Welcome to You Ask Andy

Paul Bardach, age 10, of Flushing, N.Y., for his questions

How does the sun cause evaporation?

The sun evaporates moisture from the earth, and the moisture falls down again as Rain or snow or hail. An inch of rainfall over a square mile is equal to 72,300 tons Of water. In one hour, a sharp shower can dump 117 million tons of water on Long Island, and all this moisture was evaporated from the sea or other watery places on the earth. Every second, the air drinks up about 15 million tons of water from the face of the globe. This water evaporates. It becomes vapor and mixes with the other invisible gases of the air. The total amount of vapor in the atmosphere around the globe is equal to 13 trillion tons of water. Every second, some 15 million tons of this vapor becomes rain or snow or hail and falls again down to the earth. Evaporation is caused by the sun and the winds which circulate the air around the Globe. More water evaporates when the sun is hottest and the breezes are brisk. The sun provides heat, and heat is a form of energy. When we add enough heat to a kettle, the water will boil and turn to invisible vapor. This is because water is made from molecules.

All solids, liquids and gases are made from atoms and molecules. When we add enough heat, solid iron turns to a liquid, and still more heat changes liquid iron to Iron vapor, which is a gas. The molecules in a substance use heat energy to move faster. Then we add enough heat to liquid water, the molecules move fast enough to separate and fly apart. The liquid water becomes water vapor, which is a gas.

The sun heats the surfaces of the land and sea, and they in turn heat the air above them. On a clear day, the sun heats every square yard of the ocean surface. It supplies enough heat on a summer's day to change one glass of water from each square yard of the ocean into vapor. This amount of heat allows the water molecules to get up enough speed to leave the sea and take to the air as separate molecules of gaseous vapor.

Dry air is thirstier than damp air. On a breezy day, the air near the sea is lifted up and blown away. As it drinks its fill of vapor, the surface air is removed and replaced by drier, thirstier air. Evaporation from the sea and other watery places is greatest on hot, breezy days.

Warm air tends to expand and rise upward. It cools as it expands, and some of its vapor is changed into misty droplets of cloud material. In one minute the hot sun  can evaporate enough water from a square yard of the ocean to form a cloud 10 yards long, 10 yards wide and 10 yards high. fantastic as it may sound, sooner or later this cloud will change back into only one ounce of water and come pelting down in drops of rain.

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