Precipitation Forming Processes
Before we study precipitation,we are going to look at the microscopic processes of cloud formation breifly.
If pure water is cooled below freezing it can become supercooled and not form ice because it has and energy barrier it has to over come before this can happen. For water droplets or ice crystals to form there must be small nuclei for the water to condense (or deposit) onto. These are called condensation nuclei. The condensation nuclei found in our atmosphere are dust, salt particles etc. See atmospheric impurities
The are two theories of how water vapour in the air forms into water droplets or sublimes into ice crystals big enough to overcome the up draughts in the coulds and fall as precipitation.
Although the definiton of precipitation includes sleet, hail, hoar frost, fog and rime only rain and snow provide significant totals in the hydrological cycle.
There are three main types of rainfall:
Convergent and Cyclonic Rainfall :
This from of rain is caused by the convergence of two air masses.
In the tropics, the trade winds, blowing towards the equator, meet at the inter-tropical convergence zone (ITCZ). The air is forced to rise and along with strong convection currents, forms the heavy afternoon thunderstorms asociated with the equatorial climate.
In the temperal latitudes, depressions form. Warm moist air is forced to rise over the cold air which mostly causes periods of prolonged rainfall.
When warm moist air is forced to rise, for example by a mountain barrier it is forced to cool and so clouds can be formed. Mountains reduce the water carrying capacity of the rising air by enforced cooling and can increase amounts of cyclonic rainfall by retarding the speed of depression movement.
Mountains also tend to cause air streams to converge and funnel through valleys. Rainfall totals increase where mountains are parallel to the coast because air travelling over warm ocean currents has no choice but to rise as it comes onto the land forcing clouds to form and often precipitation. (For example, air arriving in Britain from the Atlantic Ocean has travelled over the Gulf stream and is forced up over the mountains causing the West of Engalnd to have a high rainfall.
As air descends on the leeward side of the mountain range, it becomes compressed and warmed and condensation ceases, creating a rain shadow.
A rain shadow is an area of reduced rainfall on the far side of the mountains, for example Eastern England.
For further phenomena see the orographic cloud page.
When the ground surface is locally heated and the adjacent air, heated by conduction expands and rise, convectional rainfall occurs. This heating occurs daily in equatorial areas and in S.E. Engalnd in summer. During its ascent, the air mass remains warmer than the surrounding air and therefore it is likely to become unstable and large cumulo-nimbus clouds form. These unstable conditions, possibly helped by frontal or orographic uplift force the air to rise in a strong vertical updraught or 'chimney'. The updraught is maintained by the energy released through latent heat as the water vapour condenses then freezes. The top of the cloud is characterised by ice-crystas in an anvil shape. The top of the cloud is flattened by reaching the temperature at the tropopause. When the ice crystals and frozen water droplets(hail) become large enough they fall in a downdraught. The air trough which they fall remains cool as heat is absorbed by evaporation. The downdraught reduces the warm air supply to the 'chimney' and therefore limits the lifetime of the storm. Such storms are usually accompanied by thunder and lightning. One of the theories for lightning suggests that the ice crystals in the upper cloud create a positive charge while the ground has a negative charge. Lightning is what we see of the discharge of electricity between the clouds and the ground or vice-versa.
Convection is one of the processes by which surplus heat and energy from the Earth's surface are transferred vertically to the atmosphere, maintaining the Earth's heat balance.
Snow forms under the same conditions as rain except that the dew point temperatures are below freezing so the vapour condenses straight to a solid (sublimation). Ice crystals will form if there are small particles present for them to form onto. These may aggregate to form snowflakes. As warm air holds more moisture than cold air snowfalls are heaviest when the air temperature is just below feezing. As the temperatures drop further still it becomes too cold for snow. Snow is usually found with arctic maritime air or polar continental air, see British weather summary.
Sleet is a mixture of ice and snow formed when the upper air temperature is below freezing, allowing snowflakes to form and the lower air temperature is just above freezing (2 - 4 C) which allows them to partially melt.
This is the opposite of sleet and ocurs when water droplets form in the upper air which then fall and turn into ice when they come into contact with the ground, forming a layer of ice. This is known as black ice, on roads.
Hail is frozen rain drops which are bigger than 5 mm in diameter. Very large hail stones can form if they stay in the clouds for a long time. When they fall they collide with water droplets which freeze and make another layer, they can circulate up and down in the clouds for a long time until they are large enough and heavy enough to form a down-draught. Hail usually forms in cumulo-nimbus clouds, resulting from the uplift of air by convection currents or at a cold front. It is more common in areas with warm summers where there is enough heat to trigger off the uplift and is less common in colder climates.
Dew, Hoar Frost, Fog and Rime
Dew, hoar frost andradiation fog all from under calm, clear anticyclonic
conditions when the Earth is losing lots of heat at night. The ground cools
the air and this causes the moisture present to condense. If the dew point
is above freezing then dew will form and if it is below freezing then a
frost will be found. Dew and hoar frost usually come within 1m of the ground.
If the air is relatively warm and moist and the ground cools quickly then a radiation fog can form. It is call a mist if visibility is more than 1 km or a fog, visibility less than 1 km. For the fog to develop a gentle breeze is needed to stir the air and allow the cooling to affect more of the air. These fogs usually occur in valleys and are thickest around surise. The water droplets formed in them are small enough to remain in the air. The fog is likely to thicken if a temperature inversion has taken placed and the air is trapped by overlying warmer air.
Advection fog forms when warm air passes over or meets cold air, giving rapid cooling.
Rime occurs when supercooled drops of water, found in fog, come in contact and freeze on to objects.
For classification of precipitation see weather symbols.