Cloud Index: Cumulus
The fluffy cotton-wool clouds that signal convection
Welcome to the Cloud Index — a regular feature on Looking Up where we profile a type of cloud, explaining how they form, what they’re made of and how to use them to forecast the weather. You can find the full list of clouds we’ve covered in the Cloud Index index.
It seems only right to kick off the Cloud Index with cumulus. When someone says the word “cloud”, they’re the first thing you picture, and they tell us a huge amount about what’s going on in the skies above our heads.
Cumulus is latin for “heap” or “pile”, and was given its name by 18th-century meteorologist Luke Howard, who we profiled last year. Cumulus is puffy on top and flat below, and often has quite clearly defined edges. It can be made of water vapour or ice crystals, and appears low in the sky.
It’s quite rare to see just one cumulus cloud — they often form across a landscape, sometimes in lines or clusters called ‘cloud streets’ that can stretch for hundreds of kilometres. From above, they yield spectacular bumpy panoramas.
How They Form
Imagine a pot of boiling water on a stove. As the base of the pan heats, bubbles of steam form that are lighter than the fluid around them, so they rise slowly up to the surface.
Exactly the same thing happens on a warm summer’s day. The sun heats the surface of the ground , creating bubbles of air (meteorologists call them “parcels”) that are warmer than the surrounding air. These rise into the sky, slowly cooling.
The amount of water that air can hold decreases as it the air cools, so eventually the bubble gets to a temperature where it can’t hold its water as a vapour any more. At this point, the water vapour turns into millions of tiny liquid droplets, each suspended in the air. A cumulus cloud is born.
However, this process of cloud formation releases heat, allowing the air to keep rising now that some of its water has been shed. The cloud gets taller and taller until it either runs out of energy or hits the natural barrier of the bottom of the stratosphere. At this point, it’s usually become a cumulonimbus, so we’ll talk about that another day.
What they mean for the weather
You can use the height of cumulus clouds to easily forecast the weather. Flat cumulus humilis clouds indicate fair weather, but the taller the cloud grows the more likely that some of those water droplets will start falling out as rain or even hail. The bigger the droplets, the darker the cloud will appear, so colour can be a useful signal too.
The tops of cumulus clouds are very bright and reflective, so they tend to bounce a lot of sunlight back out into space. As a result, they cool our climate. A planet with more cumulus clouds and fewer cirrus clouds will be a cooler one.
Anything else?
There are lots of variations on cumulus, including altocumulus, cirrocumulus, stratocumulus, and even the volcano- and wildfire-driven pyrocumulus clouds. We’ll deal with each of those another day, as they have some interesting quirks of their own.
Oh, and Earth’s cumulus clouds aren’t unique — we see cumuliform clouds on most other planets in the solar system, including Mars, Jupiter, Saturn, Uranus and Neptune. It’s likely that they’re widespread throughout the entire Universe.
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