A、It’s defined as the result of the moisture in the Earth’ s atmosphere. B、It’ s defined as the result of the Earth’ s rotation.

We only have a few minutes left so I’d like to go over a couple of points before we move on. Remember that, although there are both horizontal and vertical movements in air. The term wind applies only to horizontal movements. And more air is involved in horizontal movements than vertical movements. And what causes the horizontal movements? Alternately, it’s the solar radiation, because the unequal heating of the earth than the atmosphere produces horizontal differences in air pressure. These differences set winds in motion. Essentially, winds are the nature’ s way of balancing uneven distribution air pressure over the earth. Secondly, let me repeat my answer to the question we had before about wind direction. Many people get confused by what they hear in weather forecasts. We talk about wind direction in terms of where the wind’ s coming from, not where it’ s blowing to. There is a reason for this. To weather broadcasters, the origin of wind is more important than its destination. The winds origin helps them predict the weather. Logically, in the northern hemisphere, a north wind tends to bring cold weather, south wind warm weather. I haven’ t forgotten vertical movement of air, but we don’ t have time today to talk about them in depth. In our next class then, I’ll begin by discussing updraft and downdraft and how they affect the wealth. I suspect most of you can guess which of the two brings warm which weather, and which brings cold.
Now listen to the following recording and answer questions 16 to 18.
16. According to the speaker, how’ s the wind defined?
17. Why does the speaker mention solar radiation?
18. According to the speaker, which weather broadcast can be confusing?