Most clouds form in the lowest part of Earth’s atmosphere, called the troposphere (it has an average height of 10km above the planet’s surface), but sometimes they can reach as high as the stratosphere (50km) and even the mesosphere (85km).
Scientists have calculated that the average cumulus cloud contains about 500 tons of water, which is the weight of about 100 elephants (weighing an average of five tons each).
In 1802 English meteorologist Luke Howard classified the types of clouds and gave them Latin names.
Other planets that have their own atmosphere – such as Venus, Jupiter (right) and Saturn – also have clouds. The word cumulus means “heap” or “pile”. The word stratus means “layer” or “sheet”. The word cirrus refers to a “curl of hair”. Cirrus clouds can move as fast as 160km/h. Some people mistakenly believe that contrails (below) – the line-shaped clouds that trail behind planes – are chemicals expelled by the plane but they are man-made clouds. Contrails form when the vapour in the hot air from the plane’s exhaust mixes with the cold air in the atmosphere and condenses to form ice crystals.
On Jupiter the Great Red Spot – a swirling storm of clouds twice as wide as Earth – has been observed on the giant planet for more than 300 years.