When was the last time you ‘cloud-gazed’? Looked up at the sky and noticed the really high, sheet-like clouds, the big fluffy clouds, the grey-tinged rain clouds off in the distance, those clouds that look like airplane trails and others that are just all bitsy and scattered.
Clouds are cool. Both as a personal opinion and as a physical fact: their average temperature ranges from 0.2 to 0.3 degrees Celsius over their lifetime with a maximum of 2-4.5 degrees C during early cloud development. Those facts come from this really interesting paper that discussed solely cloud temperatures, and interestingly, had possible error as large as 0.5 degrees Celsius, but the authors used the smallest possible error, 0.1 degrees C in their final average temperature value of 0.2 to 0.3 degrees C. The cloud will spend some time significantly colder than this though, as clouds tend to be warmer in their early life after formation and then cool far more as they rise. Interestingly, they generally stay as liquid water when they are cooler than zero because the pressure is lower with altitude and lowers the freezing point so it has to be extremely cold to actually freeze. In fact, because of the lower air pressure and the tiny size of water droplets, they can remain as liquid water at temperatures as low as -30 degrees C.
But back to the topic, clouds are made up of water droplets or tiny ice crystals. They form as part of the water cycle, as the evaporated water rises and cools until it condenses back into liquid water droplets. This condensation occurs around condensation nuclei, often bits of dust, ice, sea salt or other atmospheric particles that provide an area for the water to condense onto. These water droplets are incredibly tiny at only one hundredth of a millimetre, or 100 million droplets per cubic metre! When a lot of water condenses in one area, it will then be visible to us as a cloud.
But you might ask, if the water is condensing as droplets, why doesn’t it immediately fall as rain? The thing is, these droplets are like the dust that you see floating around when the sun’s shining in a window … they’re so tiny that they can float. To actually fall as rain, the individual droplets need to collide with each other to form larger and larger droplets until they are too heavy to float up in the sky. Then gravity does its trick and pulls them down as rain.
Clouds normally form when there are sudden changes in temperature, such as if a warm weather front meets a cool front, or when air suddenly rises (and consequently cools) when it meets hills or mountains. This is simply because the warmer the air is, the more water vapour it can contain without condensing. So if the temperature suddenly drops, the water must go somewhere to condense into liquid form.
The type of cloud that is formed will depend on the temperature, wind conditions and other variables which influence its shape, size and other properties.
Types of clouds
Clouds are generally classified into three categories based on the part of the atmosphere they are found in. Excitingly, these category are known as high cloud (above 6 km), medium cloud (between 2 and 6 km) and low cloud (under 2 km). These are further split up into the specific cloud types, based on size and shape.
These include cirrus clouds (the light, wispy clouds), cirrocumulus (the very light, spread-out, “bumpy” clouds) and cirrostratus (the very light, sheet-like clouds that cover large portions of the sky).
These are the altocumulus (they’re the small, weirdly-shaped ones), altostratus (grey, sheet-like cloud that covers large portions of sky fairly thickly) and nimbostratus (generally darker, thicker cloud that sticks around for ages with days and weeks of rain).
These are the stratus (again, flattish sort of clouds, they’re just lower in the atmosphere), stratocumulus (in-between the fluffiness of cumulus and the flatness of stratus clouds), cumulus clouds (generally medium-size, roundish white fluffy clouds) and finally the cumulonimbus, which start in the low cloud region but can extend upwards into both the medium and high cloud regions and are often rain or storm clouds.
Some other cool stuff
It’s very similar to the explanation of the sky’s colour. The sky appears blue because when light waves hit atmospheric molecules, blue light is scattered more than the other colours. When the light waves hit a cloud, they hit the individual water droplets that are larger than molecules, and so scatter the light wavelengths fairly evenly. Clouds can appear grey at the bottom because most of the light is scattered upwards or to the sides of the cloud, rather than down. So the bottom of the cloud isn’t reflecting white light, so it appears grey. The more droplets are in the cloud (ie. when the clouds are full of water, ready to rain), the more pronounced this effect is, making rainy clouds appear greyer.
Fog is like cloud, except it forms thickly and at ground level. Given the explanations of cloud formation above, it’s quickly obvious that fog forms when there is a temperature change, such as when temperatures cool overnight, when air goes up a slope, or even when cool air passes over a warm area, as the warm air is cooled and condenses suddenly. Additionally, fog forms far more easily in industrial areas because there are many dust or smog particles that can act as condensation nuclei, making it easy for water vapour to condense.
The thunderstorm process is actually quite fascinating. It occurs when warm air is trapped underneath cooler air. These weather conditions cause the warm air to cool rapidly and form large cumulus clouds and cause the water droplets to condense together and freeze into ice crystals- hail. As hail is heavy, and will naturally fall, it moves downward and bumps other hail stones. This friction causes negative electrical charge to build up at the base of the cloud and a positive charge at the top of the cloud. Having a concentration of any charge causes instability and the cloud will naturally act to try and return to a neutral charge.
So to discharge all the negative charge, the cloud has to release it to the positive upper side of another cloud or down to the Earth. We see this in both cloud-to-cloud lightning and down-to-Earth lightning. This release of energy is what we see as a lightning bolt, and it is so powerful that it causes the air to heat incredibly fast, which then causes it to expand and push outwards powerfully, creating a thunderclap.
I hope this has been a little bit interesting and provides some great information on a part of life that we often just take for granted!When was the last time you ‘cloud-gazed’? Or looked up at the sky and tried to identify the clouds and wondered where they came from? And maybe you’d just like to learn why they’re cool