Planets that have atmospheres may also have clouds. Clouds can be made up of tiny drops of liquid or tiny crystals of ice. Sometimes, during storms, there may be dust clouds, too.

Clouds Contribute to the Circulation of Liquids on a Planet

    When clouds form and then release liquids, they moisten the land and keep oceans and lakes from just evaporating away. On earth, evaporation, condensation into clouds, and rain, are called the water cycle.

The Water Cycle

    Evaporated water condenses to form clouds.
The water falls as rain.
The rainwater soaks into the ground or runs off the surface in rivers.
Water evaporates from the ground, from the transpiration of leaves, from the breath of animals, from puddles, lakes, rivers and oceans.

Notice that there is a constant reservoir of underground water. The level of this water is called the water table. Trees sink their roots down into the water table if they can, so that they have a perpetual supply of water. People sink wells into the ground and tap the water table.

Rainfall replenishes the water in the ground. If too much water is pumped out, the water table falls, i.e., is lower down in the earth. Water tables are falling as human populations increase. This makes pumping the water out more difficult, and sometimes causes the now drier earth to settle and become more compact. This may result in permanent changes in the underground water storage capability of the area.

Erosion is an Important Result of Rainfall

     Falling water and water runoff cause erosion, the wearing away of rock surfaces and the redistribution of stones, and smaller particles of sand, silt, and surface debris. On earth, large raindrops fall at eight meters per second, snow flakes at about one meter per second. While these impacts are gentle, rapidly flowing water can move even large stones. Erosion wears mountains down and spreads the debris on flat plains, preparing level areas which, on earth, are good places for farming and for large grazing animals.

Clouds Affect Planetary Temperatures

Clouds affect temperature in two ways:

First, clouds are part of the albedo of a planet -- that is, they can act like mirrors and send some of the sun's heat back into space without allowing it to reach the surface. This cools the planet.

Second, clouds can act like blankets, preventing the heat that is on the surface of the planet from escaping into space. This makes the planet warmer. These two effects reach a dynamic balance, keeping the temperatures within a "normal" range. If human activities cause important changes in the average amount of earth covered by clouds, climate could be affected.


Kinds of Clouds
     On earth, clouds are made of tiny drops of condensed or frozen water vapor. Noticing clouds can tell us about conditions in our atmosphere. Clouds are classified by height and structure:

	Cirrus High Clouds above 20,000 feet made of ice crystals wispy "mare's tails"	Altoform Medium Clouds 6,500 to 20,000 feet	Low Clouds below 6,500 feet
Cumulus tall more than wide rounded tops, flat bases	Cirrocumulus high, puffy clouds	Altocumulus puffy masses, may look like waves	Stratocumulus low, gray and sometimes lumpy
Stratus continuous horizontal sheets or lumpy layers	Cirrostratus ice crystals transparent layers, may look like haloes around sun and moon	Altostratus made of water extensive horizontal areas	Nimbostratus dense, gray, releasing rain, snow. Fog when at ground level