The condensing water vapor forms small droplets of water (0.012 mm) or ice crystals that, when surrounded with billions of other droplets or crystals, are visible as clouds. Clouds reflect all light and are white, but they can appear grey or even black if they are so thick or dense that sunlight cannot pass through.
Clouds form in areas where moist air rises and cools. This can happen
The actual form of cloud created depends on the strength of the uplift and on air stability. In unstable conditions convection dominates, creating vertically developed clouds. Stable air produces clouds created through turbulence alone, creating massive bulbous cloud forms. Frontal uplift creates various cloud forms depending on the composition of the front (ana-type or kata-type warm or cold front). Orographic uplift also creates variable cloud forms depending on air stability, although cap[?] and wave clouds[?] are specific to orographic clouds[?].
Clouds are divided into two general categories: sheet-like and layer-like. These are named stratus clouds (or stratiform, the Latin stratus means layer) and cumulus clouds (or cumiloform, cumulus means piled up). These two cloud types are divided into four more groups that distinguish the cloud's altitude.
High clouds (Family A) form above 20,000 feet in the cold region of the troposphere, and are denoted by the prefix cirro- or cirrus. At this altitude water almost always freezes so clouds are composed of ice crystals. The clouds tend to be wispy, are often transparent, and include cirrus, cirrocumulus (mackerel sky), and cirrostratus.
Low clouds (Family C) are found up to 6,500 feet and include the stratus (dense and grey), stratocumulus (broken up stratus) and nimbostratus (dense and grey with precipitation) clouds. When stratus clouds contact the ground they are called fog.
Vertical clouds (Family D), such as cumulus (white and 'fluffy'), have strong upcurrents and rise far above their bases and can form at many heights. Cumulonimbus clouds, associated with heavy precipitation and thunderstorms, can start near the ground and soar up to 75,000 feet. Mammatus clouds[?] are pockets of sinking air that may form at the base of cumulonimbus clouds. Pileus clouds[?] are thin clouds that form over the tops of cumulonimbus clouds as they expanding.
A contrail is a long thin cloud which develops as the result of the passage of a jet airplane at high altitudes.
Clouds on other planets often consist of material other than water, depending on local atmospheric conditions (what gases are present, and the temperature).