What are clouds made of?

Your question of the month in July (2016)
Emilie, 8 years old

It is a great feeling flying with a plane: floating high in the sky and sometimes in the midst of a sea of clouds. But how do clouds actually form? And why do they often look so different? We will explain this to you.

There is a lot of water on our earth: in the sea, in lakes and rivers, but the ground also stores water.

The sun warms up the earth. The water is heated up by the warmth of the sunrays. When water is heated, water particles rise upwards in the form of water vapour: the water evaporates. Maybe you have noticed yourself sometimes that hot drinks steam or that steam clouds form in a hot shower. That is water vapour.

The warm air absorbs this water vapour and rises upwards. However, the higher the air rises the colder it gets. In the cold air, the water vapour turns back into water drops. This is called condensation. That actually also happens to the water vapour on the bathroom mirror: Once it cools down, it takes on liquid form again and makes the mirror moist. If the air is particularly cold, small ice crystals even form.

Up in the cold air the individual water drops and ice crystals merge together into one increasingly bigger drop – a cloud forms. What seems to be white and fluffy to us is in reality an accumulation of billions of water drops. The radiation of the sun makes the cloud appear white to us.

Did you know that there are different kinds of clouds? Depending on how high above the ground they form, one differentiates between high clouds (5 to 13 km/3 to 8 miles above the ground), medium high clouds (2 to 7 km/1 to 5 miles above the ground), low clouds (up to 2 km/1 mile above the ground) and clouds that occur at all heights. These are the four cloud families. Depending on their shape, size and form, the cloud families can be divided up into different cloud types. There are, for example, cumulus clouds – these are large and white, like a cluster of cauliflower florets, which you can see fantastic "cloud animals" in – or stratus clouds. That is what a grey, solid cloud layer is called that covers the entire sky and usually announces bad weather. Which types of clouds will you discover on your next flight?