Clouds are ubiquitous in Earth’s atmosphere. They are scientifically challenging because aspects about their formation, evolution, distribution, and impact on Earth’s climate and the hydrological cycle are still not fully understood. Understanding them requires knowledge about both the large-scale meteorology and micro-scale cloud processes involved in the formation of cloud droplets, ice crystals, raindrops, snowflakes, graupel, and hailstones.

Clouds are an integral part of Earth’s atmosphere and are a major factor in Earth’s radiation budget. Their influence on the radiation budget differs for the solar (shortwave) and terrestrial (longwave) radiation and depends on the cloud type and altitude. Clouds also play an important role in Earth’s hydrological cycle, by transporting water vapor through the atmosphere and from the oceans to the continents, eventually delivering the water from the atmosphere to Earth’s surface as rain or snow. This chapter introduces the different cloud types and their properties. It also discusses where these different cloud types can be found in the atmosphere and their global coverage.