英語(yǔ)翻譯
Eventually this equatorial air mass ceases ti rise and spreads north and south.This high-altitude air is dry,since the moisture it once held fell as tropical rains.As this air mass flows north and south,it cools,which increases its density.Eventually it sinks back to the earth's surface at about 30 latitude and spreads north and south.This air draws moisture from the lands over which it flows and creates deserts in the process.
\x05Air moving from 30 latitude toward the equator completes an atmospheric circulation cell at low latitudes.As figure 2.5b shows,there are three such cells on either side of the equator.Air moving from 30 latitude toward the poles is part of the atmospheric circulation cell at middle latitudes.This warm,moist air flowing from the south rises as it meets cold polar air flowing from the north.As this air mass rises,moisture picked up from desert regions at lower latitudes condenses to from the clouds that produce the abundant precipitation of temperate regions.The air rising over temperate region spreads northward and southward at a high altitude,completing the middle-and high-latitude cells of general atmospheric circulation.