Joseph Lambert Jr., age 11, of Greenville, Miss., for his question:~
HOW ARE CLOUDS FORMED?
Clouds form from water that has evaporated from lakes, oceans and rivers, or from moist soil and plants. This evaporated water, called water vapor, expands and cools as it rises into the air.
Air can hold only a certain amount of water vapor at any given temperature. When the temperature drops, some of the water vapor begins to condense, or change to a liquid. Tiny droplets of water are formed.
For water vapor to condense, particles so small they can be seen only through a microscope must be present. These particles, called condensation nuclei, become the centers of the droplets. Many condensation nuclei are tiny salt particles or particles present in smoke.
A cloud often contains both water droplets and ice particles if the temperature is between 32 degrees Fahrenheit and minus 40 degrees. Water droplets do not always freeze at the normal freezing temperature of water, 32 degrees Fahrenheit. They may remain liquid down to a temperature of minus 40 degrees.
Water vapor can rise to form clouds in several ways. When the sun warms the ground, the air next to the ground is heated. Because warm air is lighter than the same volume of cooler air, the warm air rises. This rise of warm air is called a convection current, and this method of cloud formation is called convection.
As the air rises, it expands and becomes cooler. If enough water vapor is in the expanding air, the vapor will condense and form clouds.
Clouds also form by lifting. When warm, moist air moves up the side of a hill or over a mountain range, it is lifted and cools by expansion. This cooling causes the water vapor to condense and form clouds that hang over the mountains.
Weather fronts, where masses of warm and cool air meet, produce clouds by frontal activity.
In clouds caused by frontal activity, the water vapor in the rising warm air becomes cooler and condenses, creating the water droplets that form clouds.
Weather forecasters study clouds carefully because certain types often appear before storms. In many cases, a warm front or a low pressure system may be identified by these clouds, which form in a definite order over several days.
First, a few wispy cirrus clouds appear in the west. Soon more appear and gradually merge into cirrostratus clouds that cover the sky.
The cirrostratus clouds are later hidden by a lower layer of altostratus clouds that become thicker and hide the sun. Light rain or snow may begin to fall from the altostratus layer.
The base of the clouds becomes still lower as nimbostratus clouds move in with heavier rain or snow. Cumulus and cumulonimbus clouds often develop within the nimbostratus ones, and the steady rain becomes a heavy shower.
As the storm moves past, the rain or snow ends but the sky remains overcast with stratocumulus clouds. These clouds disappear as soon as fair weather returns.