平流层大约从距离地面12公里开始，天气系统例如季风通常在对流层上发生。
The stratosphere begins about 12 kilometres up, above the troposphere where weather systems like the monsoon develop.
平流层的空气一般停留在对流层上空，向两极移动，然后再下沉到高纬度带的低层大气。
Stratospheric air generally rises above tropical latitudes, spreads toward the poles, and then sinks back toward the lower atmosphere at high latitudes.
这是因为对流层的顶端通常极其寒冷，上升的水蒸汽凝结成飘落而下的冰晶，而没有继续上升进入平流层。
This is because the top of the troposphere is normally very cold, causing ascending water vapour to freeze into ice crystals that drift and fall, rather than continuing up into the stratosphere.
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