A cloud is a visible aggregate of tiny water droplets and/or ice crystals suspended in the atmosphere and can exist in a variety of shapes and sizes. Some clouds are accompanied by precipitation; rain, snow, hail or sleet.
All clouds form as a consequence of rising air. Sometimes air is forced to rise over mountains. More usually, warm air, being less dense, will rise above cold air. At fronts for example, warm air masses rise over cold air masses when they converge. At much smaller scales, columns of rising warm air may be generated by daytime heat from the Sun.
When air rises, it expands, causing cooling and a drop in temperature. As the temperature falls, the humidity (or water vapour content) of air increases towards 100%. Finally, after sufficient cooling, the air becomes saturated, and water vapour begins to condense out as tiny water droplets, forming cloud.
Clouds are classified into a system that uses Latin words to describe the appearance of clouds as seen by an observer on the ground. There are four principal classes of cloud:
Further classification identifies clouds by the height of the cloud base. For example, cloud names containing the prefix "cirr-", as in cirrus clouds, are located at high levels while cloud names with the prefix "alto-", as in altostratus, are found at middle levels.
High-level clouds form above 6,000 metres and since the temperatures are so cold at such high elevations, these clouds are primarily composed of ice crystals. High-level clouds are typically thin and white in appearance, but can appear in a magnificent array of colors when the sun is low on the horizon. Typical high-level clouds include cirrus, cirrostratus and cirrocumulus clouds.
The bases of mid-level clouds typically appear between 2,000 to 6,000 metres. Because of their lower altitudes, they are composed primarily of water droplets, although they may also be composed of ice crystals when temperatures are cold enough. Typical mid-level clouds include altostratus and altocumulus clouds.
Low clouds are of mostly composed of water droplets since their bases generally lie below 2,000 metres. However, when temperatures are cold enough, these clouds may also contain ice particles and snow. Typical low-level clouds include stratus, stratocumulus and nimbostratus clouds.
Probably the most familiar of the classified clouds is the cumulus cloud. Generated most commonly through small rising currents of warm air from the Earth's surface, these clouds are known for their flat bases and distinct outlines, with the cloud tops designating the limit of the rising air. When the air currents rise more vigorously, cumulonimbus or thunderclouds may form. These clouds can grow to heights in excess of 12,000 metres, releasing incredible amounts of energy through the condensation of water vapour within the cloud itself. Often associated with cumulonimbus clouds are heavy rain or hail showers.