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The science behind the shape of clouds — how cumulus and cirrus formations forecast the weather

The science behind the shape of clouds — how cumulus and cirrus formations forecast the weather
Ross Lazear
By Ross Lazear
23 Mar 2025
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Clouds form because of moisture in the air, and their different shapes tell you what to expect.

I’m a meteorologist, and I’ve been fascinated by weather since I was eight years old. I grew up in Minnesota, where the weather changes from wind-whipping blizzards in winter to severe thunderstorms – sometimes with tornadoes – in the summer. So, it’s not all that surprising that I’ve spent most of my life looking at clouds.

All clouds form as a result of saturation – that’s when the air contains so much water vapour that it begins producing liquid or ice.

Once you understand how certain clouds develop their shapes, you can learn to forecast the weather.

Cotton ball cumulus clouds


Clouds that look like cartoon cotton balls or cauliflower are made up of tiny liquid water droplets and are called cumulus clouds.

Often, these are fair-weather clouds that form when the sun warms the ground and the warm air rises. You’ll often see them on humid summer days.

However, if the air is particularly warm and humid, and the atmosphere above is much colder, cumulus clouds can rapidly grow vertically into cumulonimbus. When the edges of these clouds look especially crisp, it’s a sign that heavy rain or snow may be imminent.

Wispy cirrus are ice clouds


When cumulonimbus clouds grow high enough into the atmosphere, the temperature becomes cold enough for ice clouds, or cirrus, to form.

Clouds made up entirely of ice are usually more transparent. In some cases, you can see the sun or moon through them.

Cirrus clouds that form atop a thunderstorm spread outward and can form anvil clouds. These clouds flatten on top as they reach the stratosphere, where the atmosphere begins to warm with height.

However, most cirrus clouds aren’t associated with storms at all.

There are many ice clouds associated with tranquil weather that are simply regions of the atmosphere with more moisture, but not with precipitation (rain).

Fog and stratus clouds


Clouds are a result of saturation, but saturated air can also exist at ground level. When this occurs, we call it fog.

In temperatures below freezing, fog can actually deposit ice on objects at or near the ground, called rime ice.

When clouds form thick layers, we add the word “stratus”, or “layer”, to the name. Stratus can occur just above the ground, or a bit higher up – we call it altostratus then.

It can occur even higher and become cirrostratus, or a layer of ice clouds.

If there’s enough moisture and lift, stratus clouds can create rain or snow. These are nimbostratus.

How mountains can create their own clouds


There are a number of other unique and beautiful cloud types that can form as air rises over mountain slopes and other topographical features.

Lenticular clouds, for example, can look like flying saucers hovering just above, or near, mountaintops. Lenticular clouds can actually form far from mountains, as wind over a mountain range creates an effect like ripples in a pond.

Rarer are banner clouds, which form from horizontally spinning air on one side of a mountain.

Wind plays a big role


You might have looked up at the sky and noticed one layer of clouds moving in a different direction from another. Clouds move along with the wind, so what you’re seeing is the wind changing direction with height.

Cirrus clouds at the level of the jet stream – often about 10km above the ground – can sometimes move at more than 320km/h. But because they are so high up, it’s often hard to tell how fast they are moving. DM

First published by The Conversation.

Ross Lazear is an instructor in atmospheric and environmental sciences at the University at Albany, State University of New York.

This story first appeared in our weekly Daily Maverick 168 newspaper, which is available countrywide for R35.




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