Cloud
From Wikipedia, a free encyclopedia written in simple English for easy reading.
A cloud is water in the atmosphere (air) that we can see. It is where rain and snow comes from.
Water on the earth evaporates (turns into gas) and goes up into the sky. There, when it is colder, the water condenses: it changes from a gas to drops of water or crystals of ice. We see these drops of water as clouds. The drops fall back down to earth as rain, and then the water evaporates again. This is called the "water cycle".
Clouds on other planets are sometimes just collections of gases.
[edit] How clouds form
Clouds form in places where air full of water cools, usually when the air goes up. This can happen
- along warm and cold fronts (where the weather is very hot or cold),
- where air goes up the side of a mountain and cools as it goes higher
- when warm air goes over a colder thing (such as cool water in a lake).
Clouds are heavy. The water in a cloud can have a mass of several million tonnes. Each cubic metre (m3) of the cloud has only about 5 grams of water in it. Cloud droplets are also about 1000 times heavier than evaporated water, so they are much heavier than air. They do not fall, but stay in the air, because there is warm air all round the heavier water droplets. When water changes from gas to droplets, this makes heat. Because the droplets are very small, they "stick" to the warm air.
There are different sorts of cloud, because of how fast the air goes up and if the air stands still or is moving. Also, some clouds make more rain, or make thunder and lightning. These differences come from how big the water droplets are, and how they join together.
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