These get so heavy that they roll down the glass. They roll down the walls, too, and drop from the ceiling. If vapor is not turned out of doors it makes a room damp. That is just what happens when it rains. The vapor in the air goes up into the sky. When it finds a cold layer of air it rolls into large drops and falls.
Vapor is always going up. Most of it goes up from the ocean, the lakes and rivers. Three fourths of our big world is covered with water, you know. The sun warms the top layer of water and turns it into vapor. Every leaf and blade of grass on the land has water in it. The sun steals this water, too. Sometimes it takes so much and gives so little back again that the grass turns brown, and the leaves wilt. Every animal and plant drinks water and breathes it out again from the lungs, or gives it to the air through little holes in the skin called pores. You know how you perspire on a hot day? Little beads of water ooze out, all over you. You can find your pores with a mag-ni-fy-ing glass. You can find pores in green leaves, too.
Plants and animals perspire more on a hot day than on a cold one. The land and water give off more vapor in the summer than in the winter. Wring a handkerchief out of hot water, hang it in the sun and see how quickly it dries. Set a shallow pan of water in the sun and see how soon the water disappears. The air is always thirsty. It drinks like a greedy fish.
But then, it is not stingy. It gives back every bit of water it gets. But it does not always give it back where it got it. Sometimes it rises in vapor from the ocean, goes up in the sky to a layer of cold air, and falls back into the ocean almost as quick as you can say Jack Robinson. But it's a very good thing for little boys and girls and trees and bees, that all the vapor doesn't do that. The ocean doesn't need the water rightaway, and the land does. A great deal of vapor goes on long journeys. It uses the wind for horses. Haven't you seen fleecy clouds floating across the sky? They were riding on the wind. The winds find it no trouble at all to carry these vapor clouds along with them. The vapor clouds travel until they strike a cold layer of air. Then they roll into rain drops and fall.
As oceans are the great vapor tanks, so mountains are the chief rain makers. The tops of mountains are very cold, and they are so high up in the air that the vapor clouds bump right into them and turn to rain. In the winter the air is so cold that the rain, in falling, freezes into snow. Wide river valleys, islands and sea coasts, get a great deal of rain and snow, too, if the winds blow over them
