Scott Collins, age 13, of Barre, Vt., for his question:
WHAT CAUSES RAIN?
Rain is water that falls out of clouds in drops. But the formation of rain depends upon a number of processes of nature. First there is evaporation, as moisture is taken up into the air from the earth's surface, especially from the warmer parts of the ocean. The moisture, called water vapor, cannot be seen.
Water vapor is mixed with the other gases in the air and is carried upward by the wind. The moisture loaded air cools as it rises. This is because the air expands as it rises.
Once the air starts to rise it will continue to do so until its temperature is the same as that of the air surrounding it. When the temperature is equalized, the upward movement of that mass of air is halted.
As air rises and cools, the amount of water vapor it can hold decreases. If the rising and cooling continues long enough, the air will become saturated. If the air is then cooled below that point, it is said to have reached the dew point.
When air reaches the dew point, some of the water vapor the air contains condenses into tiny particles of water, so fine that they might be called water dust. This water dust is known as clouds or fog, according to whether it is high in the air or near the ground.
A still greater cooling of the air will cause the tiny cloud particles to unite into drops so large and heavy that they fall.
Floating particles in the air, called condensation nuclei, are very important in rain formation. The condensing vapor collects on these tiny particles. But the basic condition necessary for the formation of rain is the lowering of the temperature to a point where the air can no longer hold all the moisture in it.
The rate of rainfall within a certain period of time, usually an hour, determines the intensity of rain. Rain may fall as a trace, as a light rain, a moderate rain or a heavy rain.
A trace of rain means that the rainfall is so slight that it cannot be measured. A light rain is a rain that measures from a trace to .10 inch per hour. A moderate rain falls at a rate from .11 to .30 inch per hour and a heavy rain falls faster than .30 per hour.
The tropics generally have at least 100 inches of rain each year. The highest rainfall for one year in any area was 1,041 inches in Cherrapunji, India. The highest in one spot was over 600 inches on Mount Waialeali, Kauai, Hawaii.
The lowest rainfall ever recorded over a long period was three one hundredt hs of an inch in Arica, Chili, which has maintained this average over a period more than 60 years.
The chief causes for variation in rainfall are location, altitude, stance from the sea and character of the land. On slopes that are exposed to ocean winds, rainfall is generally abundant. But regions shut off from the sea by high mountains are often almost rainless.
Raindrops are often pictured as being shaped like a teardrop. But raindrops are always perfectly round. The pull of gravity on the falling raindrop does not change this round shape into a teadrop shape.