2011年英语四级阅读备考材料（13）

　　The dictionary defines the word precipitation as a "falling down", and when the meteorologist uses this word with regard to the weather he is speaking of the rain, snow, sleet, and hail which fall down to earth from the atmosphere. All of these examples of precipitation have something in common. In every case what falls to the earth is water in one form or another.

　　Evaporation. In order to understand why precipitation occurs and to appreciate some of the similarities and differences in the various types of precipitation, we must first find out how water gets into the atmosphere. This will also help us to understand some of the other weather elements like dew and frost, and clouds and fog.

　　Of course you have seen water disappear from a wet street or from the surface of a fish tank. We call this disappearance evaporation. Why does water evaporate and where does the water go?

　　Scientists know that water, like all other substances in the universe, is composed of countless billions of tiny particles known as molecules. The molecules of water are so small they cannot be seen with even the most powerful electron microscopes ever devised. There is ample proof of their existence, however, and we also know that these molecules are constantly darting about and colliding.

　　Now, consider the water in an aquarium. Throughout the tank, water molecules are speeding about and bumping into each other, The constant collisions prevent the tiny particles from getting very far. At the surface of the tank, however, many molecules can escape freely into the air. This is the process of evaporation and it is through this process that water is constantly entering the atmosphere. The molecules of water are, of course, too small to be seen, and so we speak of trie water in the air as an invisible vapor. When the temperature increases, the speed at which the molecules move also increases. Faster-moving molecules escape more rapidly and evaporation is speeded up when it is warm.

　　The sun's rays, heating down on the oceans, lakes, and rivers of the earth, cause the surface water to evaporate and enter the surrounding atmosphere. When the amount of vapor is great, we feel uncomfortable and say that it is humid. At any particular temperature the air can hold only a certain amount of water vapor. If the air is holding a maximum amount of vapor, no more water can evaporate. The meteorologist also says that the humidity at such a time is 100%.