Cumulus Clouds are found worldwide but tend to be more common in humid regions. Cumulus clouds form through convection and while the cloud base is commonly below 2,000m they can reach up to 12 miles high, and 6 miles across. Cumulus clouds are formed within columns of rising thermal air, with their base marking the level at which condensation is occurring, and this explains why all cumulus clouds in one area have their base at the same height.
As the thermal air speeds vertically upwards it draws in surrounding air at its base, and this air is cooler and drier. Cloud droplets evaporate, and cloud fragments sink, in this drier air. This prevents cumulus clouds from growing wider and stops other clouds from forming close to it.
If you look out of the window on a clear summer morning and discover that there is not a cloud to be seen in the sky, and then by mid morning there is a horde of fluffy white clouds marching across the sky, blocking out the sun, then these will be cumulus clouds. Fluffy and white, looking like cotton wool they move quickly across the sky on the breeze. Cumulus tend to be short lived, they form rapidly and they disappear rapidly.
The heat of the morning sun makes the ground warmer and so by mid-morning patches of warmed ground are overlain by larger invisible patches of warm air. This parcel of air will start to rise and carry its moisture with it, and as it rises it expands (the air pressure is lower the higher up you are) and cools and its moisture condenses into tiny cloud droplets, forming a cumulus cloud.
Sometimes a power station or an industrial chimney will introduce enough moisture into the atmosphere to cause its very own cumulus. As mentioned the clear air around surrounding the cumulus clouds is an indication of descending air, and the larger the cumulus then the larger the clear air around it. Therefore a bigger than normal patch of blue sky may indicate the approach of a sharp shower. A medium sized cumulus cloud can hold as much as 1000 tons of moisture in it.
But at the end of the day, as the sun starts to lose its heating power the convection will end, and the cumulus clouds will slowly drift, fade and disappear.
In summary, the word ‘cumulus’ comes from the Latin word for a heap or a pile, and this aptly describes their appearance. They do not tend to last very long, and consequently do not reach to great heights (unlike, for instance, cumulonimbus). Cumulus clouds are a good indication of fair weather, with perhaps the odd sharp, short shower. And finally they are more prominent in the summer months, when convection is at its greatest.
