How do clouds and rain form?

Water vapour saturation
The air in our atmosphere always contains some water vapour, whose density can only climb to a certain saturation value at any given temperature. The maximum possible water vapour content of the air however increases out of proportion to any increase in temperature. Equally, it is always possible to cool an air parcel, reducing the water vapour saturation value, to a temperature at which the water in it starts to condense. The temperature at which this starts to happen is known as the dew point. Further cooling results in the excess water vapour being deposited onto fixed surfaces (dew formation) or onto condensation nuclei, leading to the formation of droplets. Tiny particles of various origin, the so-called aerosols, are usually present in abundance in the atmosphere and serve as these condensation nuclei.

As an air parcel rises it cools, the water vapour condensing when it reaches the dew point. Sunlight is scattered from the resultant droplets so that the emergent clouds can be seen by human eyes by their white, diffuse light. The best example of this is when air parcels rise from the warm ground in summer; those with only a small buoyancy compared to the surrounding atmosphere produce ‘fine weather’ clouds, those with stronger buoyancy climb higher and form thick clouds and heavy rainfall.

In low pressure systems with their cyclonic (anti-clockwise, in the northern hemisphere) air flow, warm, humid, subtropical air is transported to the north-east. Due to its lower density it moves up over the colder northern air, cooling as it rises. This is how clouds form on the warm fronts of such pressure systems in the mid-latitudes. Conversely, on the western side of the low the cold air moves southward, pushing under the warmer subtropical air and raising it up above its condensation level to form clouds on the cold front. Clouds can also be formed from the cooling and condensation that occurs when air flows over physical obstructions like mountain ranges.

Cloud droplets are tiny, only 1/100 mm in diameter, and are thus very light and practically float freely in the air. As the droplets collide, with time some grow larger than others and start to slowly fall, falling faster as they accumulate more and more droplets. If the cloud of droplets is dense enough to form droplets greater than 1/10 mm in diameter through this process, the droplets survive the fall through the air below the cloud, despite evaporation, and reach the ground as rain. This process develops what is known as ‘warm rain’, but in our part of the world it is responsible for only the weakest drizzle from near surface clouds. In the warm tropics it is responsible for intense rainfall from clouds lower than 5 km.

More important in the formation of heavier rain is a process that involves frozen particles. Although the temperature at cloud level is often below 0 ºC, the droplets themselves remain in liquid form – this is known as ‘super cooling’. In these circumstances, a tiny disturbance (a collision, or the presence of a suitable ice nuclei) is enough to cause the droplets to suddenly freeze solid. Water vapour condenses more easily onto ice particles than onto liquid water droplets, so these ice particles grow faster than the surrounding droplets and begin to fall sooner. They grow further through the accumulation of other droplets and quickly become large enough to reach ground level. Since the ice particles encounter ever warmer layers of the atmosphere as they fall, they often melt as they fall and reach the ground as rain. If they have formed in a very thick cloud however they can become so large that they do not melt before they reach the ground and fall as granulated ice particles. Storm clouds can contain upward winds that are so strong that ice particles that have already melted at the surface are pulled back up into the cloud and freeze anew. At some point they will fall again, growing by the accumulation of other droplets and melting again. Occasionally this process is repeated a number of times, leading to the eventual formation of large, layered hailstones.

Types of Clouds:
Clouds are basically divided into two classes, based on appearance: there are single layer (stratiform) clouds and accumulated (cumuliform) clouds, which can both occur at different altitudes throughout the troposphere. Stratiform clouds are formed through the large-scale rising of entire layers of air, whilst cumulus clouds are primarily formed through the action of small scale vertical mixing processes. The following table gives a brief overview of the appellations of the main cloud types:

Clouds are further named to denote their origin, details of their appearance or their degree of development (e.g. St Fr – Stratus Fractus – Broken Stratus)..

An overview of these topics can be found in almost all introductory books on meteorology. We recommend for example: Barry/Chorley: Atmosphere, Weather & Climate, 2003. Books on assessing the weather for outdoor sports (sailing, windsurfing, climbing) often contain good and practically useful descriptions of the processes important for clouds, and are often better value than academic textbooks too!