Thomas Gibson, age 12, of Asheville, North Carolina, for his question­:

What is water vapor?

Imagine a gallon of water floating around in the air. Impossible? Not at all. Clouds are made of tiny droplets of water so small that they can float aloft. These droplets are so small that a room full of cloud would contain only enough liquid water to fill a thimble. But cloud mist is not vapor. The droplets in a cloud are giants compared with the tiny particles of water vapor.

Water vapor is a gas. We cannot see it, touch it or smell it. Yet liquid water, solid ice and gassy water vapor are all made of the same tiny particles. Water is a compound of two parts hydrogen atoms and one part oxygen atoms. Each tiny particle contains two atoms of oxygen and four atoms of hydrogen, linked together in a special way. This particle is the building block from which water, ice and water vapor are made In a single drop of water there are countless trillions of these tiny particles, all alike.

At ordinary temperatures the water particles have a strong attraction for each other. They cling together like a line of skaters playing crack‑the‑whip. When one particle floats along, a string of others follows, clinging to its coat tail. This is what makes water wet. The particles of liquid water are always on the move. But, once in a while, a single particle gets up enough speed to break away from its fellows. This happens when water is heated. The heat gives more energy to the water particles and more energy means more speed. The particles, on the surface are bombarded from below and shaken loose. These freed particles become gas and mingle with the other gasps in the air. The evaporating water becomes vapor.

There is always some water vapor in the air around us, even over an arid desert. But the atmosphere can hold dust so much water vapor anal no more. In the hot, moist tropics, up to five percent of the air may be water vapor. But usually the proportion is much, much less. Warm air can hold more water vapor than cool air. And the atmosphere is foreve changing its temperature. It may blow hot over the sea, in which case it will soak up a lot of evaporated water vapor. Sooner or later this hot air will expand. It will rise aloft and spread out. And, as it expands, it pools itself.

The vapor it carries is now too much. The atmosphere can no longer carry it as gas. The gaseous water particles are squeezed together into droplets of liquid water. A cloud is born. It rides the breezes, slowly, slowly sinking. Sooner or later its misty droplets become too heavy to stay aloft. They become rain drops and fall down to water the earth.

Every day tons of water becomes gaseous vapor. And every day tons of this vapor gel into raindrops. This merry‑go‑round is the life giving water cycle, which supplies all the thirsty plaints and land animals.