70 percent rainfall reduction during peak drought conditions, and that relative humidity declined by 2 to 7 percent compared to today. This quantitative climate data can be used to better predict how these drought conditions may have affected agriculture, including yields of the Maya's staple crops, such as maize.

Critics of the drought theory question the spatial patterns of drought and its relation to the timing of the degradation of Maya city-states. Archaeological research demonstrates that while many regions of the Maya Lowlands were indeed abandoned during the eighth to eleventh centuries CE, other regions experienced only minor disruption, or even flourished. Although the spatial patterns of societal collapse are complex, population centers continued in many coastal regions and in the northern Yucatán Peninsula, including as Chichen Itza, Uxmal, and Coba, whereas most states in the central regions collapsed and landscapes were depopulated. The reasons for this spatial heterogeneity in societal disintegration are largely unknown, but researches have hypothesised that central regions may have been more affected because of a very deep water table (which would have exacerbated the effects of drought), or that the longevity of the northern regions was likely facilitated by access to the coast, and thus trade routes.

Other critics of the megadrought theory, including David Webster, note that much of the evidence of drought comes from the northern Yucatán and not the southern part of the peninsula, where Classic Maya civilization flourished. Webster states that if water sources were to have dried up, then several city-states would have moved to other water sources. That Gill suggests that all water in the region would have dried up and destroyed Maya civilization is a stretch, according to Webster, although Webster does not have a precise competing theory explaining the Classic Maya Collapse. Since publication, further records from the more southerly states have strengthened the argument of a synchronous drought occurring across the Yucatán Peninsula.

Climatic changes are, with increasing frequency, found to be major drivers in the rise and fall of civilizations all over the world. Professors Harvey Weiss of Yale University and Raymond S. Bradley of the University of Massachusetts have written, "Many lines of evidence now point to climate forcing as the primary agent in repeated social collapse." In a separate publication, Weiss illustrates an emerging understanding of scientists:

Within the past five years new tools and new data for archaeologists, climatologists, and historians have brought us to the edge of a new era in the study of global and hemispheric climate change and its cultural impacts. The climate of the Holocene, previously assumed static, now displays a surprising dynamism, which has affected the agricultural bases of pre-industrial societies. The list of Holocene climate alterations and their socio-economic effects has rapidly become too complex for brief summary.

A number of causal mechanisms for droughts in the Maya area have been proposed, but there is no consensus among researchers regarding a single causal mechanism. Instead, it is likely that multiple mechanisms were involved, including solar variability, shifts in the position of the Intertropical Convergence Zone, changes in tropical cyclone frequency and deforestation.

The Maya are often perceived as having lived in a rainforest, but technically, they lived in a seasonal desert without access to stable sources of drinking water. The exceptional accomplishments of the Maya are even more remarkable because of their engineered response to the fundamental environmental difficulty of relying upon rainwater rather than permanent sources of water. "The Maya succeeded in creating a civilization in a seasonal desert by creating a system of water storage and management which was totally dependent on consistent rainfall." The constant need for water kept the Maya on the edge of survival. "Given this precarious balance of wet and dry conditions, even a slight shift in the distribution of annual precipitation can have serious consequences." Water and civilization were vitally connected in ancient Mesoamerica. Vernon Scarborough, an archaeologist and specialist in pre-industrial land and water usage practices, believes water management and access were critical to the development of Maya civilization.

Systemic ecological collapse model

Some ecological theories of Maya decline focus on the worsening agricultural and resource conditions in the late Classic period. It was originally thought that the majority of Maya agriculture was dependent on a simple slash-and-burn system. Based on this method, the hypothesis of soil exhaustion was advanced by Orator F. Cook in 1921. Similar soil exhaustion assumptions are associated with erosion, intensive agricultural, and savanna grass competition.

More recent investigations have shown a complicated variety of intensive agricultural techniques utilized by the Maya, explaining the high population of the Classic Maya polities. Modern archaeologists now comprehend the sophisticated intensive and productive agricultural techniques of the ancient Maya, and several of the Maya agricultural methods have not yet been reproduced. Intensive agricultural methods were developed and utilized by all the Mesoamerican cultures to boost their food production and give them a competitive advantage over less skillful peoples. These intensive agricultural methods included canals, terracing, raised fields, ridged fields, chinampas, the use of human feces as fertilizer, seasonal swamps or bajos, using muck from the bajos to create fertile fields, dikes, dams, irrigation, water reservoirs, several types of water storage systems, hydraulic systems, swamp reclamation, swidden systems, and other agricultural techniques that have not yet been fully understood. Systemic ecological collapse is said to be evidenced by deforestation, siltation, and the decline of biological diversity.

In addition to mountainous terrain, Mesoamericans successfully exploited the very problematic tropical rainforest for 1,500 years. The agricultural techniques utilized by the Maya were entirely dependent upon ample supplies of water, lending credit to the drought theory of collapse. The Maya thrived in territory that would be uninhabitable to most peoples. Their success over two millennia in this environment was "amazing."

Other explanations

Anthropologist Joseph Tainter wrote extensively about the collapse of the Southern Lowland Maya in his 1988 study The Collapse of Complex Societies. His theory about Maya collapse encompasses some of the above explanations, but focuses specifically on the development of and the declining marginal returns from the increasing social complexity of the competing Maya city-states. Psychologist Julian Jaynes suggested that the collapse was due to a failure in the social control systems of religion and political authority, due to increasing socioeconomic complexity that overwhelmed the power of traditional rituals and the king's authority to compel obedience.

Precipitation

From Wikipedia, the free encyclopedia

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

 

Mean precipitation based on global high resolution climate data (CHELSA)

 

Countries by average annual precipitation. Some parts of a country can be much wetter than others, so it is not an accurate depiction of the wettest and driest places on earth.

In meteorology, precipitation is any product of the condensation of atmospheric water vapor that falls from clouds due to gravitational pull. The main forms of precipitation include drizzle, rain, sleet, snow, ice pellets, graupel and hail. Precipitation occurs when a portion of the atmosphere becomes saturated with water vapor (reaching 100% relative humidity), so that the water condenses and "precipitates" or falls. Thus, fog and mist are not precipitation but colloids, because the water vapor does not condense sufficiently to precipitate. Two processes, possibly acting together, can lead to air becoming saturated: cooling the air or adding water vapor to the air. Precipitation forms as smaller droplets coalesce via collision with other rain drops or ice crystals within a cloud. Short, intense periods of rain in scattered locations are called showers.

Moisture that is lifted or otherwise forced to rise over a layer of sub-freezing air at the surface may be condensed into clouds and rain. This process is typically active when freezing rain occurs. A stationary front is often present near the area of freezing rain and serves as the focus for forcing and rising air. Provided there is necessary and sufficient atmospheric moisture content, the moisture within the rising air will condense into clouds, namely nimbostratus and cumulonimbus if significant precipitation is involved. Eventually, the cloud droplets will grow large enough to form raindrops and descend toward the Earth where they will freeze on contact with exposed objects. Where relatively warm water bodies are present, for example due to water evaporation from lakes, lake-effect snowfall becomes a concern downwind of the warm lakes within the cold cyclonic flow around the backside of extratropical cyclones. Lake-effect snowfall can be locally heavy. Thundersnow is possible within a cyclone's comma head and within lake effect precipitation bands. In mountainous areas, heavy precipitation is possible where upslope flow is maximized within windward sides of the terrain at elevation. On the leeward side of mountains, desert climates can exist due to the dry air caused by compressional heating. Most precipitation occurs within the tropics and is caused by convection. The movement of the monsoon trough, or intertropical convergence zone, brings rainy seasons to savannah regions.

Precipitation is a major component of the water cycle, and is responsible for depositing fresh water on the planet. Approximately 505,000 cubic kilometres (121,000 cu mi) of water falls as precipitation each year: 398,000 cubic kilometres (95,000 cu mi) over oceans and 107,000 cubic kilometres (26,000 cu mi) over land. Given the Earth's surface area, that means the globally averaged annual precipitation is 990 millimetres (39 in), but over land it is only 715 millimetres (28.1 in). Climate classification systems such as the Köppen climate classification system use average annual rainfall to help differentiate between differing climate regimes. Global warming is already causing changes to weather, increasing precipitation in some geographies, and reducing it in others, resulting in additional extreme weather.

Precipitation may occur on other celestial bodies. Saturn's largest satellite, Titan, hosts methane precipitation as a slow-falling drizzle, which has been observed as Rain puddles at its equator and polar regions.

Types

Main article: Precipitation types

 

A thunderstorm with heavy precipitation

Precipitation is a major component of the water cycle, and is responsible for depositing most of the fresh water on the planet. Approximately 505,000 km³ (121,000 cu mi) of water falls as precipitation each year, 398,000 km³ (95,000 cu mi) of it over the oceans. Given the Earth's surface area, that means the globally averaged annual precipitation is 990 millimetres (39 in).

Mechanisms of producing precipitation include convective, stratiform, and orographic rainfall. Convective processes involve strong vertical motions that can cause the overturning of the atmosphere in that location within an hour and cause heavy precipitation, while stratiform processes involve weaker upward motions and less intense precipitation. Precipitation can be divided into three categories, based on whether it falls as liquid water, liquid water that freezes on contact with the surface, or ice. Mixtures of different types of precipitation, including types in different categories, can fall simultaneously. Liquid forms of precipitation include rain and drizzle. Rain or drizzle that freezes on contact within a subfreezing air mass is called "freezing rain" or "freezing drizzle". Frozen forms of precipitation include snow, ice needles, ice pellets, hail, and graupel.

Measurement

Liquid precipitation

Rainfall (including drizzle and rain) is usually measured using a rain gauge and expressed in units of millimeters (mm) of height or depth. Equivalently, it can be expressed as a physical quantity with dimension of volume of water per collection area, in units of liters per square meter (L/m²); as 1L=1dm³=1mm·m², the units of area (m²) cancel out, resulting in simply "mm". This also corresponds to an area density expressed in kg/m², if assuming that 1 liter of water has a mass of 1 kg (water density), which is acceptable for most practical purposes. The corresponding English unit used is usually inches. In Australia before metrication, rainfall was measured in "points" which were defined as a hundredth of an inch.

Solid precipitation

A snow gauge is usually used to measure the amount of solid precipitation. Snowfall is usually measured in centimeters by letting snow fall into a container and then measure the height. The snow can then optionally be melted to obtain a water equivalent measurement in millimeters like for liquid precipitation. The relationship between snow height and water equivalent depends on the water content of the snow; the water equivalent can thus only provide a rough estimate of snow depth. Other forms of solid precipitation, such as snow pellets and hail or even sleet (rain and snow mixed), can also be melted and measured as water equivalent, usually expressed millimeters like for liquid precipitation.

How the air becomes saturated

Cooling air to its dew point

Late-summer rainstorm in Denmark

 

Lenticular cloud forming due to mountains over Wyoming

The dew point is the temperature to which a parcel of air must be cooled in order to become saturated, and (unless super-saturation occurs) condenses to water. Water vapor normally begins to condense on condensation nuclei such as dust, ice, and salt in order to form clouds. The cloud condensation nuclei concentration will determine the cloud microphysics. An elevated portion of a frontal zone forces broad areas of lift, which form cloud decks such as altostratus or cirrostratus. Stratus is a stable cloud deck which tends to form when a cool, stable air mass is trapped underneath a warm air mass. It can also form due to the lifting of advection fog during breezy conditions.

There are four main mechanisms for cooling the air to its dew point: adiabatic cooling, conductive cooling, radiational cooling, and evaporative cooling. Adiabatic cooling occurs when air rises and expands. The air can rise due to convection, large-scale atmospheric motions, or a physical barrier such as a mountain (orographic lift). Conductive cooling occurs when the air comes into contact with a colder surface, usually by being blown from one surface to another, for example from a liquid water surface to colder land. Radiational cooling occurs due to the emission of infrared radiation, either by the air or by the surface underneath. Evaporative cooling occurs when moisture is added to the air through evaporation, which forces the air temperature to cool to its wet-bulb temperature, or until it reaches saturation.

Adding moisture to the air

The main ways water vapor is added to the air are: wind convergence into areas of upward motion, precipitation or virga falling from above, daytime heating evaporating water from the surface of oceans, water bodies or wet land, transpiration from plants, cool or dry air moving over warmer water, and lifting air over mountains.

Forms of precipitation

Main article: Water cycle

 

Condensation and coalescence are important parts of the water cycle.

Raindrops

Puddle in the rain

Coalescence occurs when water droplets fuse to create larger water droplets, or when water droplets freeze onto an ice crystal, which is known as the Bergeron process. The fall rate of very small droplets is negligible, hence clouds do not fall out of the sky; precipitation will only occur when these coalesce into larger drops. droplets with different size will have different terminal velocity that cause droplets collision and producing larger droplets, Turbulence will enhance the collision process. As these larger water droplets descend, coalescence continues, so that drops become heavy enough to overcome air resistance and fall as rain.

Raindrops have sizes ranging from 5.1 to 20 millimetres (0.20 to 0.79 in) mean diameter, above which they tend to break up. Smaller drops are called cloud droplets, and their shape is spherical. As a raindrop increases in size, its shape becomes more oblate, with its largest cross-section facing the oncoming airflow. Contrary to the cartoon pictures of raindrops, their shape does not resemble a teardrop. Intensity and duration of rainfall are usually inversely related, i.e., high intensity storms are likely to be of short duration and low intensity storms can have a long duration. Rain drops associated with melting hail tend to be larger than other rain drops. The METAR code for rain is RA, while the coding for rain showers is SHRA.

Ice pellets

Main article: Ice pellets

 

An accumulation of ice pellets

Ice pellets or sleet are a form of precipitation consisting of small, translucent balls of ice. Ice pellets are usually (but not always) smaller than hailstones. They often bounce when they hit the ground, and generally do not freeze into a solid mass unless mixed with freezing rain. The METAR code for ice pellets is PL.

Ice pellets form when a layer of above-freezing air exists with sub-freezing air both above and below. This causes the partial or complete melting of any snowflakes falling through the warm layer. As they fall back into the sub-freezing layer closer to the surface, they re-freeze into ice pellets. However, if the sub-freezing layer beneath the warm layer is too small, the precipitation will not have time to re-freeze, and freezing rain will be the result at the surface. A temperature profile showing a warm layer above the ground is most likely to be found in advance of a warm front during the cold season, but can occasionally be found behind a passing cold front.

Hail

Main article: Hail

 

A large hailstone, about 6 centimetres (2.4 in) in diameter

Like other precipitation, hail forms in storm clouds when supercooled water droplets freeze on contact with condensation nuclei, such as dust or dirt. The storm's updraft blows the hailstones to the upper part of the cloud. The updraft dissipates and the hailstones fall down, back into the updraft, and are lifted again. Hail has a diameter of 5 millimetres (0.20 in) or more. Within METAR code, GR is used to indicate larger hail, of a diameter of at least 6.4 millimetres (0.25 in). GR is derived from the French word grêle. Smaller-sized hail, as well as snow pellets, use the coding of GS, which is short for the French word grésil. Stones just larger than golf ball-sized are one of the most frequently reported hail sizes. Hailstones can grow to 15 centimetres (6 in) and weigh more than 500 grams (1 lb). In large hailstones, latent heat released by further freezing may melt the outer shell of the hailstone. The hailstone then may undergo 'wet growth', where the liquid outer shell collects other smaller hailstones. The hailstone gains an ice layer and grows increasingly larger with each ascent. Once a hailstone becomes too heavy to be supported by the storm's updraft, it falls from the cloud.

Snowflakes

Main article: Snowflake

 

Snowflake viewed in an optical microscope

Snow crystals form when tiny supercooled cloud droplets (about 10 μm in diameter) freeze. Once a droplet has frozen, it grows in the supersaturated environment. Because water droplets are more numerous than the ice crystals the crystals are able to grow to hundreds of micrometers in size at the expense of the water droplets. This process is known as the Wegener–Bergeron–Findeisen process. The corresponding depletion of water vapor causes the droplets to evaporate, meaning that the ice crystals grow at the droplets' expense. These large crystals are an efficient source of precipitation, since they fall through the atmosphere due to their mass, and may collide and stick together in clusters, or aggregates. These aggregates are snowflakes, and are usually the type of ice particle that falls to the ground. Guinness World Records list the world's largest snowflakes as those of January 1887 at Fort Keogh, Montana; allegedly one measured 38 cm (15 in) wide. The exact details of the sticking mechanism remain a subject of research.

Although the ice is clear, scattering of light by the crystal facets and hollows/imperfections mean that the crystals often appear white in color due to diffuse reflection of the whole spectrum of light by the small ice particles. The shape of the snowflake is determined broadly by the temperature and humidity at which it is formed. Rarely, at a temperature of around −2 °C (28 °F), snowflakes can form in threefold symmetry—triangular snowflakes. The most common snow particles are visibly irregular, although near-perfect snowflakes may be more common in pictures because they are more visually appealing. No two snowflakes are alike, as they grow at different rates and in different patterns depending on the changing temperature and humidity within the atmosphere through which they fall on their way to the ground. The METAR code for snow is SN, while snow showers are coded SHSN.

Diamond dust

Main article: Diamond dust

Diamond dust, also known as ice needles or ice crystals, forms at temperatures approaching −40 °C (−40 °F) due to air with slightly higher moisture from aloft mixing with colder, surface-based air. They are made of simple ice crystals, hexagonal in shape. The METAR identifier for diamond dust within international hourly weather reports is IC.

Occult deposition

Occult deposition occurs when mist or air that is highly saturated with water vapour interacts with the leaves of trees or shrubs it passes over.

Causes

Frontal activity

Main article: Weather fronts

Stratiform or dynamic precipitation occurs as a consequence of slow ascent of air in synoptic systems (on the order of cm/s), such as over surface cold fronts, and over and ahead of warm fronts. Similar ascent is seen around tropical cyclones outside of the eyewall, and in comma-head precipitation patterns around mid-latitude cyclones. A wide variety of weather can be found along an occluded front, with thunderstorms possible, but usually their passage is associated with a drying of the air mass. Occluded fronts usually form around mature low-pressure areas. Precipitation may occur on celestial bodies other than Earth. When it gets cold, Mars has precipitation that most likely takes the form of ice needles, rather than rain or snow.

Convection

Convective precipitation

Convective rain, or showery precipitation, occurs from convective clouds, e.g. cumulonimbus or cumulus congestus. It falls as showers with rapidly changing intensity. Convective precipitation falls over a certain area for a relatively short time, as convective clouds have limited horizontal extent. Most precipitation in the tropics appears to be convective; however, it has been suggested that stratiform precipitation also occurs. Graupel and hail indicate convection. In mid-latitudes, convective precipitation is intermittent and often associated with baroclinic boundaries such as cold fronts, squall lines, and warm fronts. Convective precipitation mostly consist of mesoscale convective systems and they produce torrential rainfalls with thunderstorms, wind damages, and other forms of severe weather events.

Orographic effects

See also: Orographic lift and Precipitation types

 

Orographic precipitation

Orographic precipitation occurs on the windward (upwind) side of mountains and is caused by the rising air motion of a large-scale flow of moist air across the mountain ridge, resulting in adiabatic cooling and condensation. In mountainous parts of the world subjected to relatively consistent winds (for example, the trade winds), a more moist climate usually prevails on the windward side of a mountain than on the leeward or downwind side. Moisture is removed by orographic lift, leaving drier air (see katabatic wind) on the descending and generally warming, leeward side where a rain shadow is observed.

In Hawaii, Mount Waiʻaleʻale, on the island of Kauai, is notable for its extreme rainfall, as it has the second-highest average annual rainfall on Earth, with 12,000 millimetres (460 in). Storm systems affect the state with heavy rains between October and March. Local climates vary considerably on each island due to their topography, divisible into windward (Koʻolau) and leeward (Kona) regions based upon location relative to the higher mountains. Windward sides face the east to northeast trade winds and receive much more rainfall; leeward sides are drier and sunnier, with less rain and less cloud cover.

In South America, the Andes mountain range blocks Pacific moisture that arrives in that continent, resulting in a desertlike climate just downwind across western Argentina. The Sierra Nevada range creates the same effect in North America forming the Great Basin and Mojave Deserts. Similarly, in Asia, the Himalaya mountains create an obstacle to monsoons which leads to extremely high precipitation on the southern side and lower precipitation levels on the northern side.

Snow

Main article: Snow

 

Lake-effect snow bands near the Korean Peninsula in early December 2008

Extratropical cyclones can bring cold and dangerous conditions with heavy rain and snow with winds exceeding 119 km/h (74 mph), (sometimes referred to as windstorms in Europe). The band of precipitation that is associated with their warm front is often extensive, forced by weak upward vertical motion of air over the frontal boundary which condenses as it cools and produces precipitation within an elongated band, which is wide and stratiform, meaning falling out of nimbostratus clouds. When moist air tries to dislodge an arctic air mass, overrunning snow can result within the poleward side of the elongated precipitation band. In the Northern Hemisphere, poleward is towards the North Pole, or north. Within the Southern Hemisphere, poleward is towards the South Pole, or south.

Southwest of extratropical cyclones, curved cyclonic flow bringing cold air across the relatively warm water bodies can lead to narrow lake-effect snow bands. Those bands bring strong localized snowfall which can be understood as follows: Large water bodies such as lakes efficiently store heat that results in significant temperature differences (larger than 13 °C or 23 °F) between the water surface and the air above. Because of this temperature difference, warmth and moisture are transported upward, condensing into vertically oriented clouds (see satellite picture) which produce snow showers. The temperature decrease with height and cloud depth are directly affected by both the water temperature and the large-scale environment. The stronger the temperature decrease with height, the deeper the clouds get, and the greater the precipitation rate becomes.

In mountainous areas, heavy snowfall accumulates when air is forced to ascend the mountains and squeeze out precipitation along their windward slopes, which in cold conditions, falls in the form of snow. Because of the ruggedness of terrain, forecasting the location of heavy snowfall remains a significant challenge.

Within the tropics

Main article: Wet season

See also: Monsoon and Tropical cyclone

 

Rainfall distribution by month in Cairns showing the extent of the wet season at that location

The wet, or rainy, season is the time of year, covering one or more months, when most of the average annual rainfall in a region falls. The term green season is also sometimes used as a euphemism by tourist authorities. Areas with wet seasons are dispersed across portions of the tropics and subtropics. Savanna climates and areas with monsoon regimes have wet summers and dry winters. Tropical rainforests technically do not have dry or wet seasons, since their rainfall is equally distributed through the year. Some areas with pronounced rainy seasons will see a break in rainfall mid-season when the intertropical convergence zone or monsoon trough move poleward of their location during the middle of the warm season. When the wet season occurs during the warm season, or summer, rain falls mainly during the late afternoon and early evening hours. The wet season is a time when air quality improves, freshwater quality improves, and vegetation grows significantly. Soil nutrients diminish and erosion increases. Animals have adaptation and survival strategies for the wetter regime. The previous dry season leads to food shortages into the wet season, as the crops have yet to mature. Developing countries have noted that their populations show seasonal weight fluctuations due to food shortages seen before the first harvest, which occurs late in the wet season.

Tropical cyclones, a source of very heavy rainfall, consist of large air masses several hundred miles across with low pressure at the centre and with winds blowing inward towards the centre in either a clockwise direction (southern hemisphere) or counterclockwise (northern hemisphere). Although cyclones can take an enormous toll in lives and personal property, they may be important factors in the precipitation regimes of places they impact, as they may bring much-needed precipitation to otherwise dry regions. Areas in their path can receive a year's worth of rainfall from a tropical cyclone passage.

Large-scale geographical distribution

See also: Earth rainfall climatology

On the large scale, the highest precipitation amounts outside topography fall in the tropics, closely tied to the Intertropical Convergence Zone, itself the ascending branch of the Hadley cell. Mountainous locales near the equator in Colombia are amongst the wettest places on Earth. North and south of this are regions of descending air that form subtropical ridges where precipitation is low; the land surface underneath these ridges is usually arid, and these regions make up most of the Earth's deserts. An exception to this rule is in Hawaii, where upslope flow due to the trade winds lead to one of the wettest locations on Earth. Otherwise, the flow of the Westerlies into the Rocky Mountains lead to the wettest, and at elevation snowiest, locations within North America. In Asia during the wet season, the flow of moist air into the Himalayas leads to some of the greatest rainfall amounts measured on Earth in northeast India.

Measurement

See also: Rain gauge, Disdrometer, and Snow gauge

 

Standard rain gauge

The standard way of measuring rainfall or snowfall is the standard rain gauge, which can be found in 100 mm (3.9 in) plastic and 200 mm (7.9 in) metal varieties. The inner cylinder is filled by 25 mm (0.98 in) of rain, with overflow flowing into the outer cylinder. Plastic gauges have markings on the inner cylinder down to 0.25 mm (0.0098 in) resolution, while metal gauges require use of a stick designed with the appropriate 0.25 mm (0.0098 in) markings. After the inner cylinder is filled, the amount inside is discarded, then filled with the remaining rainfall in the outer cylinder until all the fluid in the outer cylinder is gone, adding to the overall total until the outer cylinder is empty. These gauges are used in the winter by removing the funnel and inner cylinder and allowing snow and freezing rain to collect inside the outer cylinder. Some add anti-freeze to their gauge so they do not have to melt the snow or ice that falls into the gauge. Once the snowfall/ice is finished accumulating, or as 300 mm (12 in) is approached, one can either bring it inside to melt, or use lukewarm water to fill the inner cylinder with in order to melt the frozen precipitation in the outer cylinder, keeping track of the warm fluid added, which is subsequently subtracted from the overall total once all the ice/snow is melted.

Other types of gauges include the popular wedge gauge (the cheapest rain gauge and most fragile), the tipping bucket rain gauge, and the weighing rain gauge. The wedge and tipping bucket gauges have problems with snow. Attempts to compensate for snow/ice by warming the tipping bucket meet with limited success, since snow may sublimate if the gauge is kept much above freezing. Weighing gauges with antifreeze should do fine with snow, but again, the funnel needs to be removed before the event begins. For those looking to measure rainfall the most inexpensively, a can that is cylindrical with straight sides will act as a rain gauge if left out in the open, but its accuracy will depend on what ruler is used to measure the rain with. Any of the above rain gauges can be made at home, with enough know-how.

When a precipitation measurement is made, various networks exist across the United States and elsewhere where rainfall measurements can be submitted through the Internet, such as CoCoRAHS or GLOBE. If a network is not available in the area where one lives, the nearest local weather office will likely be interested in the measurement.

Hydrometeor definition

A concept used in precipitation measurement is the hydrometeor. Any particulates of liquid or solid water in the atmosphere are known as hydrometeors. Formations due to condensation, such as clouds, haze, fog, and mist, are composed of hydrometeors. All precipitation types are made up of hydrometeors by definition, including virga, which is precipitation which evaporates before reaching the ground. Particles blown from the Earth's surface by wind, such as blowing snow and blowing sea spray, are also hydrometeors, as are hail and snow.

Satellite estimates

Although surface precipitation gauges are considered the standard for measuring precipitation, there are many areas in which their use is not feasible. This includes the vast expanses of ocean and remote land areas. In other cases, social, technical or administrative issues prevent the dissemination of gauge observations. As a result, the modern global record of precipitation largely depends on satellite observations.

Satellite sensors work by remotely sensing precipitation—recording various parts of the electromagnetic spectrum that theory and practice show are related to the occurrence and intensity of precipitation. The sensors are almost exclusively passive, recording what they see, similar to a camera, in contrast to active sensors (radar, lidar) that send out a signal and detect its impact on the area being observed.

Satellite sensors now in practical use for precipitation fall into two categories. Thermal infrared (IR) sensors record a channel around 11 micron wavelength and primarily give information about cloud tops. Due to the typical structure of the atmosphere, cloud-top temperatures are approximately inversely related to cloud-top heights, meaning colder clouds almost always occur at higher altitudes. Further, cloud tops with a lot of small-scale variation are likely to be more vigorous than smooth-topped clouds. Various mathematical schemes, or algorithms, use these and other properties to estimate precipitation from the IR data.

The second category of sensor channels is in the microwave part of the electromagnetic spectrum. The frequencies in use range from about 10 gigahertz to a few hundred GHz. Channels up to about 37 GHz primarily provide information on the liquid hydrometeors (rain and drizzle) in the lower parts of clouds, with larger amounts of liquid emitting higher amounts of microwave radiant energy. Channels above 37 GHz display emission signals, but are dominated by the action of solid hydrometeors (snow, graupel, etc.) to scatter microwave radiant energy. Satellites such as the Tropical Rainfall Measuring Mission (TRMM) and the Global Precipitation Measurement (GPM) mission employ microwave sensors to form precipitation estimates.

Additional sensor channels and products have been demonstrated to provide additional useful information including visible channels, additional IR channels, water vapor channels and atmospheric sounding retrievals. However, most precipitation data sets in current use do not employ these data sources.

Satellite data sets

The IR estimates have rather low skill at short time and space scales, but are available very frequently (15 minutes or more often) from satellites in geosynchronous Earth orbit. IR works best in cases of deep, vigorous convection—such as the tropics—and becomes progressively less useful in areas where stratiform (layered) precipitation dominates, especially in mid- and high-latitude regions. The more-direct physical connection between hydrometeors and microwave channels gives the microwave estimates greater skill on short time and space scales than is true for IR. However, microwave sensors fly only on low Earth orbit satellites, and there are few enough of them that the average time between observations exceeds three hours. This several-hour interval is insufficient to adequately document precipitation because of the transient nature of most precipitation systems as well as the inability of a single satellite to appropriately capture the typical daily cycle of precipitation at a given location.

Since the late 1990s, several algorithms have been developed to combine precipitation data from multiple satellites' sensors, seeking to emphasize the strengths and minimize the weaknesses of the individual input data sets. The goal is to provide "best" estimates of precipitation on a uniform time/space grid, usually for as much of the globe as possible. In some cases the long-term homogeneity of the dataset is emphasized, which is the Climate Data Record standard.

In other cases, the goal is producing the best instantaneous satellite estimate, which is the High Resolution Precipitation Product approach. In either case, of course, the less-emphasized goal is also considered desirable. One key result of the multi-satellite studies is that including even a small amount of surface gauge data is very useful for controlling the biases that are endemic to satellite estimates. The difficulties in using gauge data are that 1) their availability is limited, as noted above, and 2) the best analyses of gauge data take two months or more after the observation time to undergo the necessary transmission, assembly, processing and quality control. Thus, precipitation estimates that include gauge data tend to be produced further after the observation time than the no-gauge estimates. As a result, while estimates that include gauge data may provide a more accurate depiction of the "true" precipitation, they are generally not suited for real- or near-real-time applications.

The work described has resulted in a variety of datasets possessing different formats, time/space grids, periods of record and regions of coverage, input datasets, and analysis procedures, as well as many different forms of dataset version designators. In many cases, one of the modern multi-satellite data sets is the best choice for general use.

Return period

See also: 100-year flood

The likelihood or probability of an event with a specified intensity and duration is called the return period or frequency. The intensity of a storm can be predicted for any return period and storm duration, from charts based on historical data for the location. The term 1 in 10 year storm describes a rainfall event which is rare and is only likely to occur once every 10 years, so it has a 10 percent likelihood any given year. The rainfall will be greater and the flooding will be worse than the worst storm expected in any single year. The term 1 in 100 year storm describes a rainfall event which is extremely rare and which will occur with a likelihood of only once in a century, so has a 1 percent likelihood in any given year. The rainfall will be extreme and flooding to be worse than a 1 in 10 year event. As with all probability events, it is possible though unlikely to have two "1 in 100 Year Storms" in a single year.

Uneven pattern of precipitation

A significant portion of the annual precipitation in any particular place (no weather station in Africa or South America were considered) falls on only a few days, typically about 50% during the 12 days with the most precipitation.

Role in Köppen climate classification

Main article: Köppen climate classification

 

Updated Köppen-Geiger climate map

Af   Am   Aw/As

BWh   BWk   BSh   BSk

Csa   Csb   Csc

Cwa   Cwb   Cwc

Cfa   Cfb   Cfc

Dsa   Dsb   Dsc   Dsd

Dwa   Dwb   Dwc   Dwd

Dfa   Dfb   Dfc   Dfd

ET   EF

The Köppen classification depends on average monthly values of temperature and precipitation. The most commonly used form of the Köppen classification has five primary types labeled A through E. Specifically, the primary types are A, tropical; B, dry; C, mild mid-latitude; D, cold mid-latitude; and E, polar. The five primary classifications can be further divided into secondary classifications such as rain forest, monsoon, tropical savanna, humid subtropical, humid continental, oceanic climate, Mediterranean climate, steppe, subarctic climate, tundra, polar ice cap, and desert.

Rain forests are characterized by high rainfall, with definitions setting minimum normal annual rainfall between 1,750 and 2,000 mm (69 and 79 in). A tropical savanna is a grassland biome located in semi-arid to semi-humid climate regions of subtropical and tropical latitudes, with rainfall between 750 and 1,270 mm (30 and 50 in) a year. They are widespread on Africa, and are also found in India, the northern parts of South America, Malaysia, and Australia. The humid subtropical climate zone is where winter rainfall (and sometimes snowfall) is associated with large storms that the westerlies steer from west to east. Most summer rainfall occurs during thunderstorms and from occasional tropical cyclones. Humid subtropical climates lie on the east side continents, roughly between latitudes 20° and 40° degrees from the equator.

An oceanic (or maritime) climate is typically found along the west coasts at the middle latitudes of all the world's continents, bordering cool oceans, as well as southeastern Australia, and is accompanied by plentiful precipitation year-round. The Mediterranean climate regime resembles the climate of the lands in the Mediterranean Basin, parts of western North America, parts of western and southern Australia, in southwestern South Africa and in parts of central Chile. The climate is characterized by hot, dry summers and cool, wet winters. A steppe is a dry grassland. Subarctic climates are cold with continuous permafrost and little precipitation.

Effect on agriculture

Rainfall estimates for southern Japan and the surrounding region from July 20 to 27, 2009.

Precipitation, especially rain, has a dramatic effect on agriculture. All plants need at least some water to survive, therefore rain (being the most effective means of watering) is important to agriculture. While a regular rain pattern is usually vital to healthy plants, too much or too little rainfall can be harmful, even devastating to crops. Drought can kill crops and increase erosion, while overly wet weather can cause harmful fungus growth. Plants need varying amounts of rainfall to survive. For example, certain cacti require small amounts of water, while tropical plants may need up to hundreds of inches of rain per year to survive.

In areas with wet and dry seasons, soil nutrients diminish and erosion increases during the wet season. Animals have adaptation and survival strategies for the wetter regime. The previous dry season leads to food shortages into the wet season, as the crops have yet to mature. Developing countries have noted that their populations show seasonal weight fluctuations due to food shortages seen before the first harvest, which occurs late in the wet season.

Changes due to global warming

See also: Effects of climate change and Extreme weather

 

Extreme precipitation events have become more common in the U.S. over recent decades.

Increasing temperatures tend to increase evaporation which leads to more precipitation. Precipitation has generally increased over land north of 30°N from 1900 to 2005 but has declined over the tropics since the 1970s. Globally there has been no statistically significant overall trend in precipitation over the past century, although trends have varied widely by region and over time. In 2018, a study assessing changes in precipitation across spatial scales using a high-resolution global precipitation dataset of over 33+ years, concluded that "While there are regional trends, there is no evidence of increase in precipitation at the global scale in response to the observed global warming."

Each region of the world is going to have changes in precipitation due to their unique conditions. Eastern portions of North and South America, northern Europe, and northern and central Asia have become wetter. The Sahel, the Mediterranean, southern Africa and parts of southern Asia have become drier. There has been an increase in the number of heavy precipitation events over many areas during the past century, as well as an increase since the 1970s in the prevalence of droughts—especially in the tropics and subtropics. Changes in precipitation and evaporation over the oceans are suggested by the decreased salinity of mid- and high-latitude waters (implying more precipitation), along with increased salinity in lower latitudes (implying less precipitation, more evaporation, or both). Over the contiguous United States, total annual precipitation increased at an average rate of 6.1% per century since 1900, with the greatest increases within the East North Central climate region (11.6% per century) and the South (11.1%). Hawaii was the only region to show a decrease (−9.25%).

Changes due to urban heat island

See also: Urban heat island

 

Image of Atlanta, Georgia, showing temperature distribution, with hot areas appearing white

The urban heat island warms cities 0.6 to 5.6 °C (1.1 to 10.1 °F) above surrounding suburbs and rural areas. This extra heat leads to greater upward motion, which can induce additional shower and thunderstorm activity. Rainfall rates downwind of cities are increased between 48% and 116%. Partly as a result of this warming, monthly rainfall is about 28% greater between 32 to 64 kilometres (20 to 40 mi) downwind of cities, compared with upwind. Some cities induce a total precipitation increase of 51%.

Forecasting

Main articles: Probability of precipitation and Quantitative precipitation forecast

 

Example of a five-day rainfall forecast from the Hydrometeorological Prediction Center

The Quantitative Precipitation Forecast (abbreviated QPF) is the expected amount of liquid precipitation accumulated over a specified time period over a specified area. A QPF will be specified when a measurable precipitation type reaching a minimum threshold is forecast for any hour during a QPF valid period. Precipitation forecasts tend to be bound by synoptic hours such as 0000, 0600, 1200 and 1800 GMT. Terrain is considered in QPFs by use of topography or based upon climatological precipitation patterns from observations with fine detail. Starting in the mid to late 1990s, QPFs were used within hydrologic forecast models to simulate impact to rivers throughout the United States. Forecast models show significant sensitivity to humidity levels within the planetary boundary layer, or in the lowest levels of the atmosphere, which decreases with height. QPF can be generated on a quantitative, forecasting amounts, or a qualitative, forecasting the probability of a specific amount, basis. Radar imagery forecasting techniques show higher skill than model forecasts within six to seven hours of the time of the radar image. The forecasts can be verified through use of rain gauge measurements, weather radar estimates, or a combination of both. Various skill scores can be determined to measure the value of the rainfall forecast.