单选题 Wouldn' t it be great if you could just look up at the sky and read the weather forecast right away? Well, you can. The forecast is written in clouds. If you can read that writing, you can tell something about the atmosphere. With some practice, you can become a pretty good weather forecaster. Who knows, you might even do as well as meteorologists. Meteorologists use much more information than just the appearance of the clouds to make their forecast. They collect data from all over the world. Then they put it into powerful, high-speed computers. This does give the meteorologists an advantage, because they can track weather patterns as they move from west to east across the country. But you have an advantage, too. You can look at the sky and get your data directly. A meteorologist uses a computer forecast that' s several hours old to make a local forecast. What are you seeing when you look at a cloud? "A picture of moisture is doing in the atmosphere," says meteorologist Peter Leavitt. There' s moisture throughout the atmosphere. Most of the time you don' t see it, because it' s in the form of an invisible gas called water vapor. Sometimes, the temperature of the air gets cold enough to cause the water vapor to change to liquid water. It' s called condensation , and we see it happen all the time ( for example, when humid air from the shower hits the cold glasses of a mirror). When enough water vapor condenses, droplets(小水滴) come in the air. These droplets scatter light. A cloud is seen. Watching clouds over a day or two tells you a lot more than a single cloud about the weather to come. Changes in clouds show changes in the atmosphere. You should begin to notice patterns. Certain clouds, following each other in order, can signal an approaching storm. But don' t take our word for it: see for yourself.

单选题 What does "meteorologists" refer to______.

单选题 Meteorologists make their weather forecast by______.

单选题 The word "condensation" (Para. 3) means ______.

单选题 If you want to learn weather forecasts, you' d better______.

单选题 This passage mainly tells us about how to______.