Clouds are made up of ice crystals and droplets of water. These form when water evaporates from bodies of water, such as the oceans. Once water reaches higher altitudes in the atmosphere, it becomes liquid and solid.
Heat from the sun causes water to evaporate. Once the evaporated water, called water vapor, reaches higher altitudes where temperatures are cooler, it becomes liquid water again. At even higher altitudes, the air becomes colder, and ice crystals form instead of liquid water. Clouds form when the water droplets or ice crystals become large enough to be seen.
Droplets and ice crystals move and bump into one another, forming larger water droplets and ice crystals. When these droplets and crystals become heavy enough, they fall to the ground as precipitation.