Without the atmosphere, there would be no weather, no wind, rain, snow, or clouds. Air is all around us; we live at the bottom of a great ocean of air. It is invisible, but we can feel it when it moves. There are miles of all above us, pressing down with great weight on the earth and everything on it. Because air presses down on us from all directions at the same time, and because we are able to bear (忍受) this weight, we do not seem to feel it. But scientists have proved that air has this weight, and that anything that has weight creates (产生) a force called pressure. As changes in air pressure take place, they make air move.
Air is a gas that expands (膨胀) when heated, gets lighter, and moves upward. When air is cooled, it gets heavier, sinks close to the earth's surface, and flows like water in a great river. As warm air rises, cold air rashes to take its place. Thus winds originate (起源). The winds that blow high above us are caused by the warmer air running away from colder air. The wind we feel near the earth's surface is the heavy colder air trying hard to catch the warmer air. Changes in temperature cause the air to move. And of course there are many changes, so air movements are taking place practically all the time.

单选题 The first paragraph mainly tells us ______.

单选题 Air moves under such conditions that ______.

单选题 Which of the following pictures best tells the movements of warm air and cold air?

单选题 According to the passage, we can conclude that in some areas if farmers who grow grapes light fires in the early morning, that is because the farmers want to ______.