A cloud is a visible mass of droplets or frozen crystals floating in the atmosphere above the surface of a planet or other astronomical body. The most common form of clouds on M-class planets are made of water vapor. Low-level clouds very near or at a planet's surface are called fog.
A cloud may also refer informally to a concentration of matter drawn together by gravity. For example, nebulae are sometimes known as interstellar clouds.
A common consequence of water vapor clouds on planets is the meteorological phenomenon known as rain. Storms, lightning, and thunder are also associated with clouds.