Rainy Day Essay Wikipedia The Free

"Rainy season" redirects here. For other uses, see Rainy Season (disambiguation).

The rainy season, or monsoon season, is the time of year when most of a region's average annual rainfall occurs. It usually lasts one or more months.[1] The term "green season" is also sometimes used as a euphemism by tourist authorities.[2] Areas with wet seasons are dispersed across portions of the tropics and subtropics.[3]

Under the Köppen climate classification, for tropical climates, a wet season month is defined as a month where average precipitation is 60 millimetres (2.4 in) or more.[4] In contrast to areas with savanna climates and monsoon regimes, Mediterranean climates have wet winters and dry summers. Dry and rainy months are characteristic of tropical seasonal forests: in contrast to tropical rainforests, which do not have dry or wet seasons, since their rainfall is equally distributed throughout the year.[5] Some areas with pronounced rainy seasons will see a break in rainfall mid-season, when the intertropical convergence zone or monsoon trough moves to higher latitudes in the middle of the warm season.[6]

When the wet season occurs during a warm season, or summer, precipitation falls mainly during the late afternoon and early evening. In the wet season, air quality improves, fresh water quality improves, and vegetation grows substantially, leading to crop yields late in the season. Rivers overflow their banks, and some animals retreat to higher ground. Soil nutrients diminish and erosion increases. The incidence of malaria increases in areas where the rainy season coincides with high temperatures, particularly in tropical areas.[7] Some animals have adaptation and survival strategies for the wet season. Often, the previous dry season leads to food shortages in the wet season, as the crops have yet to mature.

Character of the rainfall[edit]

In areas where the heavy rainfall is associated with a wind shift, the wet season is known as the monsoon.[8] Rainfall in the wet season is mainly due to daytime heating which leads to diurnal thunderstorm activity within a pre-existing moist airmass, so the rain mainly falls in late afternoon and early evening in savannah and monsoon regions. Further, much of the total rainfall each day occurs in the first minutes of the downpour,[6] before the storms mature into their stratiform stage.[9] Most places have only one wet season, but areas of the tropics can have two wet seasons, because the monsoon trough, or Intertropical Convergence Zone, can pass over locations in the tropics twice per year. However, since rain forests have rainfall spread evenly through the year, they do not have a wet season.[5]

It is different for places with a Mediterranean climate. In the western United States, during the cold season from September–May, extratropical cyclones from the Pacific Ocean move inland into the region due to a southward migration of the jet stream during the cold season. This shift in the jet stream brings much of the annual precipitation to the region,[10] and sometimes also brings heavy rain and strong low pressure systems.[11] The peninsula of Italy has weather very similar to the western United States in this regard.[12]

Areas affected[edit]

Areas with a savanna climate in Sub-Saharan Africa, such as Ghana, Burkina Faso,[13][14]Darfur,[15]Eritrea,[16]Ethiopia,[17] and Botswana have a distinct rainy season.[18] Also within the savanna climate regime, Florida and South Texas have a rainy season.[19] Monsoon regions include the Indian subcontinent, Southeast Asia (including Indonesia and Philippines),[20] northern sections of Australia's North,[21]Polynesia,[22]Central America,[23] western and southern Mexico,[24] the Desert Southwest of the United States,[10] southern Guyana,[25] portions of northeast Brazil.[26]

Northern Guyana has two wet seasons: one in early spring and the other in early winter.[25] In western Africa, there are two rainy seasons across southern sections, but only one across the north.[27] Within the Mediterranean climate regime, the west coast of the United States and the Mediterranean coastline of Italy, Greece,[28] and Turkey experience a wet season in the winter months.[29] Similarly, the wet season in the Negev desert of Israel extends from October through May.[30] At the boundary between the Mediterranean and monsoon climates lies the Sonoran desert, which receives the two rainy seasons associated with each climate regime.[31]

The wet season is known by many different local names throughout the world. For example, in Mexico it is known as "storm season". Different names are given to the various short "seasons" of the year by the Aboriginal tribes of Northern Australia: the wet season typically experienced there from December to March is called Gudjewg. The precise meaning of the word is disputed, although it is widely accepted to relate to the severe thunderstorms, flooding, and abundant vegetation growth commonly experienced at this time.[32]

Effects[edit]

In tropical areas, when the monsoon arrives, high daytime high temperatures drop and overnight low temperatures increase, thus reducing diurnal temperature variation.[33] During the wet season, a combination of heavy rainfall and, in some places such as Hong Kong, an onshore wind, improve air quality.[34] In Brazil, the wet season is correlated with weaker trade winds off the ocean.[26] The pH level of water becomes more balanced due to the charging of local aquifers during the wet season.[35] Water also softens, as the concentration of dissolved materials reduces during the rainy season.[36] Erosion is also increased during rainy periods.[6]Arroyos that are dry at other times of the year fill with runoff, in some cases with water as deep as 10 feet (3.0 m).[37] Leaching of soils during periods of heavy rainfall depletes nutrients.[37] The higher runoff from land masses affects nearby ocean areas, which are more stratified, or less mixed, due to stronger surface currents forced by the heavy rainfall runoff.[38]

Floods[edit]

High rainfall can cause widespread flooding,[39] which can lead to landslides and mudflows in mountainous areas.[40] Such floods cause rivers to burst their banks and submerge homes.[41] The Ghaggar-Hakra River, which only flows during India's monsoon season, can flood and severely damage local crops.[42] Floods can be exacerbated by fires that occurred during the previous dry season, which cause soils which are sandy or composed of loam to become hydrophobic, or water repellent.[43] In various ways governments may help people deal with wet season floods. Flood plain mapping identifies which areas are more prone to flooding.[44] Instructions on controlling erosion through outreach[clarification needed] are also provided by telephone or the internet.[45]

Life adaptations[edit]

Humans[edit]

The wet season is the main period of vegetation growth within the Savanna climate regime.[46] However, this also means that wet season is a time for food shortages before crops reach their full maturity.[47] This causes seasonal weight changes for people in developing countries, with a drop occurring during the wet season until the time of the first harvest, when weights rebound.[48] Malaria incidence increases during periods of high temperature and heavy rainfall.[49]

Animals[edit]

Cows calve, or give birth, at the beginning of the wet season.[50] The onset of the rainy season signals the departure of the monarch butterfly from Mexico.[51] Tropical species of butterflies show larger dot markings on their wings to fend off possible predators and are more active during the wet season than the dry season.[52] Within the tropics and warmer areas of the subtropics, decreased salinity of near shore wetlands due to the rains causes an increase in crocodile nesting.[53] Other species, such as the arroyo toad, spawn within the couple of months after the seasonal rains.[54]Armadillos and rattlesnakes seek higher ground.[55]

See also[edit]

References[edit]

  1. ^Glossary of Meteorology (2013). Rainy season.Archived 2009-02-15 at the Wayback Machine. American Meteorological Society. Retrieved on 2008-12-27.
  2. ^Costa Rica Guide (2005). When to Travel to Costa Rica. ToucanGuides. Retrieved on 2008-12-27.
  3. ^Michael Pidwirny (2008). CHAPTER 9: Introduction to the Biosphere. PhysicalGeography.net. Retrieved on 2008-12-27.
  4. ^"Updated world Köppen-Geiger climate classification map"(PDF). 
  5. ^ abElisabeth M. Benders-Hyde (2003). World Climates. Blue Planet Biomes. Retrieved on 2008-12-27.
  6. ^ abcJ. S. 0guntoyinbo and F. 0. Akintola (1983). Rainstorm characteristics affecting water availability for agriculture.Archived 2009-02-05 at the Wayback Machine. IAHS Publication Number 140. Retrieved on 2008-12-27.
  7. ^"Malaria Fact Sheet". The World Health Organization. April 2016. Retrieved April 24, 2016. 
  8. ^Glossary of Meteorology (2009). Monsoon.Archived 2008-03-22 at the Wayback Machine. American Meteorological Society. Retrieved on 2009-01-16.
  9. ^Robert A. Houze Jr (1997). Stratiform Precipitation in Regions of Convection: A Meteorological Paradox? Bulletin of the American Meteorological Society, pp. 2179. Retrieved on 2008-12-27.
  10. ^ abJ. Horel (2006). Normal Monthly Precipitation, Inches.Archived 2006-11-13 at the Wayback Machine. University of Utah. Retrieved on 2008-03-19.
  11. ^Norman W. Junker (2008). West Coast Cold Season Heavy Rainfall Events. Hydrometeorological Prediction Center. Retrieved on 2008-03-01.
  12. ^BBC Weather (2009). Country Guide: Italy. British Broadcasting Company. Retrieved on 2008-12-27.
  13. ^Patrick Laux et al. (2008): Predicting the regional onset of the rainy season in West Africa. International Journal of Climatology, 28 (3), 329–342.
  14. ^Patrick Laux et al. (2009): Modelling daily precipitation features in the Volta Basin of West Africa. International Journal of Climatology, 29 (7), 937–954.,
  15. ^David Vandervort (2009). Darfur: getting ready for the rainy season. International Committee of the Red Cross. Retrieved on 2009-02-06.
  16. ^Mehari Tesfazgi Mebrhatu, M. Tsubo, and Sue Walker (2004). A Statistical Model for Seasonal Rainfall Forecasting over the Highlands of Eritrea. New directions for a diverse planet: Proceedings of the 4th International Crop Science Congress. Retrieved on 2009-02-08.
  17. ^Alex Wynter (2009). Ethiopia: March rainy season "critical" for southern pastoralists. Thomson Reuters Foundation. Retrieved on 2009-02-06.
  18. ^The Voice (2009). Botswana: Rainy Season Fills Up Dams. allAfrica.com. Retrieved on 2009-02-06.
  19. ^Randy Lascody (2008). The Florida Rain Machine.National Weather Service. Retrieved on 2009-02-06.
  20. ^OCHA Partnership for Humanity (2008). OCHA Field Situation Report: Indonesia – Rainy Season 1 December 2008.Archived 18 February 2009 at the Wayback Machine. United Nations Office for the Coordination of Humanitarian Affairs. Retrieved on 2009-02-06.
  21. ^Burarra Gathering (2006). Burarra Gathering. Burarra Gathering. Retrieved on 2009-02-06.
  22. ^Tahiti Sun Travel Network (2007). About Bora Bora Island. Retrieved on 2009-02-06.
  23. ^Joint Typhoon Warning Center (2006). 2.4 Analysis & Forecasting "Thumb Rules" for the Rainy Season.United States Navy. Retrieved on 2009-02-06.
  24. ^Remote Sensing for Migratory Creatures (2002). Phenology and Creature Migration: Dry season and wet season in West Mexico. Arizona Remote Sensing Center. Retrieved on 2009-02-06.
  25. ^ abHorace Burton (2006). The climate of Guyana.Archived 2009-01-24 at the Wayback Machine. Caribbean Institute for Meteorology and Hydrology. The Outfield, August 2006, pp. 3. Retrieved on 2009-02-08.
  26. ^ abJames Brian Elsner (1988). Analysis of Wet Season Rainfall Over the Nordeste of Brazil, South America. University Of Wisconsin-Milwaukee. Retrieved on 2009-02-06.
  27. ^C. H. Mari, G. Cailley, L. Corre, M. Saunois, J. L. Attie, V. Thouret, and A. Stohl (2007). Biomass burning plumes during the AMMA wet season experiment. Atmospheric Chemistry and Physics Discussions, pp. 17342. Retrieved on 2009-02-06.
  28. ^Greek Embassy London (2008). Welcome to Greece. Government of Greece. Retrieved on 2009-02-06.
  29. ^D. Bozkurt, O.L. Sen and M. Karaca (2008). Wet season evaluation of RegCM3 performance for Eastern Mediterranean. EGU General Assembly. Retrieved on 2009-02-06.
  30. ^Ron Kahana; Baruch Ziv; Yehouda Enzel & Uri Dayan (2002). "Synoptic Climatology of Major Floods in the Negev Desert, Israel"(PDF). International Journal of Climatology. 22: 869. Bibcode:2002IJCli..22..867K. doi:10.1002/joc.766. Archived from the original(PDF) on 2011-07-19. 
  31. ^Michael J. Plagens (2009). What and Where is the Sonoran Desert? Arizonensis. Retrieved on 2009-02-07.
  32. ^"The Six Seasons". Australian Government, Department of the Environment. Retrieved April 25, 2016. 
  33. ^Official Web Site of District Sirsa, India (2001). District Sirsa.Archived 2010-12-28 at the Wayback Machine. National Informatice Center. Retrieved on 2008-12-27.
  34. ^Mei Zheng (2000). The sources and characteristics of atmospheric particulates during the wet and dry seasons in Hong Kong. University of Rhode Island. Retrieved on 2008-12-27.
  35. ^S. I. Efe, F. E. Ogban, M. J. Horsfall, E. E. Akporhonor (2005). Seasonal Variations of Physico-chemical Characteristics in Water Resources Quality in Western Niger Delta Region, Nigeria. Journal of Applied Scientific Environmental Management. Retrieved on 2008-12-27.
  36. ^C. D. Haynes, M. G. Ridpath, M. A. J. Williams (1991). Monsoonal Australia. Taylor & Francis, pp. 90. ISBN 978-90-6191-638-3. Retrieved on 2008-12-27.
  37. ^ abUnited States War Department (1909). Road Notes, Cuba. 1909. United States Department of War. Retrieved on 2009-01-16.
  38. ^K.W. Choi and J.H.W. Lee (2000). Wet Season Tidal Circulation and flushing in Three Fathoms Cove.Archived 2009-02-27 at the Wayback Machine. 4th International Conference on Hydro-Science and Engineering. Retrieved on 2008-12-27.
  39. ^Overseas Security Advisory Council (2009). Warden Message: Guyana Rainy Season Flood Hazards.[permanent dead link] Overseas Security Advisory Council. Retrieved on 2009-02-05.
  40. ^National Flood Insurance Program (2009). California's Rainy Season.Federal Emergency Management Agency. Retrieved on 2009-02-05.
  41. ^AFP (2009). Bali Hit By Wet Season Floods.ABC News. Retrieved on 2009-02-06.
  42. ^"Sirsa District Disaster Management Plan, 2015-2016"(PDF). District Sirsa. Government of Haryana Department of Revenue and Disaster Management, Haryana Institute of Public Administration. 2015. Retrieved April 25, 2016. 
  43. ^Jack Ainsworth & Troy Alan Doss. Natural History of Fire & Flood Cycles. California Coastal Commission. Retrieved on 2009-02-05.
  44. ^FESA (2007). Flood.Archived 2009-05-31 at the Wayback Machine. Government of Western Australia. Retrieved on 2009-02-06.
  45. ^King County Department of Development and Environmental Services (2009). Erosion and Sediment Control for Construction Sites. King County, Washington Government. Retrieved on 2009-02-06.
  46. ^Charles Darwin University (2009). Characteristics of tropical savannas.Archived 2009-02-17 at the Wayback Machine. Charles Darwin University. Retrieved on 2008-12-27.
  47. ^A. Roberto Frisancho (1993). Human Adaptation and Accommodation. University of Michigan Press, pp. 388. ISBN 978-0-472-09511-7. Retrieved on 2008-12-27.
  48. ^Marti J. Van Liere, Eric-Alain D. Ategbo, Jan Hoorweg, Adel P. Den Hartog, and Joseph G. A. J. Hautvast. The significance of socio-economic characteristics for adult seasonal body-weight fluctuations: a study in north-western Benin. British Journal of Nutrition: Cambridge University Press, 1994.
  49. ^African Centre of Meteorological Application for Development (2008). Ten Day Climate Bulletin: Dekad of 01 to 10 April, 2008.Archived 2009-02-27 at the Wayback Machine. ACMAD. Retrieved on 2009-02-08.
  50. ^John P. McNamara, J. France, D. E. Beever (2000). Modelling Nutrient Utilization in Farm Animals. CABI, pp. 275. ISBN 978-0-85199-449-9. Retrieved on 2009-02-06.
  51. ^Dr. Lincoln Brower (2005). Precipitation at the Monarch Overwintering Sites in Mexico. Journey North. Retrieved on 2009-02-06.
  52. ^Paul M. Brakefield and Torben B. Larsen (1983). The evolutionary significance of dry and wet season forms in some tropical butterflies.Biological Journal of the Linnean Society, pp. 1–12. Retrieved on 2008-12-27.
  53. ^Phil Hall (1989). Crocodiles, Their Ecology, Management, and Conservation. International Union for Conservation of Nature and Natural Resources Crocodile Specialist Group, pp. 167. Retrieved on 2008-12-27.
  54. ^San Diego Natural History Museum (2009). Bufo californicus: Arroyo Toad. San Diego Natural History Museum. Retrieved on 2009-01-16.
  55. ^Linda Deuver (1978). Dry season, wet season.Archived 2009-01-20 at the Wayback Machine. Audubon Magazine, November 1978, pp. 120–130. Retrieved on 2009-02-06.

Template:Watershed

Wet season storm at night in Darwin, Australia.

For other uses, see Rain (disambiguation) and Rainy (disambiguation).

Rain is liquid water in the form of droplets that have condensed from atmosphericwater vapor and then becomes heavy enough to fall under gravity. Rain is a major component of the water cycle and is responsible for depositing most of the fresh water on the Earth. It provides suitable conditions for many types of ecosystems, as well as water for hydroelectric power plants and crop irrigation.

The major cause of rain production is moisture moving along three-dimensional zones of temperature and moisture contrasts known as weather fronts. If enough moisture and upward motion is present, precipitation falls from convective clouds (those with strong upward vertical motion) such as cumulonimbus (thunder clouds) which can organize into narrow rainbands. In mountainous areas, heavy precipitation is possible where upslope flow is maximized within windward sides of the terrain at elevation which forces moist air to condense and fall out as rainfall along the sides of mountains. On the leeward side of mountains, desert climates can exist due to the dry air caused by downslope flow which causes heating and drying of the air mass. The movement of the monsoon trough, or intertropical convergence zone, brings rainy seasons to savannahclimes.

The urban heat island effect leads to increased rainfall, both in amounts and intensity, downwind of cities. Global warming is also causing changes in the precipitation pattern globally, including wetter conditions across eastern North America and drier conditions in the tropics.[citation needed] Antarctica is the driest continent. The globally averaged annual precipitation over land is 715 mm (28.1 in), but over the whole Earth it is much higher at 990 mm (39 in).[1]Climate classification systems such as the Köppen classification system use average annual rainfall to help differentiate between differing climate regimes. Rainfall is measured using rain gauges. Rainfall amounts can be estimated by weather radar.

Rain is also known or suspected on other planets, where it may be composed of methane, neon, sulfuric acid, or even iron rather than water.

Formation

Water-saturated air

Air contains water vapor, and the amount of water in a given mass of dry air, known as the mixing ratio, is measured in grams of water per kilogram of dry air (g/kg).[2][3] The amount of moisture in air is also commonly reported as relative humidity; which is the percentage of the total water vapor air can hold at a particular air temperature.[4] How much water vapor a parcel of air can contain before it becomes saturated (100% relative humidity) and forms into a cloud (a group of visible and tiny water and ice particles suspended above the Earth's surface)[5] depends on its temperature. Warmer air can contain more water vapor than cooler air before becoming saturated. Therefore, one way to saturate a parcel of air is to cool it. The dew point is the temperature to which a parcel must be cooled in order to become saturated.[6]

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.[7] 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,[8] 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.[9] 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.[10]

The main ways water vapor is added to the air are: wind convergence into areas of upward motion,[11] precipitation or virga falling from above,[12] daytime heating evaporating water from the surface of oceans, water bodies or wet land,[13] transpiration from plants,[14] cool or dry air moving over warmer water,[15] and lifting air over mountains.[16] Water vapor normally begins to condense on condensation nuclei such as dust, ice, and salt in order to form clouds. Elevated portions of weather fronts (which are three-dimensional in nature)[17] force broad areas of upward motion within the Earth's atmosphere which form clouds decks such as altostratus or cirrostratus.[18]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.[19]

Coalescence and fragmentation

Coalescence occurs when water droplets fuse to create larger water droplets. Air resistance typically causes the water droplets in a cloud to remain stationary. When air turbulence occurs, water droplets collide, producing larger droplets.

As these larger water droplets descend, coalescence continues, so that drops become heavy enough to overcome air resistance and fall as rain. Coalescence generally happens most often in clouds above freezing, and is also known as the warm rain process.[20] In clouds below freezing, when ice crystals gain enough mass they begin to fall. This generally requires more mass than coalescence when occurring between the crystal and neighboring water droplets. This process is temperature dependent, as supercooled water droplets only exist in a cloud that is below freezing. In addition, because of the great temperature difference between cloud and ground level, these ice crystals may melt as they fall and become rain.[21]

Raindrops have sizes ranging from 0.1 to 9 mm (0.0039 to 0.3543 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. Large rain drops become increasingly flattened on the bottom, like hamburger buns; very large ones are shaped like parachutes.[22][23] Contrary to popular belief, their shape does not resemble a teardrop.[24] The biggest raindrops on Earth were recorded over Brazil and the Marshall Islands in 2004 — some of them were as large as 10 mm (0.39 in). The large size is explained by condensation on large smoke particles or by collisions between drops in small regions with particularly high content of liquid water.[25]

Rain drops associated with melting hail tend to be larger than other rain drops.[26]

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.[27][28]

Droplet size distribution

The final droplet size distribution is an exponential distribution. The number of droplets with diameter between and per unit volume of space is . This is commonly referred to as the Marshall–Palmer law after the researchers who first characterized it.[23][29] The parameters are somewhat temperature-dependent,[30] and the slope also scales with the rate of rainfall (d in centimeters and R in millimetres per hour).[23]

Deviations can occur for small droplets and during different rainfall conditions. The distribution tends to fit averaged rainfall, while instantaneous size spectra often deviate and have been modeled as gamma distributions.[31] The distribution has an upper limit due to droplet fragmentation.[23]

Raindrop impacts

Raindrops impact at their terminal velocity, which is greater for larger drops due to their larger mass to drag ratio. At sea level and without wind, 0.5 mm (0.020 in) drizzle impacts at 2 m/s (6.6 ft/s) or 7.2 km/h (4.5 mph), while large 5 mm (0.20 in) drops impact at around 9 m/s (30 ft/s) or 32 km/h (20 mph).[32]

Rain falling on loosely packed material such as newly fallen ash can produce dimples that can be fossilized.[33] The air density dependence of the maximum raindrop diameter together with fossil raindrop imprints has been used to constrain the density of the air 2.7 billion years ago.[34]

The sound of raindrops hitting water is caused by bubbles of air oscillating underwater.[35][36]

The METAR code for rain is RA, while the coding for rain showers is SHRA.[37]

Virga

Main article: Virga

In certain conditions precipitation may fall from a cloud but then evaporate or sublime before reaching the ground. This is termed virga and is more often seen in hot and dry climates.

Causes

Frontal activity

Main article: Weather fronts

Stratiform (a broad shield of precipitation with a relatively similar intensity) and dynamic precipitation (convective precipitation which is showery in nature with large changes in intensity over short distances) occur as a consequence of slow ascent of air in synoptic systems (on the order of cm/s), such as in the vicinity of cold fronts and near and poleward of surface warm fronts. Similar ascent is seen around tropical cyclones outside the eyewall, and in comma-head precipitation patterns around mid-latitude cyclones.[38] 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.[39] What separates rainfall from other precipitation types, such as ice pellets and snow, is the presence of a thick layer of air aloft which is above the melting point of water, which melts the frozen precipitation well before it reaches the ground. If there is a shallow near surface layer that is below freezing, freezing rain (rain which freezes on contact with surfaces in subfreezing environments) will result.[40]Hail becomes an increasingly infrequent occurrence when the freezing level within the atmosphere exceeds 3,400 m (11,000 ft) above ground level.[41]

Convection

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.[38][42]Graupel and hail indicate convection.[43] In mid-latitudes, convective precipitation is intermittent and often associated with baroclinic boundaries such as cold fronts, squall lines, and warm fronts.[44]

Orographic effects

Main articles: Orographic lift, Precipitation types (meteorology), and United States rainfall climatology

Orographic precipitation occurs on the windward 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.[16]

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 mm (460 in).[45] Systems known as Kona storms affect the state with heavy rains between October and April.[46] 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.[47]

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.[48] The Sierra Nevada range creates the same effect in North America forming the Great Basin and Mojave Deserts.[49][50]

Within the tropics

See also: Monsoon and Tropical cyclone

Main article: Wet season

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.[51] The term green season is also sometimes used as a euphemism by tourist authorities.[52] Areas with wet seasons are dispersed across portions of the tropics and subtropics.[53]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.[54] 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.[27] 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,[55]freshwater quality improves,[56][57] and vegetation grows significantly.

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 counter clockwise (northern hemisphere).[58] 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.[59] Areas in their path can receive a year's worth of rainfall from a tropical cyclone passage.[60]

Human influence

See also: Global warming and Urban heat island

The fine particulate matter produced by car exhaust and other human sources of pollution forms cloud condensation nuclei, leads to the production of clouds and increases the likelihood of rain. As commuters and commercial traffic cause pollution to build up over the course of the week, the likelihood of rain increases: it peaks by Saturday, after five days of weekday pollution has been built up. In heavily populated areas that are near the coast, such as the United States' Eastern Seaboard, the effect can be dramatic: there is a 22% higher chance of rain on Saturdays than on Mondays.[61] The urban heat island effect 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 km (20 to 40 mi) downwind of cities, compared with upwind.[62] Some cities induce a total precipitation increase of 51%.[63]

Increasing temperatures tend to increase evaporation which can lead to more precipitation. Precipitation generally increased over land north of 30°N from 1900 through 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. 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 and/or more evaporation). Over the contiguous United States, total annual precipitation increased at an average rate of 6.1 percent since 1900, with the greatest increases within the East North Central climate region (11.6 percent per century) and the South (11.1 percent). Hawaii was the only region to show a decrease (−9.25 percent).[64]

Analysis of 65 years of United States of America rainfall records show the lower 48 states have an increase in heavy downpours since 1950. The largest increases are in the Northeast and Midwest, which in the past decade, have seen 31 and 16 percent more heavy downpours compared to the 1950s. Rhode Island is the state with the largest increase, 104%. McAllen, Texas is the city with the largest increase, 700%. Heavy downpour in the analysis are the days where total precipitation exceeded the top 1 percent of all rain and snow days during the years 1950–2014[65][66]

The most successful attempts at influencing weather involve cloud seeding, which include techniques used to increase winter precipitation over mountains and suppress hail.[67]

Characteristics

Patterns

Main article: Rainband

Rainbands are cloud and precipitation areas which are significantly elongated. Rainbands can be stratiform or convective,[68] and are generated by differences in temperature. When noted on weather radar imagery, this precipitation elongation is referred to as banded structure.[69] Rainbands in advance of warm occluded fronts and warm fronts are associated with weak upward motion,[70] and tend to be wide and stratiform in nature.[71]

Rainbands spawned near and ahead of cold fronts can be squall lines which are able to produce tornadoes.[72] Rainbands associated with cold fronts can be warped by mountain barriers perpendicular to the front's orientation due to the formation of a low-level barrier jet.[73] Bands of thunderstorms can form with sea breeze and land breeze boundaries, if enough moisture is present. If sea breeze rainbands become active enough just ahead of a cold front, they can mask the location of the cold front itself.[74]

Once a cyclone occludes, a trough of warm air aloft, or "trowal" for short, will be caused by strong southerly winds on its eastern periphery rotating aloft around its northeast, and ultimately northwestern, periphery (also known as the warm conveyor belt), forcing a surface trough to continue into the cold sector on a similar curve to the occluded front. The trowal creates the portion of an occluded cyclone known as its comma head, due to the comma-like shape of the mid-tropospheric cloudiness that accompanies the feature. It can also be the focus of locally heavy precipitation, with thunderstorms possible if the atmosphere along the trowal is unstable enough for convection.[75] Banding within the comma head precipitation pattern of an extratropical cyclone can yield significant amounts of rain.[76] Behind extratropical cyclones during fall and winter, rainbands can form downwind of relative warm bodies of water such as the Great Lakes. Downwind of islands, bands of showers and thunderstorms can develop due to low level wind convergence downwind of the island edges. Offshore California, this has been noted in the wake of cold fronts.[77]

Rainbands within tropical cyclones are curved in orientation. Tropical cyclone rainbands contain showers and thunderstorms that, together with the eyewall and the eye, constitute a hurricane or tropical storm. The extent of rainbands around a tropical cyclone can help determine the cyclone's intensity.[78]

Acidity

See also: Acid rain

The phrase acid rain was first used by Scottish chemist Robert Augus Smith in 1852.[79] The pH of rain varies, especially due to its origin. On America's East Coast, rain that is derived from the Atlantic Ocean typically has a pH of 5.0–5.6; rain that comes across the continental from the west has a pH of 3.8–4.8; and local thunderstorms can have a pH as low as 2.0.[80] Rain becomes acidic primarily due to the presence of two strong acids, sulfuric acid (H2SO4) and nitric acid (HNO3). Sulfuric acid is derived from natural sources such as volcanoes, and wetlands (sulfate reducing bacteria); and anthropogenic sources such as the combustion of fossil fuels, and mining where H2S is present. Nitric acid is produced by natural sources such as lightning, soil bacteria, and natural fires; while also produced anthropogenically by the combustion of fossil fuels and from power plants. In the past 20 years the concentrations of nitric and sulfuric acid has decreased in presence of rainwater, which may be due to the significant increase in ammonium (most likely as ammonia from livestock production), which acts as a buffer in acid rain and raises the pH.[81]

Köppen climate classification

Main article: Köppen climate classification

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).[83] A tropical savanna is a grasslandbiome located in semi-arid to semi-humid climate regions of subtropical and tropicallatitudes, 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.[84] The humid subtropical climate zone is where winter rainfall is associated with large storms that the westerlies steer from west to east. Most summer rainfall occurs during thunderstorms and from occasional tropical cyclones.[85] Humid subtropical climates lie on the east side continents, roughly between latitudes 20° and 40° degrees away from the equator.[86]

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.[87] The Mediterranean climate regime resembles the climate of the lands in the Mediterranean Basin, parts of western North America, parts of Western and South Australia, in southwestern South Africa and in parts of central Chile. The climate is characterized by hot, dry summers and cool, wet winters.[88] A steppe is a dry grassland.[89] Subarctic climates are cold with continuous permafrost and little precipitation.[90]

Measurement

Gauges

See also: Rain gauge, Disdrometer, and Snow gauge

Rain is measured in units of length per unit time, typically in millimeters per hour,[91] or in countries where imperial units are more common, inches per hour.[92] The "length", or more accurately, "depth" being measured is the depth of rain water that would accumulate on a flat, horizontal and impermeable surface during a given amount of time, typically an hour.[93] One millimeter of rainfall is the equivalent of one liter of water per square meter.[94]

The standard way of measuring rainfall or snowfall is the standard rain gauge, which can be found in 100-mm (4-in) plastic and 200-mm (8-in) metal varieties.[95] 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 it 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.[96] 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.[97] 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.[98]

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.[99][100] If a network is not available in the area where one lives, the nearest local weather or met office will likely be interested in the measurement.[101]

Remote sensing

See also: Weather radar

One of the main uses of weather radar is to be able to assess the amount of precipitations fallen over large basins for hydrological purposes.[102] For instance, river flood control, sewer management and dam construction are all areas where planners use rainfall accumulation data. Radar-derived rainfall estimates compliment surface station data which can be used for calibration. To produce radar accumulations, rain rates over a point are estimated by using the value of reflectivity data at individual grid points. A radar equation is then used, which is,

,

where Z represents the radar reflectivity, R represents the rainfall rate, and A and b are constants.[103]

Rain falling on a field, in southern Estonia
The shape of rain drops depending upon their size
Rainfall distribution by month in Cairns showing the extent of the wet season at that location
Image of Atlanta, Georgia showing temperature distribution, with blue showing cool temperatures, red warm, and hot areas appearing white.
Mean surface temperature anomalies during the period 1999 to 2008 with respect to the average temperatures from 1940 to 1980
Updated Köppen-Geiger climate map[82]

  Af

  Am

  Aw

  BWh

  BWk

  BSh

  BSk

  Csa

  Csb

  Cwa

  Cwb

  Cfa

  Cfb

  Cfc

  Dsa

  Dsb

  Dsc

  Dsd

  Dwa

  Dwb

  Dwc

  Dwd

  Dfa

  Dfb

  Dfc

  Dfd

  ET

  EF

Twenty-four-hour rainfall accumulation on the Val d'Irène radar in Eastern Canada. Zones without data in the east and southwest are caused by beam blocking from mountains. (Source: Environment Canada)

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