Jeffrey Marsh, age 10, of Lansing, Michigan, for his question:

How can a 40 ton cloud float?

If all that water were in one big gob, it would come pouring down like a falling stone. But it floats aloft in tiny round droplets. However, the water in a misty cloud cannot float on high forever. Its tiny droplets are sinking, slowly sinking. Sometimes an updraft of rising Air helps to hold up a wet cloud, but sooner or later that moisture must come down.

Actually, a 40 ton cloud is a skinny little wisp. A young thunderhead may hold 4,000 tons of water several miles above the ground. A large storm cloud may float aloft with 300,000 tons of water. The average cloud is a misty mixture of droplets suspended in the air. We need microns to measure these ball shaped droplets of water. There are 1,000 microns in a little millimeter and almost a million microns in a yard.

Such tiny spheres do fall through.the air, but they sink more slowly than other falling objects. Someone figured out the slower falling rates for micron sized cloud droplets. Most of them average about five microns in width, but the assortment in a cloud ranges from half a micron to 260 microns. For the midgets, the falling rate is six microns per second and it takes them about an hour to sink down an inch. Under normal conditions, the largest drops fall nearly as fast as stones. But average cloud droplets, five microns wide, fall at 0.7 millimeters per second, and take about an hour to fall ten feet. Most clouds form several miles above the ground, when gaseous vapor changes to droplets of liquid water. It may take a floating cloud several days to sink this distance down through the air.

However, a friendly updraft from below may hoist it even higher, or at least delay its sinking droplets. Meantime, the temperature and other changing weather conditions may remodel the misty cloud. Zillions of droplets may clog together and form large heavy drops of rain. Then down splashes the watery rain. If the air gets warmer, it may evaporate the liquid droplets into gaseous vapor. Then the cloud disappears into thin air. While it lives, the floating cloud, with its zillions of spheres of water, is blown along by the breezy winds. During its lifetime, it may carry hundreds of tons of water for hundreds of miles over land and sea.

It is hard to grasp the small sizes and the enormous numbers of droplets in an average cloud. Magnified 200 times, they look like large, medium and small pinheads. In a thimbleful of misty cloud material there are 70 to perhaps 700 of these droplets in assorted sizes    with plenty of space between them. The water content in a shoebox full of misty cloud may weigh as much as an aspirin. But even a small cloud has enough of this moist mist to fill a box a mile wide on every side. Its tons and tons of water can float on high because it is separated into tiny spheres that sink slowly, very slowly through the air.