What is known as heavy rainfall

rain

Rain is the most common form of liquid precipitation. It helps close the water cycle, which is a crucial factor for life on earth. In contrast to frozen precipitation such as hail, sleet or snow, rain consists of liquid water that falls from clouds to the earth as a result of gravity. The type of rain is differentiated according to its origin, duration, intensity, effect and geographical occurrence. The starting point of every rain are clouds, which, depending on altitude and temperature, consist of either 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 sublimation (resublimation) or cloud droplets with the help of condensation nuclei through condensation. Depending on the length of time they have stayed in the cloud, these can bind to further water vapor, other cloud droplets or even ice crystals and thereby grow. If ice crystals come into a warmer environment, they melt back into drops. If the weight of the drops is so great that they can no longer be kept "in suspension" by the air currents (updrafts) that prevail in a cloud, they begin to sink to the ground due to gravity and the rain we know occurs. The growing together of many small water droplets into larger and heavier ones accelerates this process and also increases the speed of the raindrops falling. As a rule, the rain hitting the ground consists of drops with a diameter of 0.6 to 3 mm.

More information on this process can be found in the Precipitation chapter, drop formation section.

 

Raindrops

Rain is as diverse as the processes that lead to its creation. Land rain, drizzle, downpour, rain shower or downpour: rain is not just rain, as everyone has experienced firsthand. Once it falls from the sky in slow motion, the other time the drops patter like small water bombs on the ground.

The type of rain is mainly determined by the size of the drops and the speed at which they fall. The speed of fall does not result from the height from which the rain falls, but from the size and thus the weight of the droplets: the smaller, the slower they fall to the ground because they are very effectively slowed down by the resistance of the air. In "normal" rain, the drops reach a speed of 5 m / s and a size of approx. 2 mm. But raindrops cannot grow to be arbitrarily large. Their growth limit is physically approx. 5 mm, since the air resistance when falling atomizes all larger drops into smaller ones into several smaller drops. Apart from the surface tension of the water, this is the reason why the water from a fountain or a lawn sprinkler never reaches the ground as a jet, but instead breaks up into drops beforehand.

Drops are never tear-shaped in nature, as some might expect. Rather, they keep their spherical shape (ball) up to about 1 mm in diameter, but then they begin to flatten out more and more on their underside. As a result, the shape of the drop changes from an initial spherical shape to a hemispherical shape with increasing size and thus greater falling speed. As a result, the air resistance continues to increase, while the falling speed remains almost constant. This hemispherical shape can be retained until the pressure caused by the air resistance on the underside (the side facing the direction of fall) is so great that it overcomes the surface tension of the water. The largest drop had a diameter of 9 mm, but drops usually burst from 5 mm.

  1. In reality, raindrops do not have this teardrop shape.
  2. Very small water droplets are approximately spherical.
  3. Larger raindrops are flattened on the underside due to air resistance.
  4. Large raindrops experience so much drag that they become unstable.
  5. Very large raindrops are split up into several smaller drops by air resistance.

 

 

Nimbostratus virga

Virga with remnants of cloud

Drizzle or drizzle

Drizzle or drizzle is a uniform precipitation that falls from a cloud and consists of the smallest, fine, densely distributed water droplets with a diameter between 0.1 and 0.5 mm. It usually falls from deep and often relatively thin stratus clouds, as the distance in the cloud for coalescence is insufficient to form larger raindrops and can take hours to days. Drizzle also occurs in thick, weeping fog. The view is often limited or poor.

Drizzle is characteristic of moist warm air masses, which flow in from warmer to colder areas and are gradually cooled through contact with the colder surface. But drizzle does not come from ice crystals. This is because these are absent in drizzling weather conditions, both in the stratus clouds and in the fog. This is because the temperature is above 0 ° C or the lack of suitable freezer cores. In such sliding air masses, the relative humidity of the air is high, which is why the droplets do not evaporate on their way to the ground. This can certainly happen with drops of the same size falling from an ordinary rain cloud. You either only see falling strips (Virga) or they evaporate so strongly during their fall that they finally arrive at the bottom with the same drop radius as a drizzle drop. The spray drops have a low falling speed of 0.25 - 2 m / s.

The intensity of the drizzle is generally low and consequently the amount of precipitation is negligible. With a precipitation intensity of up to 0.2 mm per hour one speaks of a light, with 0.2-0.5 mm per hour of a moderate and more than 0.5 mm per hour of a heavy drizzle.

On the other hand, rain falls over a large area from the nimbostratus clouds of a warm front, which in German is often referred to as "land rain". If the temperatures are already below zero, in the cases described, snow falls instead of rain, as the coalescence of small ice crystals instead of water droplets takes place in the cloud. The formation of hail, which can occur in connection with cumulonimbus clouds during thunderstorms, is particularly dangerous for aviation.

Rain and rain showers

rain shower
Precipitation in 10 minutes

rain
Precipitation in 1 hour

definition

quantity

definition

quantity

light

0.1-0.4 mm

light

0.1-0.5 mm

moderate

0.4 - 2 mm

moderate

0.5 - 4 mm

strong

2 - 8 mm

strong

4 - 10 mm

very strong

from 8 mm

very strong

over 10 mm

Raindrops are usually 0.5-5 mm in diameter. Drops of various sizes therefore fall in ordinary rain. Their speed of fall is between 2 and 9 m / s. Since the humidity below the cloud base is generally below 100%, the droplets partially evaporate on their way to the earth's surface. At the beginning of a rain or rain shower, this evaporation can be so great that the rain does not reach the ground, but can only be seen as a multitude of dark falling strips (virga) under the cloud (see pictures above left). In the case of weaker rain or in the early stages of rain, the droplets that reach the surface of the earth can therefore be as small as drizzle droplets.

In our middle latitudes, the formation of precipitation by ice crystals is the most important process for rain. The formation of precipitation by ice cores requires that both ice crystals and cloud droplets are present in the cloud. The saturation vapor pressure is namely lower over ice than over water. The water vapor condenses earlier on ice, while pure water droplets evaporate. The ice crystals grow at the expense of the water droplets. These become larger and larger due to entanglement and contact with simultaneous freezing until their weight can no longer be borne by the updrafts that prevail in a cloud. They begin to fall, melt at lower altitudes with temperatures above 0 ° C and finally reach the ground as ordinary raindrops.

The cloud must therefore reach well beyond the 0 ° C limit and therefore - at least in summer - have a great vertical thickness. Such powerful clouds can either be formed by deep convection in combination with a moisture-unstable stratification and warming from below or by slowly lifting a correspondingly thick layer of air. In the first case, clouds of the cumulonimbus type are formed, in the latter cloud systems from Nimbostratus and associated foothills from Altostratus, Cirrostratus and Cirrus. The slow general lifting of an air mass usually takes place over a front surface, with the warmer air flowing upwards over the cold air wedge or air flowing up an orographic obstacle, such as a large mountain range. Such an elevation of an entire layer of air causes the air mass to become unstable and powerful layer clouds are formed.

Here in Central Europe, nimbostratus and cumulonimbus are the typical rain clouds. These two types of clouds have a sufficient vertical thickness and therefore extend in height above the -10 to -15 ° C level, so that ice crystals can form.

A rain shower as rain from cumulonimbus clouds (Cb) is generally only of short duration. It takes barely more than 45 minutes and is enough to soak everything with its up to 5 mm large droplets. The precipitation intensity can change within seconds and the precipitation field is less than 10 km2 regionally very limited, as the cloud has only a small horizontal extent. The typical rain shower. It is caused by strong vertical air movements (convection) and is therefore also known as convective precipitation. Due to the strong updrafts, large droplets can form in this cloud. The shower, also known as a downpour, almost always falls from the towering cumulonimbus clouds. These imposing "cumulus clouds" often have the shape of an anvil and have the fibrous structure typical of ice clouds. On the upper floor they consist of ice crystals, while on the lower floor they consist of water droplets.

The "ordinary" rain falls from Nimbostratus clouds (Ns). There the updrafts are rather moderate, which is why the drop size and the intensity of the rain are rather moderate. The duration is much longer because of the large horizontal extent of the Nimbostratus clouds and the area affected by the rain is correspondingly larger. The result for us is long-lasting and mostly evenly falling rain. Viewed from the ground, these typical dark rain clouds appear as a compact layer of clouds with an unclearly defined underside. These extended mixed-phase clouds of ice and supercooled water reach up to the topmost cloud store at a height of 10 km. This gives the cloud droplets enough time on their way to the ground to form medium-sized droplets between 0.5 and 2 mm, which fall at a maximum speed of 5 m / s.

Continuous rain or rain is a rainfall in Central Europe, which lasts at least 6 hours uninterrupted and brings more than 0.5 mm of precipitation per hour. In the moderate latitudes it usually occurs in connection with a warm front passage.

As Heavy rain are large amounts of rain that fall in a short time. Heavy rain is thus defined according to its intensity and duration. In the German-speaking area, heavy rain is spoken of in excess of 5 mm in 5 minutes, more than 10 mm in ten minutes or more than 17 mm per hour. However, a heavy rain event can also be much more severe. Cloudbursts during thunderstorms, in which 30 mm fall in 30 minutes, are relatively rare in Central Europe, but under certain circumstances can already lead to flooded cellars. The stronger and longer lasting these events, the less likely they are to occur. In Central Europe, short but heavy rainfall is more likely than prolonged heavy rainfall, which can bring up to 200 mm of rainfall in a few days. Long-lasting heavy rain falls in Europe especially in Vb weather conditions (pronounced "5 b").

 

 

Downpour refers to rain that only rains over a small area, often less than a square kilometer, and usually only lasts a few minutes. As a rule, it is neither foreseeable nor can it be explained in retrospect why this single cloud is raining down here and now, but not a neighboring cloud. Typical weather conditions that favor 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 the rain falling from clouds that are stable in themselves. Downpours can be very heavy (heavy rain) and are then difficult to distinguish from the shower. In contrast to the various types of shower, the downpour is not recognizable early on from its cloud development. Colloquially, there is usually no distinction between showers and downpours. In the weather forecasts on radio and TV, torrential rain is often announced with formulations such as "bright to cloudy with individual showers" or as "local shower tendency".

 

Freezing rain

Freezing rain gets its name from its effect on the ground. A neologism for this phenomenon has been the so-called "Black ice“, Which has since been used across the board for every form of freezing moisture. Freezing rain occurs in the temperate latitudes and subpolar regions and can last a few minutes to a few hours. It has a temperature of over 0 ° C, so it is not undercooled and only freezes after it hits a much colder surface. This also distinguishes freezing rain from black ice, because with black ice the ice only forms in the course of time due to the water present - for example due to melted snow in the course of a day due to the warming of the day. Freezing rain, on the other hand, freezes immediately and forms a thin and particularly insidious layer of ice. Motorists know this because the freezing rain forms a thin layer of ice on the vehicle, so that you have to repeatedly scrape the vehicle's windows free.

Freezing rain is also dangerous for aircraft, as the layer of ice obstructs the view, makes the aircraft heavier and changes the wing profile, which significantly reduces lift. This ice build-up can occur if the temperature of the outer skin of the aircraft is below 0 ° C after a flight in cold air and is then flown into a rainy area.

The dangers of aircraft icing are discussed in more detail in the chapter on icing.

 

 

Hypothermic rain (freezing rain)

Supercooled rain or freezing rain is a liquid precipitation consisting of supercooled raindrops that freeze immediately upon impact. This distinguishes it from freezing rain, the drops of which are warmer than 0 ° C. "Freezing rain" and "freezing rain" are often wrongly used synonymously. Supercooled rain occurs when raindrops fall through cold layers of air, where they cool down to well below 0 ° C, but remain liquid due to a lack of crystallization nuclei. Such a situation occurs often, for example when, after a long winter high pressure situation, the layer of air close to the ground has cooled down far below 0 ° C due to nocturnal radiation and then a relatively mild Atlantic bad weather zone ("masked" cold front ") with positive temperatures in higher layers (and thus rain) over the surface near the ground Cold air slides on. This weather situation, which is particularly dangerous for aviation, is usually ended when the subsequent mild and mostly strong westerly wind clears out the cold air near the ground. If such a supercooled raindrop hits a solid obstacle, it uses this as a crystallization nucleus and freezes suddenly, so that rainy objects are covered by a compact layer 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.

Ice or freezing rain must be differentiated from 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.

The dangers of aircraft icing are discussed in more detail in the chapter on icing.

Interesting publication by AOPA Germany on the subject of aircraft icing.

 

Weather map symbols

Rain weather map symbols

symbol

number

description

21

Rain (not freezing and not as a shower) stopped in the last hour

24

Freezing rain or drizzle stopped in the last hour

25

The rain shower stopped in the last hour

58

light rain mixed with drizzle

59

moderate or heavy rain mixed with drizzle

60

light, non-freezing rain with interruptions

61

light, non-freezing rain without interruptions

62

moderate, non-freezing rain intermittently

63

moderate, non-freezing rain without interruptions

64

heavy, non-freezing rain intermittently

65

heavy, non-freezing rain without interruptions

66

light freezing rain

67

moderate or heavy freezing rain

80

light rain shower

81

moderate or heavy rain showers

82

very heavy rain shower

91

Thunderstorms during the last hour, but now only light rain at the time of observation

92

Thunderstorms during the last hour, but now only moderate or heavy rain at the time of observation

 

Weather map symbols rain and snow

symbol

number

description

23

Sleet or ice grains (not as a shower) stopped in the last hour

26

Snow or snow showers stopped in the last hour

68

light rain or drizzle mixed with snow

69

moderate or heavy rain or drizzle mixed with snow

83

light snow shower

84

moderate or heavy snow showers