<Li> <P> Particles in the atmosphere and the sun's angle enhance colors of stratocumulus cumulogenitus at evening twilight </P> </Li> <P> Particles in the atmosphere and the sun's angle enhance colors of stratocumulus cumulogenitus at evening twilight </P> <P> Clouds exert numerous influences on Earth's atmosphere and climate . First and foremost, they are the source of precipitation, thereby greatly influencing the distribution and amount of precipitation . Because of their differential buoyancy relative to surrounding cloud - free air, clouds can be associated with vertical motions of the air that may be convective, frontal, or cyclonic . The motion is upward if the clouds are less dense because condensation of water vapor releases heat, warming the air and thereby decreasing its density . This can lead to downward motion because lifting of the air results in cooling that increases its density . All of these effects are subtly dependent on the vertical temperature and moisture structure of the atmosphere and result in major redistribution of heat that affect the Earth's climate . </P> <P> The complexity and diversity of clouds is a major reason for difficulty in quantifying the effects of clouds on climate and climate change . On the one hand, white cloud tops promote cooling of Earth's surface by reflecting shortwave radiation (visible and near infrared) from the sun, diminishing the amount of solar radiation that is absorbed at the surface, enhancing the Earth's albedo . Most of the sunlight that reaches the ground is absorbed, warming the surface, which emits radiation upward at longer, infrared, wavelengths . At these wavelengths, however, water in the clouds acts as an efficient absorber . The water reacts by radiating, also in the infrared, both upward and downward, and the downward longwave radiation results in increased warming at the surface . This is analogous to the greenhouse effect of greenhouse gases and water vapor . </P>

The bottom of clouds show us where the