Daily changes in the weather affect all of us, no matter what our job or profession. Howev

er,【B1】there's very little we can do to modify the weather, we can at least know what kind of weather to expect. For this we are indebted(感激的) to our【B2】weather-watchers.

Although it may sound funny to call scientists weather-watchers, there's【B3】funny about what they do. Specifically, weather-watchers are meteorologists(气象学者)【B4】work for the United States Weather Bureau. They study the ocean of【B5】that surrounds the earth and, with the information they get, predict our weather.

The atmosphere【B6】in height from 300 to 700 miles above the earth and is divided into a series of layers or shells. The layer that is most interesting【B7】the meteorologist is the troposphere(对流层) or the layer closest to the earth.【B8】is in the troposphere that all our weather is formed.

【B9】causes our weather is really a very simple process. Endless streams of hot air push up from the earth. As they rise, they are【B10】by the freezing temperatures of the【B11】troposphere. Once it has become cold and heavy, the air【B12】back toward the earth, but it【B13】move down because of the rising hot air. In the meantime, planetary winds, which blow around the earth, or air currents keep both the cold air mass as well as the hot air mass in【B14】.

There are two additional factors that influence these moving air masses.【B15】is water vapor that the rising hot air soaks up like a sponge. The other is temperature Which, by cooling the hot air, forces it to【B16】out its water vapor in the form. of rain, snow, or sleet(雨夹雪).

These air masses【B17】push against each other with tremendous pressure. When a high-pressure mass of air succeeds【B18】pushing out a low-pressure air mass, we have clearing conditions and【B19】weather. When a low-pressure air mass forces out a high-pressure mass of air, stormy weather can be【B20】.

【B1】

A．when

B．as

C．while

D．if