Clouds

When I’m away from home in various parts of the country I am always interested in the cloudscapes I find there. The thing is that they are often typical of that region and different from other regions. This may seem obvious and trivial, however it is interesting to consider the influences that contribute to the cloudscape and how they work in different atmospheric conditions. I live in the relatively hilly and mountainous region of Wellington, and the important cloud forming processes are related to the arrangement of the hills and the higher mountains, and the all-important (for Wellington) wind direction. The terrain of other regions influences the cloudscapes of those regions, and that is how the differences arise.

There is a very useful booklet called “Cloud Forms” by my ex-colleague, Ray Smith. This is now out of print, but a few copies are still about. In it he says that there are basically three types of cloud; heap, layer and streak. These are the result of different cloud formation processes. From another approach, although there is an infinite range of cloud shapes and sizes, any cloud observer can identify these three types. The classical cloud naming system was proposed by the London pharmacist Luke Howard in 1803, and many people are familiar with the basic cloud names; “cumulus” forms for heap types, “stratus” forms for the layer types, and “cirrus” forms for the streak types. With these simple categories and some elementary meteorological knowledge, you can observe the clouds and work out what is going on in the atmosphere around you. There are also low, middle and high cloud categories, and the cloud forms that appear in those altitude ranges depend on the meteorological conditions that occur there.

To interpret the cloud forms you see, the basic meteorological elements of knowledge you need are that air cools as it rises, and that if it cools enough that the temperature of the air reaches its dew point, the water vapour will condense and become visible – a cloud. It is interesting to realise that with perfectly clean air (only the constituent gases and no aerosols) the temperature needs to decrease well below the dew point before condensation will occur. In the real atmosphere there are plenty of condensation nuclei among the aerosols that are naturally present, so condensation readily occurs when saturation is reached; that is when the air temperature is the same as the dew point and the Relative Humidity is near 100%.