cloud

A hydrometeor consisting of a visible mass of tiny liquid water droplets, ice crystals, or both at once, suspended in the atmosphere, usually located at a certain height above the Earth's surface, although it can be in contact with it. The cloud can also contain larger water or ice particles - which constitute other hydrometeors in themselves, such as rain, snow, or hail -, as well as non-aqueous liquid or solid particles, which may originate from smoke, dust, or industrial gases. The appearance of a cloud in the sky is the result of the formation of liquid water or ice, as a consequence of the condensation or sublimation of the water vapor present in the air. Clouds have very varied shapes and sizes, and can display a wide range of colors (not only the white with which they are commonly identified), depending on their composition, the time of day, and the observer's position. Their official (Latin) nomenclature and classification follow the criteria established by the WMO International Cloud Atlas. Most clouds are located in the troposphere, which is conventionally divided into three levels (high, middle, and low), in each of which only certain cloud genera appear, although some of them can occupy several levels at once. The boundaries of these levels are not fixed, as they vary with latitude and also with the time of year.