Formation of Clouds Clouds are formed when air containing water
vapor is cooled below a critical temperature called the dew
point and the resulting moisture condenses into droplets on
microscopic dust particles (condensation nuclei) in the
atmosphere. The air is normally cooled by expansion during its
upward movement. Upward flow of air in the atmosphere may be
caused by convection resulting from intense solar heating of the
ground; by a cold wedge of air (cold front) near the ground
causing a mass of warm air to be forced aloft; or by a mountain
range at an angle to the wind. Clouds are occasionally produced
by a reduction of pressure aloft or by the mixing of warmer and
cooler air currents. Sections in this article: More on cloud
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