Precipitation

If, as myth has it, the Inuit have 36 words for ‘snow’, then the English-language has a good few for the way moisture falls from the sky. The term Precipitation covers all forms of naturally falling H2O, whether frozen or liquid.

Rain

Water molecules need a core to condense around. Small dust particles in the atmosphere usually do the job. Rain drops get larger by accumulating more and more condensate as they move in all direction within a cloud. Once they are heavy enough to overcome rising thermal currents, they fall to the ground. Raindrops usually range in size from 0.5mm to 6mm in tropical conditions.

Drizzle

Finer rain, with individual droplets less than 0.5mm in size. Usually form under less turbulent conditions than Raindrops with fewer opportunities to amass, and weaker thermals to overcome. Drizzle usually falls from moisture-heavy low-level stratus clouds in Britain. Though small, the droplets are still larger than those than form clouds.

Mist

A less dense version of Fog, mist is encountered up mountains as you enter the cloud base, or arises from the formation of clouds at ground level. The individual droplets of water are too small to fall and are held in suspension by wafting air currents. Mist often occurs in Spring and late Autumn when long nights allow the ground to lose heat and become cold, chilling the immediate layer of air above and reducing its capacity to hold water in an uncondensed state. Dew point is reached and invisible water vapour condenses into visible water droplets to form mist or fog. It disperses either when drier air is blown in, or daytime ground-heating transfers radiation to the lower levels of air, allowing water droplets to evaporate back into water vapour.

Sleet

A falling mix of rain and snow at ground level. Much rainfall starts its journey as snow or hail, melting as it falls through successive layers of warmer air. Sleet is when the melting point is at ground level and some, though not all, the snow melts into rain.

Hail

Frozen raindrops can cause substantial damage. Hailstones form in cumulonimbus clouds which extend into high altitude areas where raindrops freeze. Inside the cloud the turbulence is so great that substantial accretions of raindrop can accumulate, rise, freeze, fall, gain more layers of liquid water, rise and freeze again in an enduring cycle. Because the updraught of air can be so strong, hailstones can grow to significant size, with layers of frozen water like tree rings, before they fall to the ground. Inside a thunder cloud the process of condensation of water vapour into liquid drops from moist water vapour releases latent heat that adds to the internal turbulence of rising warmer air, giving the cloud its characteristic ‘billowing’ appearance.

Snow

Ice crystals form high up in clouds, developing around a condensation nucleus of a grain of sand or dust. As they waft around they merge and coalesce into larger flakes. In lower-moisture conditions or extremely cold conditions they often remain small and give powdery, ‘dry’ snow. In more moist conditions and/or falling through air that is slightly above 0C they may melt around their edges and freeze to other crystals forming larger ‘snowflake’ shapes.

The conditions that give rise to rainfall (and associated precipitation) are where moist air is chilled. Invisible water vapour then undergoes a phase of condensation such that visible droplets of moisture are formed. There are three main causes to this:

Relief Rainfall

When moisture-laden winds (often having crossed a body of water) rise up over a range of mountains. The rising air chills, the water vapour condenses, and ‘relief’ or ‘orographic’ rain falls on the mountain summits.

Convectional Rainfall

When the driving force behind the chilling of moist air is convection currents. These are driven by intense heating of the ground. If there is a supply of water vapour (moist ground or woodland) this will be taken up by the rising thermals to higher altitudes where the air is cooler. Moist air condenses into water droplets as high altitudes are reached.

Frontal/cyclonic/depressional Rainfall

When two masses of air meet they tend to remain discrete (distinct) bodies of atmosphere. It is likely they will be of two different temperatures; the warmer will slide up over the cooler, denser body of air. The effective ‘lifting off the surface’ of the warmer, lighter, body will cause any water vapour within it to be raised to higher, cooler altitudes where it will condense into water droplets. This is what happens at a warm front and cold front in a typical depression, or area of low pressure. (The low pressure occurs because the warm sector of air is rising up, resulting in less downward pressure of air at the surface).