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
The water falls as rain.
The rainwater soaks into the ground or runs off the surface in
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:
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
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:
above 20,000 feet
made of ice crystals
wispy "mare's tails"
6,500 to 20,000 feet
below 6,500 feet
tall more than wide
rounded tops, flat bases
high, puffy clouds
puffy masses, may
look like waves
low, gray and
continuous horizontal sheets
or lumpy layers
transparent layers, may look like
haloes around sun and moon
made of water
extensive horizontal areas
dense, gray, releasing rain, snow.
Fog when at ground level
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types of clouds.