Composed of tiny water droplets and ice crystals, clouds can exist as wisps spanning just a few tens of meters, as herds of puffy sheep, as towering thunderstorms, and as giant weather systems spanning hundreds and even thousands of kilometers. For centuries, clouds have provided backdrops for paintings, stories, poems, and songs. They also determine the Earth’s climate.

Clouds form when water vapor condenses as liquid water or freezes as ice, lifting moisture evaporated from the surface and distributing it within the atmosphere. Tiny droplets and ice crystals grow in clouds to form rain, snow, and hail. Chemical reactions in water droplets and on the surfaces of ice crystals help cleanse the atmosphere of pollutants, while some reactions lead to acid rain. Heat is released from the condensation and freezing of cloud water. Evaporation of liquid water, sublimation of ice, and emission of infrared radiation cool clouds. These heating and cooling processes drive turbulence. As a result, clouds change constantly.

Clouds weigh in as the largest contributor to the sunlight reflected by the Earth and to the greenhouse effect of the Earth’s atmosphere. Our inability to reliably predict clouds remains the largest stumbling block hindering accurate assessments of climate change. Researchers within CEOAS study clouds with aircraft, satellite and surface-based measurements; model individual clouds and ensembles of clouds; and characterize the feedbacks between clouds and the climate system.

Cumulus over the plains of the Midwest (photo from the University Corporation for Atmospheric Research)