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Cumulus clouds (Source: David C Tomlinson/Getty Images)

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How much do clouds weigh?

Tuesday 22nd September 2015 10:47 am

Some clouds are light and fluffy, while others are dark and foreboding and dump rain on us. So do light clouds and dark clouds have different weights? And if they have weight, what holds them up there?

Now the name ‘cloud’ comes from the 13th century Old English word ‘clud’ or ‘clod’, meaning a mass of rock, or a hill. And sure enough, a very dark cumulus cloud can look like a bit like a hill.

Clouds are not unique to our planet Earth — other planets have clouds.

The second planet out from the Sun, Venus, has very thick clouds made of sulfur dioxide.

On Earth (that’s the third rock from the Sun) our clouds are made from molecules of water — good old H2O in the form of liquid droplets.

At the fourth rock from the Sun, Mars, the clouds are made from water — it’s not liquid water, but solid water, otherwise known as ice.

At planets five and six, Jupiter and Saturn, the clouds are multi-layered. Going down from the edge of space, the clouds of these giant gas planets are made of successive layers of ammonia, ammonium hydrosulfide and underneath them, water.

At planets seven and eight, Uranus and Neptune, the clouds are made of methane. And just to round things off, on Titan (a moon of Saturn that happens to be so big that it’s larger than the planet Mercury) there are clouds of methane, as well as rivers and oceans of methane.

On Earth, clouds can range from the barely visible thin wispy cirrus, right up to the monstrous cumulonimbus thunderclouds.

Clouds must have weight, because they are made from water — and water has weight. A cloud is made of many, many tiny ‘clumps’ of water, either liquid or frozen. The liquid droplets are about two microns across (smaller than the thickness of a human hair, which is about 50-70 microns — with a micron being a millionth of a metre). Some of these tiny droplets are so small that it would take a billion of them to make a single raindrop.

A typical cumulus cloud carries about half-a-gram of water in each cubic metre — that’s the weight of a big garden pea.

A big fat dark cumulonimbus cloud could carry up to six times as much water (say, three grams of water per cubic metre). At the other extreme, a thin wispy cirrus cloud would carry about one tenth as much, say a twentieth of gram of water in each cubic metre.

So let’s get back to our ‘average’ cumulus cloud — with half-a-gram of water in each cubic metre. Its size might be one kilometre by one kilometre by one kilometre. So if you do the numbers, this cumulus cloud could carry about 500 tonnes of water. That’s around the weight of our largest passenger jets.

How come this water stays there, floating in the sky?

There are three main causes.

First, when it’s directly above, the Sun dumps about one kilowatt of power on each square metre. Some of this power goes to warm the ground, which then creates rising currents of air.

Second, think of an advancing storm, or a weather front. As it runs into other air, it can push the air that it runs into upwards — and that goes for any clouds in that air.

Third, air carrying water vapour can run into a mountain. The air rises and cools, which condenses the water vapour into water droplets.

So talking about lift, the only thing holding up your speeding 500-tonne passenger jet is air — admittedly moving very fast — over the relatively small wings.

Let’s just reverse the situation, with regard to speed and size.

So now we’ve got a lot of slowly moving air, but covering a huge surface area — say one kilometre by one kilometre, our so-called ‘average’ cumulus cloud. If you do the physics, that slow air can easily hold up a 500-tonne cloud.

This so-called ‘slow air’ is no ‘hot air’, but actually slow air — and it carries great weight.

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This blog first appeared on Dr Karl's Great Moments in Science

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