Clouds are formed in Earth's atmosphere when water evaporates into vapor from oceans, lakes, and ponds or by evapotranspiration over moist areas of Earth's land surface. The vapor rises up into colder areas of the atmosphere due to convective, orographic, or frontal lifting. The water vapor attaches itself to condensation nuclei which could be anything from dust to microscopic particles of salt and debris. Once the vapor has been cooled to saturation, the cloud becomes visible. All weather-producing clouds form in the troposphere, the lowest major layer of the atmosphere. However very small amounts of water vapor can be found higher up in the stratosphere and mesosphere and may condense into very thin clouds if the air temperatures are sufficiently cold. One branch of meteorology is focused on the study of nephology or cloud physics.

Tropospheric clouds can be divided into three main categories with names based on Latin root words that indicate physical structure and process of formation. Clouds of the cirriform category are generally thin and occur mostly in the form of filaments. The other two categories are stratiform with clouds that are mostly sheet-like in structure, and cumuliform that appear heaped, rolled, and/or rippled.