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The physics behind cumulus clouds

An explanation of how cumulus clouds form and grow in the atmosphere

When you think of a cloud, it is most likely a cumulus cloud that pops into your head, with its distinct fluffy, cauliflower-like shape. The word ‘cumulus’ means ‘heaped’ in Latin, and aptly describes the clumpy shape of these detached clouds. They are one of the lowest clouds in the sky at altitudes of approximately 600 to 1000 metres, while the highest clouds form nearly 14 km up in the atmosphere. Depending on the position of the clouds in relation to the sun, they can appear in a brilliant white colour, or in a more foreboding grey colour. Cumulus clouds are classified into four different species: cumulus humilis clouds which are wider than they are tall, cumulus mediocris which have similar widths and heights, cumulus congestus which are taller than they are wide, and finally, cumulus fractus which have blurred edges as this is the cloud in its decaying form. Cumulus clouds are often associated with fair weather, with cumulus congestus being the only species that produces rain.

So, how do cumulus clouds form, and why are they associated with fair weather? To understand the formation of these clouds, think of a sunny day. The sun shines on the land and causes surface heating. The warm surface heats the air above it which causes this air to rise in thermals, or convection currents. The air in the thermal expands and becomes less dense as it rises through surrounding cool air. The water vapour that is carried upwards in the convection current condenses when it gets cool enough and forms a cumulus cloud.

Due to the varying properties of different surface types, some types are better at causing thermals than others. For example, the sun’s radiation will warm the surface of land more efficiently than the sea, leading to the formation of cumulus clouds over land rather than the sea. This is because water has a higher heat capacity than land, meaning it will take more heat to warm the water than the land. As cumulus clouds form on the top of independent thermals, they appear as individual floating puffs.

But, what happens when cumulus clouds are knocked off the perch of their thermal by a breeze? How do they keep growing from an innocent, lazy cumulus humilis to a dark cumulus congestus, threatening rain showers? Latent heat gives us the answer. This is the energy that is absorbed, or released, by a body when it changes state. A cumulus cloud forms at the top of a thermal as the water molecules condense (changing state from a gas to a liquid) to form water droplets. When this happens, the warmth given off by the latent heat of condensation heats up the surrounding air causing it to expand and rise further, repeating the cycle and forming the characteristic cauliflower mounds of the cloud.

The development of a cumulus humilis to cumulus congestus depends on the available moisture in the atmosphere, the strength of the sun’s radiation to form significant thermals, and whether there is a layer of warmer air higher up in the atmosphere that can halt the rising thermals. If the conditions are right, a cumulus congestus can keep growing and form a cumulonimbus cloud, which is an entirely different beast, more than deserving of its own article. So, the next time you see a cumulus cloud wandering through the sky, you will know how it came to be there.

Written by Ailis Hankinson

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