Al Norton, age 8, of Huntsville, Alabama, for his question:

Why doesn't heat rise and warm the clouds?

They tell us that warm air rises up from the ground, and so it does. Then surely it should warm the clouds and melt the frosty white snow on top of the mountains. But it doesn't. A few miles above the ground the air is very cold, even when people down here are sweltering in the summer heat. The clouds are very cool, especially those curly white feathers that fly so high in the sky. So something must happen to all the heat that rises up from the surface of the earth. And so it does.

This is what happens. The air is warmed when it touches the warm land and sea. Air, as we know, is made of gases. And gases have their own ways of doing things. When they become warm, they spread out and get thinner. As they get thinner, they lose their heat and become cooler. The warm air near the ground expands and rises aloft, growing cooler as it goes. When it gets as high as the clouds, it has no heat left to warm them.