# Rain

Falling rain

Rain is liquid water in the form of droplets that have condensed from atmospheric water vapor and then become 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 savannah climes.

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

Rain falling on a field, in southern Estonia

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

The shape of rain drops depending upon their size

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.

Black Rain Clouds

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]

A raindrop on a leaf

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 ${\displaystyle d}$ and ${\displaystyle D+dD}$ per unit volume of space is ${\displaystyle n(d)=n_{0}e^{-d/\langle d\rangle }dD}$ d 1 = 41 R 0.21 {\displaystyle \langle d\rangle ^{-1}=41R^{-0.21}} (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, called raindrop impressions.[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

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.

Other Languages
адыгабзэ: Ощхы
Afrikaans: Reën
Alemannisch: Regen
አማርኛ: ዝናብ
Ænglisc: Regn
العربية: مطر
aragonés: Plevia
ܐܪܡܝܐ: ܡܛܪܐ
armãneashti: Ploai
অসমীয়া: বৰষুণ
asturianu: Lluvia
Avañe'ẽ: Ama
Aymar aru: Jallu
azərbaycanca: Yağış
تۆرکجه: یاغیش
bamanankan: Sanji
বাংলা: বৃষ্টি
Bahasa Banjar: Hujan
Bân-lâm-gú: Hō͘
башҡортса: Ямғыр
беларуская: Дождж
беларуская (тарашкевіца)‎: Дождж
भोजपुरी: बरखा
български: Дъжд
Boarisch: Reng
བོད་ཡིག: ཆར་པ།
bosanski: Kiša
brezhoneg: Glav
буряад: Бороо
català: Pluja
Чӑвашла: Çумăр
čeština: Déšť
Cymraeg: Glaw
dansk: Regn
Deutsch: Regen
dolnoserbski: Dešć
eesti: Vihm
Ελληνικά: Βροχή
emiliàn e rumagnòl: Piòva
эрзянь: Пиземе
español: Lluvia
Esperanto: Pluvo
estremeñu: Lluvia
euskara: Euri
فارسی: باران
Fiji Hindi: Paani (barse)
français: Pluie
Fulfulde: Toɓo
Gaeilge: Fearthainn
Gaelg: Fliaghey
galego: Chuvia

گیلکی: وارؤن
ગુજરાતી: મેઘ

한국어: 비 (날씨)
հայերեն: Անձրև
हिन्दी: वर्षा
hornjoserbsce: Dešć
hrvatski: Kiša
Ido: Pluvo
Ilokano: Tudo
Bahasa Indonesia: Hujan
interlingua: Pluvia
ᐃᓄᒃᑎᑐᑦ/inuktitut: ᒥᓂ
isiXhosa: Imvula
íslenska: Rigning
italiano: Pioggia
עברית: גשם
Basa Jawa: Udan
ಕನ್ನಡ: ಮಳೆ
ქართული: წვიმა
қазақша: Жаңбыр
Kiswahili: Mvua
коми: Зэр
Kreyòl ayisyen: Lapli
kurdî: Baran
Кыргызча: Жамгыр
Latina: Pluvia
latviešu: Lietus
Lëtzebuergesch: Reen
lietuvių: Lietus
Limburgs: Raenger
lingála: Mbúla (mái)
lumbaart: Piöva
magyar: Eső
македонски: Дожд
Malagasy: Orana
മലയാളം: മഴ
मराठी: पाऊस
მარგალური: ჭვემა
مازِرونی: وارش
Bahasa Melayu: Hujan
Mìng-dĕ̤ng-ngṳ̄: Ṳ̄
монгол: Бороо
မြန်မာဘာသာ: မိုး
Nāhuatl: Quiyahuitl
Nederlands: Regen (neerslag)
Nedersaksies: Regen
नेपाली: वर्षा
नेपाल भाषा: वा (जलवायु)

norsk: Regn
norsk nynorsk: Regn
Nouormand: Pllie
occitan: Pluèja
ଓଡ଼ିଆ: ବର୍ଷା
oʻzbekcha/ўзбекча: Yomgʻir
ਪੰਜਾਬੀ: ਮੀਂਹ
پنجابی: مینہ
Patois: Rien
Перем Коми: Зэр
Piemontèis: Pieuva
polski: Deszcz
português: Chuva
română: Ploaie
Runa Simi: Para
русиньскый: Додж
русский: Дождь
саха тыла: Ардах
sardu: Proina
Scots: Rain
shqip: Shiu
sicilianu: Chiuvuta
සිංහල: වැස්ස
Simple English: Rain
سنڌي: برسات
slovenčina: Dážď
slovenščina: Dež
Soomaaliga: Roob
کوردی: باران
српски / srpski: Киша
srpskohrvatski / српскохрватски: Kiša
Basa Sunda: Hujan
svenska: Regn
Tagalog: Ulan
தமிழ்: மழை
татарча/tatarça: Яңгыр
తెలుగు: వర్షం
ไทย: ฝน
тоҷикӣ: Борон
ᏣᎳᎩ: ᎠᎦᏍᎬ
ತುಳು: ಬರ್ಸೊ
Türkçe: Yağmur
Türkmençe: Ýagyş
українська: Дощ
اردو: بارش
Vahcuengh: Fwn
vèneto: Pióva
vepsän kel’: Vihm
Tiếng Việt: Mưa
Võro: Vihm
walon: Plouve
Winaray: Uran

ייִדיש: רעגן
Yorùbá: Òjò

Zazaki: Varıt
žemaitėška: Lītos

ГӀалгӀай: ДогӀа
Kabɩyɛ: Tɛʋ