Greg Lane, age 10, of Visalia, California, for his question:

What are clouds mostly made of?

Perhaps the main ingredient is air, the ordinary air that gets thinner higher above the earth. All the ingredients in a cloud are mixed with air and some cloudy ingredients use molecules of air to create such things as snowflakes. The second main ingredient is moisture. And the moisture in a cloud usually weighs a lot more than its air. All this cloudy moisture can float aloft because it is separated into the tiniest of tiny water droplets.

What we see in a cloud is tons of misty moisture. In an ordinary cauliflower cloud, the misty droplets are very far apart and too small for our eyes to se e, one by one. In a dark rain cloud, zillions of droplets get together and form drops large enough to be seen. Also in a cloud there may be a little salt from the sea, some smoke from factory chimneys and no doubt traces of other pollutants. But always the main ingredients are enormous masses of air and many tons of water droplets.