# rain

Rain is the most common form of liquid precipitation from clouds . It consists of water that falls to the earth as a result of gravity after condensation of water vapor .

Raindrops bind dust and aerosols that have risen into the atmosphere. These components determine the pH of the rain.

The forms of rain are differentiated according to origin, duration, intensity, effect and geographical occurrence. Solid precipitation, e.g. B. hail , sleet or snow , consists of frozen water and condensation nuclei and occurs mixed with rain.

Rain front over the Müritz

## etymology

That is common. Word mhd. Rain , ahd. Regan is probably of Latin origin: regare - 'irrigate, wet'.

## Emergence

Dimensions (germ, cloud and raindrops)
Sound recording: rain falls on a car roof

The condensation of water vapor in the atmosphere occurs through cooling and aerodynamics . In addition, the dust content and the aerosols determine the dew point, deviating from the phase diagram of theoretical thermodynamics .

### General

The starting point of every rain are clouds , which consist of fine ice crystals or cloud droplets (water droplets with a diameter of 5 to 10 μm). They are formed as a result of the cooling of a moist air mass when rising in the earth's atmosphere when the dew point is undershot. Depending on the altitude and the prevailing temperature, either ice crystals form on crystallization nuclei through resublimation or cloud droplets with the help of condensation nuclei through condensation . These primary bodies can, depending on the length of time spent in the cloud, bind further water vapor, other cloud droplets or even ice crystals and thus grow. When ice crystals reach a warmer environment, they melt back into drops. If the weight of the droplets is so great that they cannot be kept "in suspension" by air friction (friction in the fluid according to Stokes' law ) or by the air currents prevailing in a cloud (updrafts), they slowly begin to close due to gravity The ground sinks, and the rain we know occurs. The growing together of many small water droplets into larger and heavier ones accelerates this process and increases the speed of fall. As a rule, the rain that hits the ground consists of drops with a diameter of 0.6 to 3 mm.

### Droplet growth in warm clouds

Raindrops on an airplane window with clearly visible ice cores

Warm clouds are clouds that only contain liquid water. Depending on the nature of the condensation nuclei , water can remain liquid below 0 ° C; These supercooled clouds are also called warm clouds and, by definition, are water clouds.

The growth of cloud droplets into rain droplets simply by collecting water vapor (vapor diffusion) is not very efficient and quite slow. Therefore, the collision and subsequent confluence ( coalescence ) of cloud droplets are further decisive steps in the formation of rain. Collisions occur because large drops sink faster than small ones. However, a collision does not always occur; often small droplets are diverted by the air current around rapidly falling large droplets. If coalescence also occurs after a collision, this is referred to as accretion , that is, growth through collection. Not every collision necessarily leads to coalescence. In this context, one speaks of collision and coalescence efficiency. The collision efficiency is very high for droplets of a similar size with a radius of at least 30 µm, whereas the coalescence efficiency is higher for droplets with different radii. Large droplets therefore collide more frequently, but they mostly remain unchanged, whereas small droplets tend to grow together with large ones. The product of collision and coalescence efficiency is also called accretion efficiency ; it is a parameter for the rain probability of clouds. The bigger the drops get, the faster they grow. This process is favored by a high moisture content in the air (tropics, subtropics) or large condensation nuclei, such as in maritime air masses.

### Growth in cold clouds

If clouds consist entirely or only partially of ice particles during their life cycle , they are called cold clouds . Mixed forms are sometimes also referred to as cool clouds . However, the mechanism of growth is completely different from that in warm clouds .

There is a balance between subcooled water droplets and the water vapor in their environment. If the air contains a lot of moisture, the drops grow, in dry air they give off water. There is a similar balance between ice crystals and water vapor, with the difference that ice crystals absorb moisture more efficiently and are less prone to releasing it again. If there are ice crystals in a cloud in the vicinity of supercooled water droplets, then a saturation of the water vapor with regard to the water droplets represents a supersaturation with regard to the ice crystals. The ice crystals grow by sublimation (technical resublimation) at the expense of the drops ( Bergeron-Findeisen process ). A chain reaction ensues, the ice crystals fall faster and break into splinters when they collide with water droplets, which in turn grow and thus lead to the icing of the supercooled water cloud. Most often clouds freeze in a temperature range of around −5 ° C to −15 ° C, in this range there is the maximum difference between the saturation vapor pressure above water and that above ice (see also: Crystal formation ). So that rain (and not snowfall, sleet etc.) occurs when the clouds are icy, the ice particles have to pass warmer layers of air and thaw again as they sink. In doing so, they may again cross layers of clouds with cloud droplets (in the case of undercooled clouds or thunderstorm clouds) or water vapor. By accretion, they continue to grow there, the process then resembles that in warm clouds .

## Measurement

When measuring rain that has fallen, the amount is given in liters per square meter or, as is common in meteorology, the height in " millimeters ". The following applies:

${\ displaystyle {\ frac {1 \, \ mathrm {l}} {1 \, \ mathrm {m} ^ {2}}} = {\ frac {1 \, \ mathrm {dm} ^ {3}} { 1 \, \ mathrm {m} ^ {2}}} = {\ frac {0 {,} 001 \, \ mathrm {m} ^ {3}} {1 \, \ mathrm {m} ^ {2}} } = 0 {,} 001 \, \ mathrm {m} = 1 \, \ mathrm {mm}}$

One millimeter of precipitation corresponds to one liter of precipitation on one square meter. This specification also corresponds to the height by which the water level in a collecting vessel (e.g. an empty tin can) would rise.

The classic rain gauge is therefore also a vessel that catches falling raindrops. Precipitation can be calculated from the amount of water in the container and the size of its opening. The first rain measurements were made in India about 2000 years ago, and rain collectors were first used in Europe in the 17th century. In the 1670s, Pierre Perrault used a rain gauge to prove the relationship between rain and the amount of runoff from streams and rivers. In strong winds, rain collectors are imprecise and tend to capture insufficient amounts of rain, as turbulence carries raindrops away from the collector. The first rain gauges were installed on roofs, today they are placed near the ground to reduce the influence of the wind. The size and shape of the official rain gauges vary from country to country. Their standardization has so far failed because it would limit the comparability with long-term records.

Newer methods of rain measurement are precipitation radar and weather satellites , but a dense network of rain collectors provides more precise values.

There are also rain sensors - these are not used for measurement, but only to control technical processes.

## Rain forms

 definition amount definition amount Definition solely according to quantity and time Rain showers Precipitation in 10 minutes Rainfall amount in one hour light 0.1 to 0.4 mm light 0.1 to 0.5 mm moderate 0.4 to 2 mm moderate 0.5 to 4 mm strong 2 to 8 mm strong 4 to 10 mm very strong from 8 mm

Depending on meteorological and geographic conditions, there are different forms of rain. Rain can be classified according to duration or intensity or according to its origin, spatial occurrence, effect on the ground or the perception of a viewer. One and the same rain event can be classified into different categories, depending on the perspective of the observer - here are some examples:

### Definition according to the primary development process

In general, rain comes about through a primary process, after which the form of rain can be named. The following forms are possible:

#### Incline rain (orographic rain)

Slope precipitation falls as slope rain or as snow, depending on the starting temperature of the incoming air masses and the height of the mountain range. The cloud formation depends on this and on the content of gaseous water vapor in the air masses and how productive the slope precipitation is then.

Slope rain (slope precipitation ) or damming rain (liquid form of orographic precipitation) occurs when the wind lets moist air rise from the sea or flat land on mountain ranges or other orographic elevations ( windward side ). Slope rain occurs in the tropics, subtropics and temperate zones. It can last from hours to a few days, in rare cases even several weeks.

The air is cooled further with increasing altitude, but its water vapor capacity also decreases and the air temperature continues to approach the dew point . First, the air cools down by one degree Celsius per 100 meters of altitude using the principle of dry adiabatic cooling ( adiabatic means “without heat exchange with the environment”, see adiabatic change of state ). As soon as a relative humidity of 100 percent is reached, the air cools down only by about 0.6 ° C per 100 meters according to the principle of moist adiabatic cooling. Here condenses , releasing the water vapor of the air mass of latent heat to water (cloud droplets), resulting in cloud formation leads. Depending on the intensity of the upward current, the result is often heavy rainfall . These are concentrated on the respective orographic obstacles, where high amounts of precipitation are often reached.

According to the thermodynamic foehn theory , the further course can look like this: On the leeward side, the descending air, if it is completely rained out, warms up again dry adiabatically by one degree Celsius per 100 meters, i.e. faster than the cooling occurred during the ascent. At low altitudes, this can lead to a warmer air flow on the leeward side, known as the foehn .

Due to these processes and the generally prevailing main wind directions, mountains and other geographic elevations usually have a rainy or weather side with increased precipitation intensity. This can also lead to the formation of rainforests in the temperate latitudes , one then speaks of the temperate rainforest . The areas on the leeward side of the mountain range receive less rainfall, they are in the rain shadow due to the leeward effect .

#### Convection rain

Creation of convection rain

Convection rain is rain from clouds that form due to convection currents. Convection rain occurs mainly in the tropics and subtropics, but also in the temperate latitudes during the warmer months, including Germany, Austria and Switzerland. Depending on the geographical location, it can last between several minutes (cloudbursts) and several days (tropical rain).

In warm weather, large amounts of the water present in the ground or on water surfaces evaporate. The resulting moist air masses close to the ground are transported upwards due to air currents (heat flows) also caused by the heat on the ground. When they reach their saturation, clouds form. The size and type of clouds formed depend on the intensity of the currents, the air mass and its humidity, the temperature and the nature of the soil (geography). Under optimal conditions, very strong convection thunderstorms often form in just a few hours. These occur mainly in tropical, but also in many other areas of the world (especially during the warm season), often in the early to late afternoon. Depending on the intensity, the number of condensation nuclei in the air and the existing moist air masses, small clouds or extremely large thunderclouds can form.

#### Front rain (stratiform rain)

Formation of front rain

Front rain (cyclone rain, stratiform rain) occurs in a warm or cold front and occurs in the subtropics and temperate zones. The duration of the rain at the front is directly dependent on the length of time the front stays above the observation site and the temperature difference at the front. Front rain moves with the front, front rain also occurs on the back of cloud fields, which are pushed into a front system by winds of different mean temperatures . Then the rain sets in briefly when the sky clears up again shortly before the end of the passage through the cloud field.

Front rain occurs when warm and humid air masses (often from tropical areas) meet cold ( polar ) air masses. In the case of a warm front, the lighter warm air slides onto the heavier cold air; in the case of a cold front, the heavier cold air is pushed under the existing warm air.

When ascending, the warm, moist air cools down, the stored water vapor condenses, clouds form and it rains. The principle of formation is similar to that of incline rain, with the difference that air masses instead of solid obstacles cause the humid air to be transported upwards.

### Complementary forms

These forms mostly describe the effect and the sensation by the observer on the ground, the primary development process is mostly neglected during the observation.

#### Continuous rain (land rain)

A long-lasting precipitation event is called continuous rain or land rain. In the temperate latitudes, it falls almost entirely from Nimbostratus clouds. Continuous rain can be observed in the tropics, subtropics and temperate latitudes and last for several hours to days, but rarely for several weeks. In the temperate latitudes it usually occurs in connection with a warm front passage. The respective definition of continuous rain can vary depending on the climatic area . In Central Europe one speaks of continuous rain when it continues with uninterrupted rainfall and a severity of more than 0.5 millimeters of precipitation per hour for a period of at least six hours.

Heavy rain from a storm front over Koh Samui, Thailand (May 2005)

#### Heavy rain

In meteorology, heavy rain is used to describe large amounts of rain that fall in a short time. This type of rain is thus defined according to its intensity and duration. Heavy rain occurs in the tropics, subtropics and temperate latitudes and can last from a few minutes to a few hours. The following general official definition:

“Rain, which has a high precipitation intensity in relation to its duration and therefore rarely occurs, z. B. on average no more than twice a year. "

Heavy rain with a shower relay over the Swabian Alb (August 2003)

is specified for the German-speaking area using a calculation formula. One speaks of heavy rain when the rain height (in millimeters ) is above a certain limit depending on the rain duration (in minutes): ${\ displaystyle h}$${\ displaystyle t}$

${\ displaystyle h> {\ sqrt {5 \ cdot t- \ left ({\ frac {t} {24}} \ right) ^ {2}}}}$

Thus a precipitate is considered heavy rain when at least 5  liters on the square in 5 minutes or more than 17 liters per square meter fell in an hour. Heavy rain events can, however, be much more severe. Events in thunderstorms , where 30 liters fall in 30 minutes on the square, are relatively rare in Central Europe, but can already possibly flooding lead to flooded basements. The stronger and longer lasting these events, the less likely they are to occur. Short but heavy rainfall is more likely than prolonged heavy rainfall, which can bring up to 200 mm of rainfall in Central Europe in a few days. Long-lasting heavy rain falls in Europe especially in Vb weather conditions (pronounced “5 b”).

Examples: On July 3, 1975, in Shangdi in the " Inner Mongolia " region of China, 401 liters per square meter fell in one hour and on November 26, 1970, 38 liters of rain per square meter fell in one minute on Basse-Terre , an island part of Guadeloupe . On June 6, 2011, 80 liters of precipitation per square meter fell in some districts of Hamburg within 45 minutes. On July 28, 2014, it rained 292 l / m² in Münster within seven hours.

##### Heavy rain in the climatic zones

In the tropics, the tendency to heavy rain is very high, especially during the rainy season in the intra- tropical convergence zone . Even tropical cyclones cause heavy rainfall, especially over the sea and on the coasts. In areas with little vegetation in the humid tropics and dry subtropics , heavy rain (“back rain”) can trigger catastrophic, heavily erosive stratification floods . In Europe, subcontinental or continental areas are affected. In the coastal or maritime climates of the temperate zone, heavy rain events occur very rarely.

The atmospheric rivers , which have been known since 1998, can shift large amounts of water to non-tropical coastal areas of the oceans and generate extremely heavy rain there, such as those processed in the Arkstorm scenario of the United States Geological Survey .

##### Heavy rain and climate change

In connection with global climate change as a result of the anthropogenic increase in greenhouse gases , a possible increase in heavy rain events is also being discussed. In 2011, a study by the German Insurance Association (GDV) showed that there are regional differences. An increase can be observed in southern Germany, while the number of events in northern Germany is falling.

#### Downpour

Downpour illuminated by the evening sun

Downpour refers to rain that only rains over a small area in terms of time and space. It usually only lasts a few minutes and often affects less than a square kilometer. Due to its topography, the area is not particularly predestined for rain, so that it is usually neither predictable nor explainable in retrospect why this single cloud is raining down here and now, but not a neighboring cloud. Typical weather conditions that promote the occurrence of downpours are the backs of slowly moving cold fronts, which cause remnants of cloud to rain down from the warm front. Likewise, strong vertical movements of the air can lead to rain falling from clouds that are stable in themselves. Downpours can be very heavy (heavy rain) and are then difficult to separate from the shower . In contrast to the different types of shower, the downpour cannot be recognized early on from its cloud development. Depending on regional custom, there is no colloquial distinction between showers and downpours . In popular scientific weather forecasts, downpours are often announced with expressions such as “clear to cloudy with possible local shower activity” or “local shower tendency”. For weather forecasts , there is a symbol of showers and one for rain.

#### drizzle

Drizzle or drizzle is defined according to its shape. It occurs in the Antarctic , the tropics, subtropics and temperate latitudes and can last hours to days, depending on the main event. Drizzle is made up of small droplets that usually fall from stratus clouds . The droplets have a diameter that is less than 0.5 millimeters. The view is often restricted when it is drizzling. Drizzle showers only occur over the lake, fall from stratocumulus clouds and are also known as miniature showers. With a precipitation intensity of up to 0.2 millimeters per hour one speaks of a light , with 0.2 to 0.5 millimeters per hour of a moderate and more than 0.5 millimeters per hour of a heavy spray. In Austria , especially in the Salzkammergut , a drizzle that usually lasts longer because of the north-facing location is also known as strangling rain .

#### Hypothermic rain (clear ice)

Ice crust around the branches of a bush after freezing rain

Supercooled rain (generally freezing rain and black ice , like the following) consists of supercooled raindrops, which are much colder than 0 ° C, fall in a liquid state and which freeze immediately upon impact ( clear or black ice in the narrower sense, glaze ). It is defined according to its shape and effect on the ground and is most likely to occur in the subpolar regions, in winter also in the moderate latitudes.

Supercooled droplets arise when clean raindrops fall through cold and very pure air layers, where they cool down to well below 0 ° C, but remain liquid due to the lack of crystallization nuclei . If such a supercooled raindrop hits a solid obstacle, it uses this as a crystallization nucleus and freezes suddenly, so that irrigated objects are covered by a compact sheet of ice up to several centimeters thick after a short time.

On roadways, freezing rain, like freezing rain, leads to dangerous slippery roads , on which even cars with winter tires can barely stop. Overhead line pylons occasionally break after freezing rain if they are no longer able to withstand the additional weight of the ice armor on the conductor cables.

If some of the supercooled raindrops meet crystal nuclei (for example grains of dust) in the air, the freezing rain is interspersed with grains of ice.

In contrast to ice or freezing rain, there is already frozen precipitation such as hail , sleet and grizzly or snow . These arise already in the clouds and fall to the ground as solid precipitation.

#### Freezing rain (rough ice)

Freezing rain (general freezing rain and black ice , as the foregoing) takes its name from the action on the ground, it is created when hitting the ground then sudden rime ice .

Such events primarily occur in temperate latitudes and subpolar regions and can last from a few minutes to a few hours. In the tropics and subtropics, freezing rain can only occur in the mountains. Freezing rain has a temperature of over 0 ° C, so it is not supercooled and only freezes after hitting a much colder surface. This often forms a soil that is not isolated by a blanket of snow , which has cooled down over a longer period of time in severe frosts down to the deeper layers. “Freezing rain” and “freezing rain” are often mistaken for the same meaning. In the moderate latitudes, massive black ice on roadways is often caused by freezing rain (but far more often by freezing humidity or fog ). Freezing rain is also dangerous for aircraft, as the layer of ice makes the aircraft heavier (and thereby increases the consumption of the fuel that was calculated before take-off and the landing weight ) and changes the wing profile , which reduces the lift of the wings (see aircraft de-icing ).

#### Warm rain

Warm rain is defined according to its effect on the ground (temperature felt by the observer). It occurs when low-lying, warm and humid air masses only have to be raised slightly in order to reach their saturation and hardly cool down in the process. This phenomenon is most likely to be observed in the tropics and subtropics, in the summer months occasionally also in temperate latitudes. In moderate latitudes, warm rain usually occurs when it rains from the front or when it rains uphill. In the tropics, on the other hand, it can also form independently of the fact that warm, ground-level and humid air masses are raised again by low local currents (or convection).

A warmer climate should therefore result in more warm rain, which favors more extreme weather events. According to a study, warm rain currently accounts for around 31 percent of total global precipitation, and 72 percent in the tropics.

### Local forms

Local forms are rain events or rain forms that are tied to very specific areas of the earth.

#### Tropical rain

In general, tropical rain is understood to be long-lasting warm rain with moderate intensity that occurs in the tropics or subtropics. It can be formed by different processes, but the main causes are considered to be zenith rains in the ITC and incline or convection processes in the secondary ITC , in some cases also extensions of tropical cyclones . Alexander von Humboldt described tropical rain as convection rain that only occurs within the tropics. According to its definition and the general definition, tropical rainforests are located in the area of ​​tropical rain. In the literature, however, warm rain is sometimes equated with tropical rain.

#### Monsoon rains

Monsoon rain is caused by the monsoon and occurs mainly in the area of ​​the Indian Ocean (India, Bangladesh , Eastern Australia , Eastern Africa , the Dhofar on the Arabian Peninsula ). The term tropical rain is often used for monsoon-like rain. According to the definition, monsoon rain is a long-term event that, according to its form of formation, is most likely to be assigned to damming rain. Monsoon rains fall over a period of several weeks. Several separate and intense rain events lasting a few hours are typical during the day. However, it can occur as light continuous rain (see regional monsoon phenomena ).

## composition

The main component of rain is water in liquid form. The water can have a temperature between −40  ° C (supercooled but not frozen) and over 20 ° C. In addition, the rain can contain other chemical elements and compounds depending on where it originates. The enrichment of the rain with additional substances cleans the air, but can cause the rainwater to be contaminated with undesirable substances.

The substances contained in rain can be of natural origin as well as anthropogenic , i.e. caused by humans.

When the spray is blown up , Na + , Cl - , Mg 2+ and K + enter the atmosphere as sea salt aerosol. In rainwater, the concentrations of these ions decrease inland. In contrast, Ca 2+ , NH 4 + , HCO 3 - and NO 3 - in precipitation come mainly from the dust that is blown over land surfaces. Due to the dissolved carbon dioxide , unpolluted rainwater has a pH value of 5.6. The traces of oxygen, nitrogen, ozone, pollen and some organic compounds contained in rainwater are primarily of natural origin . B. formic acid .

Humans also release further emissions into the atmosphere, such as dust, smoke and combustion gases from industry, traffic and domestic fires . They can influence the composition of rainwater directly or in the form of their conversion products. Radioactive emissions in the form of particles and gas can also be carried with the wind and washed out of the air and knocked down days later with rain, e.g. B. after the 1986 Chernobyl nuclear disaster in parts of Europe.

### acid rain

The predominantly man-made emissions also contain substances that can form a new compound with water and make rain a slightly acidic solution. Sulfur oxides (SO 2 ) form sulfurous acid (H 2 SO 3 ) with water , nitrogen oxides (NO 2 ) form nitric acid (HNO 3 ). This phenomenon is known as acid rain ; about two thirds of it can be attributed to the pollution with sulphurous acid and one third to the content of nitric acid. In Central Europe, acid rain intensity has decreased since the early 1980s. At the measuring stations of the German Federal Environment Agency, the pH value of the rainwater collected rose again between 1982 and 2014 from 4.1–4.6 to 5.1–5.2.

### Alkaline rain

Alkaline rain is precipitation with a pH value higher than the pH value that is established in pure water due to the natural carbon dioxide content of the earth's atmosphere (pH = 5.6). Alkaline rain is limited in terms of location and time and is the counterpart to acid rain. The cause of alkaline rain is usually the emission of large amounts of alkali dust into the atmosphere. This can e.g. B. caused by:

## Rate of fall

The condensing water vapor initially forms very fine droplets, which become heavier with increasing size. Depending on its size, a raindrop has a different rate of sinking speed in air. In clouds there are zones with rising (updraft) or falling (downdraft) air currents. A raindrop only falls to the earth's surface when the speed of the rising air current is less than its rate of descent. Its speed of impact on the earth's surface depends on its rate of descent and on the air current in which it is located. There is a heavy downpour in a falling air current.

The Stokes' law can be used for small drops to 1 mm can be used with good approximation. The sinking speed of a drop with a diameter of 1 mm is approx. 6 m / s. Larger drops change their shape due to air resistance and are flattened, they fall in a turbulent manner. In this case the drag coefficient ( c w value) is speed dependent. It changes constantly during acceleration . The air resistance of the drops increases with the square of the falling speed until the weight and drag forces have become the same, then the raindrop falls at (almost) constant speed.

### Teardrop shape

Division of the drops from 5 mm in size

Drops up to about 1 mm in diameter keep their spherical shape (ball), but then they begin to deform more and more due to the air speed as they fall. This also increases the air resistance and the speed of fall remains almost constant. The droplet size is variable (A), the largest droplet photographed so far had a diameter of 9 mm, but as a rule droplets burst from 6 mm to smaller ones. The drop shape is initially spherical (B), with increasing size and the resulting falling speed it changes to a spherical shell-shaped ( parachute-like or hamburger- shaped ) body (C). This shape can be maintained (D) until the pressure (caused by the air resistance ) on the inside (that is the side facing the direction of fall) is so great that it overcomes the surface tension of the water (E). The maximum achievable size of a drop is therefore also dependent on the composition and temperature of the water that forms it.

### calculation

The balance of forces between weight and friction at a constant falling speed forms the basis for the calculation using the C w value, or using Stokes' law. For the sake of simplicity, no signs or vectors are used, the direction of fall is always towards the earth and the air resistance counteracts this. Additional influences such as air currents (buoyancy), temperature, surface tension of the drop (material properties) or changing shape of the drop are not taken into account here.

Balance of forces, approach for the following considerations:

${\ displaystyle {\ begin {matrix} {\ text {weight}} & = & {\ text {frictional force}} \\ F _ {\ mathrm {g}} & = & F _ {\ mathrm {r}} \\\ end {matrix}}}$
Striking raindrop
Fossil raindrop funnels in the sedimentary rock

The following sizes are used:

Formula symbol description SI unit Default values
${\ displaystyle F _ {\ mathrm {r}}}$ Frictional force N
${\ displaystyle F _ {\ mathrm {g}}}$ Weight force N
${\ displaystyle g}$ Acceleration due to gravity m / s² (9.81 m / s²)
${\ displaystyle c _ {\ mathrm {w}}}$ Drag coefficient of the drop (~ 0.35 to 1.3, depending on the speed)
${\ displaystyle A _ {\ mathrm {T}}}$ Circle area of the drop as resistance area
${\ displaystyle m _ {\ mathrm {T}}}$ Mass of the drop kg
${\ displaystyle \ rho _ {\ mathrm {L}}}$ Density of air kg / m³ (~ 1.3 kg / m³)
${\ displaystyle \ rho _ {\ mathrm {T}}}$ Density of the drop (water) kg / m³ (~ 990 kg / m³)
${\ displaystyle v _ {\ mathrm {T}}}$ Speed ​​of the drop m / s
${\ displaystyle r}$ Radius of the drop m (0.0001 to 0.003 m)
${\ displaystyle \ eta _ {\ mathrm {L}}}$ Viscosity of the air Pa · s (~ 17.1 µPas)

The falling speed of particles up to ~ 1 millimeter according to Stokes' law results from the following equation of forces:

${\ displaystyle 6 \ cdot \ pi \ cdot \ eta _ {\ mathrm {L}} \ cdot r \ cdot v _ {\ mathrm {T}} = (\ rho _ {\ mathrm {T}} - \ rho _ { \ mathrm {L}}) \ cdot g \ cdot {\ frac {4} {3}} \ cdot \ pi \ cdot r ^ {3}}$

If so , then it follows for the speed: ${\ displaystyle \ rho _ {\ mathrm {L}} \ ll \ rho _ {\ mathrm {T}}}$

${\ displaystyle v _ {\ mathrm {T}} = {\ frac {2} {9}} \ cdot {\ frac {\ rho _ {\ mathrm {T}} \ cdot g} {\ eta _ {\ mathrm { L}}}} \ cdot r ^ {2}}$

For and we get a speed of . ${\ displaystyle \ rho _ {\ mathrm {T}} = 1000 \, \ mathrm {kg / m ^ {3}}, \; g = 9 {,} 8 \, \ mathrm {m / s}, \; \ eta _ {\ mathrm {L}} = 17 \ cdot 10 ^ {- 6} \, \ mathrm {Pa \ cdot s}}$${\ displaystyle r = 10 ^ {- 3} \, \ mathrm {m}}$${\ displaystyle 128 \, \ mathrm {m / s}}$

This formula can also be used to calculate the rate of descent of dust particles in the air. These can reach high altitudes (up to 30 kilometers) in the atmosphere through strong winds (desert storms), volcanic eruptions, nuclear weapon tests or meteorite impacts. If you stay for a long time due to the slow sink rate and a large amount of particles, it can cool down considerably. In the case of major events (super volcano eruption, large meteorite impacts, nuclear war), one also speaks of the nuclear winter .

Example: According to the above formula, the sinking time of a dust particle with a size of one µm that was hurled at a height of 20 kilometers is 1.8 years. This agrees quite well with general observations.

The force equation must be adapted for the falling speed of particles between ~ 1 millimeter and 3 millimeters. Depending on the weight and the shape of the drop - which itself depends on the speed - the C w value varies between 0.35 (sphere) and 1.3 (parachute-like or open hemisphere), from:

${\ displaystyle m _ {\ mathrm {T}} \ cdot g = c _ {\ mathrm {w}} \ times A _ {\ mathrm {T}} \ cdot {\ frac {\ rho _ {\ mathrm {L}}} {2}} \ cdot {v} ^ {2} \ quad {\ text {with}} \ quad A _ {\ mathrm {T}} = \ pi \ cdot r ^ {2}}$ (cross-sectional area of ​​a hemisphere flowed around)

follows for the speed:

${\ displaystyle v = {\ sqrt {\ frac {8 \ cdot r \ cdot g \ cdot \ rho _ {\ mathrm {T}}} {3 \ cdot c _ {\ mathrm {w}} \ cdot \ rho _ { \ mathrm {L}}}}}}$

The following rule of thumb is recommended as a rough estimate: Falling speed in m / s ≈ 6 · droplet diameter in millimeters (only approximately correct in a range of 0.5 to max. 1.5 mm droplet size). A 1 mm drop falls at a speed of around 6  m / s ≈ 20  km / h .

## effect

Gravel path destroyed by heavy rain in the Großer Höllental
light continuous rain

Rain is the most common form of precipitation and helps close the water cycle , which is a critical factor for life on earth. In the long term, the rain-fed streams and rivers will erode entire mountains. With appropriate geological conditions, ravines and canyons can arise. Rain cleans the air and washes out dust, pollen and other particles. It also releases oxygen , nitrogen , carbonic acid , sulfuric acid and nitric acid from the air. The dissolved substances lead to increased erosion and weathering of rock and soil, as well as increased rain erosion in buildings, machines and systems (for example on aircraft wings). Rain also loosens minerals from rock and soil that serve as nutrients for plants and other life forms.

In the long term, excessive rain can lead to a change in the local climate ( microclimate and mesoclimate ), and thus also to a change in fauna and flora . This can also result in rinsing (denudation) or extensive erosion or watering of the soil. Short-term excessive rain can locally lead to torrents and flooding. On slopes and in the mountains, it can cause slope or landslides and debris avalanches.

The lack of rain leads to drought in the long term and thus to a change in the local climate, which can also cause changes in fauna and flora. This process promotes desertification . Due to the reduced rain erosion , however, structures, systems and machines may remain in place for longer; the Giza pyramids are an example of low erosion over millennia. Short-term lack of rain ( dehydration ) does not change the local climate and therefore does not pose a threat to fauna and flora.

## Cultural history

### Hydraulic companies

Karl August Wittfogel's thesis of the hydraulic society has long shaped the idea of ​​societies in which the distribution and regulation of water resources and rare rainfall was central. Central to these societies was a state cult (with a powerful civil service and priesthood) and centralized typical forms of rule of "hydraulic despotism".

He named the ancient Chinese empire to tame the Huang Hes, the high culture that appeared early in the Punjab on the Indus, the regulation of the Euphrates and Tigris in Mesopotamia (cf. Babylonian Empire), the Egyptian pharaohs on the middle and lower Nile and - with Smears - the Aztec Empire in Mexico (cf. Tenochtitlán) and Inca Empire in Peru before their destruction by Spanish imperialism. Technical knowledge in water management as well as in the field of astronomy (or astrology ) played a central role.

In ancient oriental regions and epochs, thunderstorms and storms were perceived as numinous violence, with important differences in the respective mythology. In Babylonia, which is characterized by irrigation agriculture , the weather god played less of a role as a source of rain, but more as the master of the storms. In the regions of the ancient Orient that were more strongly influenced by rain -fed agriculture, i.e. in Upper Mesopotamia, Syria, Anatolia and also in Assyria, he occupied a more important position among the great deities than in Babylonia.

In China , the rain was a symbol of fertility and procreation . According to ancient mythological ideas, the dragon created it with the help of balls. At that time in China, the play of clouds and rain was also understood to mean the union of men and women .

### Rainmaker and hail protection

Rainmaker, cactus tube, approx. 75 cm long, 5 cm diameter

The beginning of modern meteorology is dated to the construction of the first thermometer by Galileo Galilei around 1600. Previously, one tried to help the hoped-for precipitation through magical practices; an example of this are the rain dances of various African and indigenous peoples . The rainmaker is a musical instrument derived from such practices in Chile. Jokingly described since the last century is technical rainmaking through with hail airmen spread silver iodide also called. At the 2008 Summer Olympics in Beijing, silver iodide was rocketed into rain clouds to keep them from disrupting the opening ceremony. In Germany, the rain production in the district of Rosenheim and in Austria in southern, western and eastern Styria is regularly used to ward off hail. In Thailand, the production of fon luang ( Thai : ฝน หลวง , royal rain ) , which goes back to an initiative of King Bhumibol, plays a central role in relation to the local monarchy.

### Folklore aspects

In Germany, Münster is known for its often rainy weather. Although the annual mean rainfall is not out of line, the proverbial rule is “It's raining in Münster or the bells are ringing, and if both is, it's Sunday”. In addition, with meimeln in the local dialect Masematte denotes a volatile easier continuous rain. In the Lower Bavarian rain , the local rival Zwiesel is often quoted as follows: "In Zwiesel konns reign, aba in Reign konns nit zwieseln." Proverbially, "Rain follows sunshine" is used nationwide. In the Middle Ages the theory that held aphids in the summer rain, the so-called nephew rain heaped crops infested.

In Austria , Salzburg and the neighboring Salzkammergut in particular are known for their long sleet of rain .

In Catholic Christianity in particular, St. George is one of the fourteen helpers in need and is responsible, among other things, for good weather, the tradition of riding Georgi goes back to this among other things. The rainbow plays a central role as a symbol of hope and a special natural phenomenon in many cultures , in Christianity as the central promise of God not to repeat the flood and to renew the covenant with the people.

## literature

• Cynthia Barnett: Rain. A natural and cultural history . Crown Publications, New York 2015, ISBN 978-0-8041-3709-6 .
• Wolfgang Kühr: The private pilot, part 2: Basics of aviation meteorology. Luftfahrtverlag Schiffmann, Bergisch Gladbach 1991, ISBN 3-921270-08-1 .
• Gösta H. Liljequist, Konrad Cehak: General Meteorology. Vieweg, Braunschweig, 1984 (3rd revised edition), ISBN 3-528-23555-1 .
• Klaus Lüders, Gebhard von Oppen: Textbook of Experimental Physics, Vol. 1: Mechanics, Relativity, Warmth. Walter de Gruyter, Berlin 2008 (12th revised edition), ISBN 3-11-016837-5 .
• Wilhelm Raith (ed.): Textbook of Experimental Physics, Vol. 7: Earth and Planets. Walter de Gruyter, Berlin 2001 (2nd updated edition), ISBN 3-11-016837-5 .
• Dieter Walch: This is how the weather works. blv, Munich 2000 (2nd edition), ISBN 3-405-15945-8 .
• Berthold Wiedersich (Ed.): Pocket Atlas Weather. Klett-Perthes, Gotha 2003, ISBN 3-623-00021-3 .

Wiktionary: Regen  - explanations of meanings, word origins, synonyms, translations
Wiktionary: convection rain  - explanations of meanings, word origins, synonyms, translations
Commons : Rain  - Collection of Images
Wikiquote: Rain  Quotes

## Individual evidence

1. ^ The dictionary of origin (=  Der Duden in twelve volumes . Volume 7 ). 5th edition. Dudenverlag, Berlin 2014 ( p. 685 ). See also DWDS ( "Regen" ) and Friedrich Kluge : Etymological Dictionary of the German Language . 7th edition. Trübner, Strasbourg 1910 ( p. 369 ).
2. ^ Bergmann Schaefer: Textbook of Experimental Physics: Earth and Planets , Vol. 7, 2nd edition, Walter de Gruyter, 2001, page 191.
3. a b Wolfram Mauser: Internet lecture: Introduction to hydrology - precipitation. ( Memento from October 28, 2011 in the Internet Archive ) Ludwig Maximilians University Munich , November 10, 2008.
4. ^ Bergmann Schaefer: Textbook of Experimental Physics: Earth and Planets . Vol. 7. 2nd edition. Walter de Gruyter, 2001. page 192.
5. ^ A b c R. C. Ward, M. Robinson: Principles of Hydrology , 3rd Edition, McGraw-Hill Book Company, London 1989, ISBN 0-07-707204-9 .
6. Paul Koppe, Alfred Stozek: Municipal wastewater . 4th edition. Vulkan-Verlag GmbH, Essen 1999.
7. Monjo, R. (2016): Measure of rainfall time structure using the dimensionless n-index . Climate Research, 67: 71-86, DOI: 10.3354 / cr01359 .
8. a b Joachim Blüthgen, Wolfgang Weischet: General climate geography . 3. Edition. Walter de Gruyter, 1980.
9. Reinhard Joachim Süring, Julius von Hann: Guide to Meteorology: according to Hann-Süring's textbook of meteorology . Tauchnitz, 1927.
10. scinexx: A little rain lesson - land rain. Springer-Verlag, Heidelberg, 2004.
11. Berlin (Germany, Federal Republic) (1994): DIN 4049-3, October 1994. Hydrology - Part 3: Terms for quantitative hydrology , Beuth Verlag GmbH.
12. ^ Ernst Heyer: Weather and Climate. (2nd edition) Potsdam 1971, p. 186
13. MeteoSchweiz: Heavy rain, ( Memento from June 25, 2009 in the Internet Archive ) MeteoSchweiz, Basel 2009.
14. WMO - World Meteorological Organization (1995): "Annual Report of the World Meteorological Organization 1994", ISBN 92-63-10824-2 .
15. See information from the severe weather center . The record precipitation led to heavy flooding in the entire city area and in neighboring Greven .
16. MZ: [1]  ( page no longer available , search in web archivesInfo: The link was automatically marked as defective. Please check the link according to the instructions and then remove this notice.   , Downpour in Munster
17. ^ WN: [2] , storms in the Münsterland, traffic chaos due to heavy rain
18. ^ Yong Zhu and Reginald E. Newell (1998) A proposed algorithm for moisture fluxes from atmospheric rivers . Monthly Weather Review, 126, 725-735.
19. Philip Bethge (February 4, 2013) Sea of ​​Mud ; Der Spiegel 6/2013
20. Effects of climate change on the damage situation in the German insurance industry ( Memento from August 21, 2017 in the Internet Archive )
21. ^ Daniel Lingenhöhl: Weather phenomenon: Why is it drizzling despite the freezing cold? In: Spektrum.de . November 8, 2019, accessed November 18, 2019 .
22. scinexx: A little rain lore - drizzle , Springer-Verlag, Heidelberg 2004.
23. MeteoSchweiz: Eisregen ( Memento of May 9, 2009 in the Internet Archive ) , MeteoSchweiz, Basel 2008 (link no longer available).
24. Freezing Rain - supercooled droplets freezing on impact and Cyclones and Fronts - the development of freezing rain , entries in University of Illinois: WW2010 .
25. German Weather Service: Eisregen ( Memento from December 8, 2014 in the Internet Archive ) , DWD Weather Lexicon (accessed on May 26, 2019).
26. Joachim Blüthgen, Wolfgang Weischet: General climate geography. Volume 2 by Wolfgang Weischet (Ed.): Textbook of general geography . Walter de Gruyter, 1980, ISBN 978-3-11-006561-9 , p. 283 ( limited preview in Google book search).
27. Frost and rain, a dangerous mix!
28. Weather dictionary: Freezing rain , Wolfgang Winkelbauer, Vienna 2009.
29. scinexx: More warm rain in a warmer world. NASA / Goddard Space Flight Center, 2004.
30. Springer: More Warm Rain in a Warmer World , NASA / Goddard Space Flight Center, 2004.
31. ^ Wilhelm Constantin Wittwer: Alexander von Humboldt , Weigel, 1860.
32. ^ German Weather Service: Tropen, Wetterlexikon, DWD 2009.
33. wetter.net: Weather dictionary: Monsunregen , Q.met GmbH, Wiesbaden 2009.
34. Weather dictionary: Monsunregen , Wolfgang Winkelbauer, Vienna 2009.
35. Federal Environment Agency: Recording of the wet deposition , June 22, 2016.
36. Ulrich Fölsche: Lecture: EF Meteorology WS 08/09. Karl Franzens University Graz , 2008, archived from the original on February 19, 2013 ; accessed on May 26, 2019 .
37. scinexx: From Molecule to Raindrop - Hamburger in Free Fall , Springer-Verlag, Heidelberg 2004.
38. Bergmann Schaefer: Textbook of Experimental Physics: Mechanics, Relativity, Warmth , Vol. 1, 11th Edition, Walter de Gruyter, 1998.
39. ^ Max Lippitsch: Lecture: History of Physics ( Memento from September 27, 2013 in the Internet Archive ) , Karl-Franzens-Universität Graz , 2007.
40. discovery.de History of Meteorology ( Memento from December 21, 2004 in the Internet Archive ), accessed on March 20, 2009.
41. a b Mara Schneider: The weather can only be controlled to a limited extent. (News article) news.de, February 19, 2009, archived from the original on April 29, 2016 ; accessed on May 26, 2019 .
42. It's raining in Münster ( memento of December 26, 2010 in the Internet Archive ) (PDF; 8.2 MB) It's raining in Münster - frequency distribution of precipitation in Münster in a nationwide comparison. Thesis for the teaching post of secondary level II, 2007, subject and supervisor Otto Klemm, presented by Frank Weritz.
 This version was added to the list of articles worth reading on March 9, 2009 .