2011年英语四级阅读备考材料（13）_第3页

The freezing point of water is 32 degrees Fahrenheit. If the temperature should be below freezing when condensation takes place on the grass or on the ground, dew will not form. Instead of growing into dewdrops, the water molecules become tiny crystals of ice. A mass of ice crystals that form in this way is known as frost. It is important to remember that frost is not dew which has frozen. The frost particles form directly from the molecular state when the temperature is sub-freezing. Dew formation and frost formation are essentially the same. The only difference between them is the temperature at which each takes place.

　　Rain and Snow. You will remember that the particles which make up a cloud are very tiny. In fact, the water droplets are so small they float about without falling to the ground. At times, however, these water particles tend to get larger. This may happen when several droplets join with others over and over again. Or, growth may occur if the process of condensation continues to take place. In either event, the original droplet may grow a million times its size into a full sized drip. Of course, the air can no longer keep such large drips afloat and precipitatio'n takes place in the form of rain.

　　1. Precipitation can be controlled to transform deserts into productive lands.

　　2. Rain, snow, sleet and hail are different forms of precipitation.

　　3. The passage is mainly about precipitation processes and forms of precipitation.

　　4. Evaporation is the process of water molecules escapes from the surface of lakes, oceans, rivers or tanks and getting into the atmosphere.

　　5. A fog is a mass of moist air falling to the earth.

　　6. Dew is different from frost in that the former takes place on the cool blades.

　　7. Rain forms when clouds become thick and moist enough.

　　8. The maximum amount of vapor in the air is known as______.

　　9. Water droplets in the air are too small and light to be affected by______and therefore

　　they are afloat miles above the earth.

　　10. A cloud is identical to a fog in______.

　　I. NG 2. Y 3. Y 4. Y 5. N 6. N 7. N 8. moisture saturation 9. gravity 10. composition