Cloud formation
Clouds are formed when humid air at the surface of the Earth rises and cools. Because cooler air cannot hold as much gaseous water as warmer air, the gaseous water (water vapor) will eventually condense into suspended water droplets, which we see as clouds. The height at which water vapor will begin to condense is known as the lifted condensation level, and this can be used as a good approximation of the cloud base. If the air is very humid, then the cloud base is likely to be very low. When the air is both humid and cold, the cloud base is at ground level and fog is formed.
Cloud Types
Clouds are classified according to the height at which they form and their shape. Low-level clouds form within 2 km of the surface, mid-level clouds between 2 and 7 km, and high-level clouds between 7 and 13 km. The three main cloud shapes are stratus, cirrus, and cumulus.
Cumulus clouds are characterized by a flat base with a puffy top. Formed by a process known as convection, cumulus can occur at any level in the troposphere and are one of the few cloud types that can have a large vertical extent. Cumulus clouds that are wider than they are tall are known as cumulus humilis, or fair weather cumulus. When individual cumulus clouds are approximately as tall as they are wide, they are known as cumulus mediocris. These clouds usually do not produce any rain but they do indicate a certain degree of atmospheric instability. For example, if cumulus mediocris are observed in the morning, then there is a good chance of rain and potentially thunder in the afternoon. Tall cumulus clouds are known as cumulus congestus. They are indicative of considerable instability in the atmosphere and commonly develop into towering cumulonimbus.
Cumulonimbus usually start at low levels and extend through all three cloud layers, frequently reaching the top of the troposphere and forming a classic anvil shaped top as the cloud begins to spread horizontally. Cumulonimbus are a sign of a highly unstable atmosphere and can result in severe rain, hail, wind, thunderstorms, tornadoes, and waterspouts. While a single cumulonimbus is relatively short lived (usually no more than an hour), the conditions that produce a single cloud can also produce many more, forming squall lines and supercells that may last for hours. If a cumulonimbus is heading your way, you should expect severe weather.
Accessory clouds that occasionally form in conjunction with cumulonimbus can provide some indication as to how severe the weather associated with the cloud is likely to be. For example, if cumulonimbus is preceded by a roll or a shelf cloud, expect very strong winds. If cumulonimbus is accompanied by mammatus clouds, which are lumpy pouches on the underside of a cumulonimbus, then convection within the cloud is particularly strong and you should expect severe conditions, particularly hail. Tuba clouds, which look like cones on the underside of a cumulonimbus, are the precursors to tornadoes and waterspouts.
Stratus clouds are characterized by flat and featureless continuous layers. They are commonly associated with the passage of a warm front and can generate prolonged light rain. When stratus clouds form at low levels, they are referred to simply as stratus. If they produce rain, stratus clouds are known as nimbostratus, while mid and high level stratus clouds are referred as altostratus and cirrostratus, respectively.
Cirrus clouds are thin and wispy cloud strands which are composed of ice crystals. They can occur at mid and high altitudes and are formed following the dissipation of cumulonimbus or contrails in high winds. The prefix cirro- can also refer to any cloud that forms at altitude. For example, cirrocumulus are high altitude cumulus clouds that can provide an early warning for the impending approach cumulonimbus.
Beyond these broad definitions, there are many different cloud species that are named based on their formation and shape. There are more than 40 named cloud species.
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