Answer:
The troposphere undergoes circulation because of convection. Convection is a mode of heat transfer wherin the heat moves with the material. Warm air, becuase it is less dense than cooler air, rises and cold air sinks back toward the surface. Convection in the atmosphere is mainly the result of the fact that more of the Sun's heat energy is received by parts of the Earth near the Equator than at the poles.
Thus air at the equator is heated reducing its the density. Lower density causes the air to rise. At the top of the troposphere this air spreads toward the poles.
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If the Earth were not rotating, this would result in a convection cell, with warm moist air rising at the equator, spreading toward the poles along the top of the troposphere, cooling as it moves poleward, then descending at the poles, as shown in the diagram above. Once back at the surface of the Earth, the dry cold air would circulate back toward the equator to become warmed once again.
Areas where warm air rises and cools are centers of low atmospheric pressure. In areas where cold air descends back to the surface, pressure is higher and these are centers of high atmospheric pressure.
