Mary Lou Lawrence, age 12, of Niagara Falls, Ontario, Canada, for her question:
Exactly how do clouds form?
Clouds are children of the oceans and continents and their life stories are governed by the sun and the air. The beaming sun evaporates moisture from the surface of the earth, changing liquid water into gaseous vapor that rises up and mingles with the other gases of the atmosphere. Clouds are born when cooler temperatures chill the air. This compels some of the gaseous vapor to change back into liquid moisture. When this happens above the ground the tiny droplets of misty moisture form a cloud.
The rules of this game are governed by air and temperature. The gaseous atmosphere grows hot and cool as its global winds blow here and there over land and sea. According to its nature, the air absorbs moisture from the earth and its temperature determines just how much water vapor it can hold. Warm air is allowed a higher percentage of moisture than cool air. But when saturated warm air cools, it has more than its quota of invisible water vapor. Clouds are formed when this surplus moisture turns. back into misty droplets of liquid water.
Something like this happens when you stand a glass of ice water in a warm room. The sides of the glass chill the warm air around it. The cooler air changes invisible vapor into sweaty drops of water on the sides of the glass. In the atmosphere, all sorts of weather conditions can create wet clouds from gaseous vapor.
When the sun warms a patch of ground, the ground shares some of its heat with the air above it. Air molecules use this heat energy to speed up and spread apart. Some of this thin, light air rises and spreads up into cooler regions aloft. There the cooler temperature forces gaseous water molecules to form liquid droplets aid clouds are born high above the ground.
Other clouds form because the world wide oceans tend to be cooler than theland during the summer and warmer during the winter. Winter winds gather gaseous moisture as they blow over the seas. When they reach the cooler land, some of this vapor may change into clouds. Other clouds are born when warm, moist winds bash into a mountain barrier. As they blow up the slopes, they grow cooler. Often their moisture forms clouds that shed rain on one side of the up slopes. Many other clouds form in stormy cells where masses of warm air mix and mingle with masses of cooler air. Clouds of all sorts may form over land or sea, aloft or near the surface whenever moist warm air is slightly chilled.
Meteorologists measure the air's temperature and vapor content and compare these findings with a chart of relative humidity. Air is saturated at 100 per cent humidity ¬but temperature makes a difference. For example, air at 86 degrees Fahrenheit can hold more than twice as much vapor than air at 61 degrees Fahrenheit. Suppose the 86 degree temperature drops to 61 degrees. Then about half the gaseous vapor must be changed into misty droplets of liquid.