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Clouds are usually the most common feature of the sky, especially over Britain. A cloud is a visible accumulation of minute droplets of water or ice crystals, or of both, suspended in the air. Clouds may also include other solid particles, such as those present in fumes, smoke or dust. While the typical size of a single minute cloud droplet (20 micrometers) is one hundred times smaller than the typical size of a rain drop, which is two millimeters, the size of the cloud itself might range from a few meters (cumulus humilis) to the order of a few 10s of kilometers (thundercell). However, large-scale convection can form clusters on the scale of 100s-1000 km and are easily recognisable from satellite imagery.
Cloud formation is generally due to vertical motion of the air above the condensation level. This might be the ascent of humid air over an orographic barrier, or forced uplift along a front, or thermal convection. Sometimes other processes, such as turbulence and reduced air pressure may be involved. Thus a cloud is created by condensation or freezing of water vapour, and with its base above the ground. However, clouds may form at or be in contact with the ground surface, too. In fact, such a cloud would be known as fog, ice fog, or mist.
Clouds usually occur within the troposphere, and some cirrus do even form at lower stratosphere levels. However, the true 'upper class' of the clouds are the rare noctilucent (left) or polar stratospheric clouds, forming at even greater altitudes way above 20 km.
Clouds and fog reflect weather patterns and interact with the ground, sun and atmosphere thus playing an important role in what the weather does. In addition to their obvious role as sources of precipitation, clouds also can affect the temperatures of the places below them. Clouds not only block incoming sunlight during the day, which cools the air, but they can also block outgoing radiation from the Earth, which can warm temperatures.
Clouds come in many sizes and an endless variety of shapes, which refelect their origin and history. Their varied forms are desrcibed in standardized ways by the WMO cloud classification scheme. Cloud may be described by its dimensions, shape, structure texture and opaqueness, but they are always white. Its only their own shade or the low sun that gives them dark or reddish colours.