Weather Reference - Precipitation

Location: London (Default) Lat: 51.5N Sunrise: 06:24GMT   Lon: 0E Sunset: 17:56GMT

Rain | Hail | Sleet | Snow | Disclaimer

Rain

Rain is precipitation that reaches the ground in liquid form. The rain is formed in a cloud as either water droplets or ice crystals. Under the influence of gravity, one the ice crystals have grown big enough, they fall from the cloud, the ice crystals melt on the way down.

If a dark fringe hanging from the base of the cloud can be seen it is usually a sign that ice crystals or water droplets have evaporated mid air- this is known as virga, and occurs when there is a deep layer of dry air, or a shallow layer of extremely dry air beneath the cloud. Virga can not be classified as precipitation as it does not reach the ground, but the evaporation that produces it increases the water vapour content in the layer of dry air making it likely that subsequent falls will reach the ground.

Rain is normally classified by the cloud that it comes from and as a general rule, precipitation from a stratiform cloud, particularly stratus and altostratus clouds will be rain as the clouds cover a wide area so the precipitation will be widespread and long lasting. Cululiform clouds normally produce showers as the fall tends to be localized and may only last a minute or two.

Flooding is caused by both rain from stratiform clouds and showers from Cululiform clouds. Persistent rain may lead to flooding over an extensive area , while heavy showers are more likely to cause flash floods.

Hail

Hail are frozen lumps of ice produced by thunder storms and are made when super cooled water droplets are circulated with the updraft area of a cumulonimbus cloud. As the droplets pass through areas of differing temperatures and humidity, different types of ice build up on them. When temperatures are just below freezing and there are many super cooled water droplets, layers of clear ice form. In colder parts of the cloud, where there are fewer and smaller water droplets, freezing so rapidly that bubbles of air are trapped forming opaque rime ice. This layering effect is enhanced as the hailstones alternately melts and freezes as it encounters warmer and colder air.

Most hailstones are the size of a pea however they can grow as large as a baseball.

Signs of hail in an approaching storm include a greenish tinge at the base of the cloud, or a whitening of any rainfall.

Sleet

When the temperature at cloud level is below zero, any water droplets that fall from clouds will be super cooled. This means that they are likely to freeze as soon as they encounter a colder layer of air or a surface that is below zero degrees.

Snow

Snow begins as ice crystals that from a cloud when water vapour freezes around minute particles in the middle and upper levels of the atmosphere, where the temperature is below zero degrees. The individual ice crystals gradually bond, forming snow flakes. Once the flakes are heavy enough they fall to the ground.

Snow that falls from a cloud often melts as it descends and reaches the ground as rain. However the melting process extracts latent heat from the surrounding air, causing the air temperature to cool and making it increasingly likely that subsequent snow will reach the ground.

The ideal conditions for snow are temperatures close to or just below zero rather than colder temperatures. This is because the warmer the snow, the more moisture it will contain and so the flakes will be bigger, and because a temperature close to zero degrees will allow the snow to melt, refreeze and combine into larger flakes. As a result, very slight changes in temperature can mean the difference between snow or rainmaking forecasting difficult.

Disclaimer

All information is presented in good faith. We cannot accept any liability for any incorrect or incomplete information. You are strongly advised to seek clarification on any information presented. If you find any errors or omissions, please contact us to let us know so that we can put it right.