“The heat of the sun,” said Kate, “causes a vapor to go up into the air from the ocean, and lakes, and rivers, and from everything that contains water. This is called evaporation. You can’t see this vapor, as it flies away into the air, but when the atmosphere grows cold, it forms clouds, and falls in rain.”
“I should think the vapor would all dry up, and be lost, when the air is so warm,” said Ronald.
“What do you mean by drying up?” inquired Marcus.
“Why, you know,—I mean drying up,—I can’t think of any other way to explain it,” replied Ronald.
“When the water in a puddle dries up,” said Marcus, “it flies into the air, in the form of a vapor, and that is evaporation. That is all the drying up there is about it. The air steals the water from the puddle, and then keeps it a close prisoner till the cold releases it. The water doesn’t dry up again in the air, but remains there. The warmer the air, the more water it will hold. In the tropics, where they have such fierce heats, the air is always full of moisture, and the plants draw it out by means of their large leaves, and so they manage to flourish the year round, although they have no rain or even clouds for months in succession. It is so with us, on a very sultry day,—there is more water than usual in the air, at such a time, although we cannot see it. Now, Kate, can you explain why this vapor which heat produces, flies away to the region of the clouds?”
“Because it is lighter than the air,” replied Kate.
“Right,” said Marcus.
“You said you couldn’t see this moisture rise from the earth,” said Aunt Fanny, “but that is not always the case. We see it in our breath, on a cold day, when it looks like steam issuing from our mouths. I have seen a river steaming as though there were a fire under it, in a very cold day, before ice had formed over it. We see this process going on, too, in the vapors or fogs which often collect over ponds, and rivers, and the ocean. But commonly, as Kate says, we see nothing of these vapors until they are condensed into clouds by the cold air above, although they are continually flying off from our bodies, and from the ground, and every thing that grows in it. When we hang out our clothes to dry, after washing them, the water in them goes to help make clouds.”
“There is one other agent in this business, that has not been mentioned,” said Marcus. “The sun draws the water, the atmosphere holds it as in a sponge, and the cold squeezes the sponge and returns the water to the earth. But the rain is not needed where it is first collected—it must be transported to distant parts of the earth; and how is this done?”
“By the winds,” replied Oscar.