Clouds are water droplets and ice crystals that float in Earth’s troposphere and move with the wind. They form as a result of water vapor condensing around small particles of dust. Condensation occurs when warm air meets cool air. Cool air can hold less water vapor than warm air; therefore, when cool air and warm air combine, excess water vapor condenses into either water or ice. Cool air and warm air are likely to meet when warm air moves over something colder (such as a body of water), or when warm air moves up the side of a mountain and cools.
There are different types of clouds, named according to how high they are, how they look, and whether they produce rain. “Cirro” means high in the atmosphere; “alto” means mid-level (low-level clouds do not have a prefix). “Nimbo” means that the cloud can produce rain, whereas “cumulo” means the clouds are piled up and “strato” refers to flat, layered clouds.
Cumulo-nimbus clouds are clouds that can cause storms that include lightning, thunder, rain, hail, and strong winds. These clouds are vertical, beginning very low to the ground and ranging up to 75,000 feet—higher than most other clouds. Scientists look for these high, tall clouds to predict storms. Nimbo-stratus clouds are low, dark clouds that also cause rain.