托福阅读：World climatic pattens

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托福阅读：World climatic pattens
　Climate is the general pattern of atmospheric conditions, seasonal vanations, and weather extremes in a region over a period of decades. One major factor determining the　uneven patterns of world climates is the variation in tfie amount of solar energy striking　different parts ofthe earth. The amouni: ofincoming solar energy reaching the earth's　surface varies with latitude, the distance north or south from the equator. Air in the　troposphere is heated more at the equator (zero latitude), where the sun is almost directly overhead than at the high-latitude poles, where the sun is lower in the sky and strikes the　earth at a low angle.
　　The large input of heat at and near the equator warms large masses of air. These warm masses rise and spread northward and southward, carryirtg heat from the equator toward　the poles. At the poles, the warm air becomes cool and falls to the earth. These cool air　masses then flow back toward the equator near ground level to fill the space left by rising　warm air masses. This general air circulation pattern in the troposphere results in warm average temperatures near the equator, cold average temperatures near the poles, and　moderate average temperatures at the middle latitudes.www.examｗ.com
& y* b: ^; L$ D0 `4 u　　The larger input of solar energy near the equator evaporates huge amounts of water from the earth's surface into the troposphere. As the warm, humid air r:ises, it cools rapidly and ioses most ofits moisture as rain near the equator. The abundant rainfall and the　constant warm temperatures near the equator create the world's tropical rain forests.
　　Two major factors cause seasonal changes in climate. One is the earth's annual orbit around the sun; the other is the earth's daily rotation around its tilted axis, the imaginary　Iine connecting the two poles. When the North Pole leans toward the sum the sun's rays　strike the Northern Hemisphere more directly per unit of area, bringing summer to the　northem half of the earth. At the same time, the South Pole is tilted away from the sun;　thus, winter conditions prevail throughout the Southern Hemisphere. As the earth makes　its annual rotation around the sun, these conditions shift and cause a change of seasons.
　　As the earth spins around its axis, the general air circulation pattern between the equator and each pole breaks into three separate belts of moving air, or prevailing surface winds, which affect the distribution ofprecipitation over the earth.