John Russell Jr., age 12, of Casper, Wyo, for his question:
JUST WHAT IS EVAPORATION?
Evaporation is the process which changes a substance from the liquid or solid state into a vapor or gas. The warmer and drier the air, the pore rapidly this process of evaporation goes on.
We generally use the word evaporation when the process occurs at everyday temperatures and use other words, such as boiling or vaporization for higher temperatures.
Water left standing in an open dish in a warm room will soon evaporate. Damp clothes hung on a line in the sunshine will lose all their moisture in a short time. Heat in the air changes the water in the dish and the clothes to water vapor.
In warm weather, the air may contain a great deal of vapor. At 90 degrees Fahrenheit there may be almost twice as much as at 70 degrees. Air contains little vapor at the freezing point. But some moisture always remains in the air.
Nothing can live in moisture free air. When air has all the vapor it can hold at a given temperature, it is saturated, or at dew point. This seldom happens, except when it rains.
Air that contains much moisture is said to be humid. But humid air usually has much less moisture than the amount needed to saturate it.
The humidity is extremely high in tropical regions, especially near the sea, while in cooler climates it decreases greatly. A person can stand much more cold when the air contains only a little vapor. This explains why cold winds that blow over water seem more biting than winds farther inland, even though the temperature is the same.
Water evaporates from oceans, lakes, rivers, ponds and the moist earth, and then later falls as rain on these areas. Clouds often show the humidity is increasing. If the cloud area becomes smaller, the air probably is becoming drier.
Evaporation in plants is called transpiration. Plants absorb moisture through their roots and lose it by evaporation through their leaves, or transpiration.
The more leaf surface exposed, the more rapid is the transpiration. The structure of such leaves as pine needles prevents rapid transporation.
Evaporation takes place from the surface of a liquid at any temperature. It also takes place from solids, for even ice and snow send off vapor. This can often be noticed a day or two after a snowstorm. The snow disappears, even though the temperature has never gone above freezing. The water evaporates directly from the solid without first becoming a liquid. Formation of vapor in this way is called sublimation.
When a liquid evaporates from the surface of something, that surface becomes much cooler because it requires heat to change a liquid into a vapor or gas. An electric fan cools us off because its current of air makes perspiration evaporate quickly. The heat needed for this evaporation is taken from our own bodies.
The same rule works when we sprinkle the sidewalk on a hot day. As the water evaporates, it uses up heat and cools the air above the walk.