Jim Smith, age 16, of Monterey Park, Calif., for his question:

What causes clouds to form?

Clouds are made of water, either in liquid or solid form. Most of t'f'..em are fine, misty droplets of liquid. Some, the high‑flying, whispy cirrus clouds, are fine fragments of ice. Sooner or later all clouds bear heavy drops of rain, pellets of ice or flakes of snow. For, sooner or later, their moisture must return to the surface of the earth.

The moisture from which clouds are made evaporates from the earth, from the sea and from other watery surfaces. This evaporated moisture becomes vapor, which is a gas, It mixes with other gases of the atmosphere. The atmosphere can hold so much vapor and no more. It is surplus vapor in the air which forms the clouds.

The air, like most things on this luxury planet, dust obey strict physical laws. Warm air can hold more vapor than cool air. Warm air must expand and, as it expands, it thins out and cools itself. Those are the two laws which cause the formation of most clouds.

There is nothing haphazard in the rules about how tech moisture air at a certain temperature can hold. The limit of vapor in the air is called saturation point. And we can measure how many grams of vapor are present in a cubic meter of air. At saturation point a cubic meter of air at 86 degrees Fahrenheit contains 30,4 grams of vapor, At 68 degrees, saturation point is 17,31 grams and at 61 degrees the air is saturated when it contains 13.65 grams of vapor per cubic meter.

411t saturation point, the humidity in the air is 100. No more moisture can be held as vapor. Surplus vapor must change from gaseous to liquid form. It may become misty droplets and a cloud. is born.

Now think of the law which says that warm air must expand and cool itself. The air gets it heat from the earth, so we find the hottest air near the surface. It rises as it expands and this warm air near the surface has drawn up perhaps its whole quota of vapor.

Let's choose a hot summer morning near. the Great Lakes. The air reaches 86 degrees. It soaks up plenty of moisture and reaches saturation point with 30.4 grams of vapor per cubic meter. As it rises it cools, more than five degrees every 1, 000 feet. Let's follow it up almost a mile. The temperature, say, drops to 68 degrees. Air at t1l". temperature is saturated with 17.31 grams of vapor per cubic meter. This, means that 13.09 grams of vapor per cubic meter is now surplus: This surplus changes from gas to liquid droplets and clouds form above the earth.

This is the process by which most clouds form. However, there are other ways by which humid air may be cooled and forced to yield. up its surplus vapor. Air cools when forced. up the side of a mountain. i mass of cool air may under‑run a mass of warm air, forcing its temperature to drop.

When the temperature becomes low enough to leave the air with a surplus of vapor, this surplus forms the clouds.