The troposphere extends from the ground to the tropopause (the boundary between the troposphere and the stratosphere). Temperature in the troposphere decreases with height, because most of its heat comes off the surface of the earth (which absorbs heat from the sun), and because as air rises to elevations with lower pressure, it expands, and the expansion lowers its temperature. Meanwhile, in the lower stratosphere, temperature is fairly constant. The tropopause can be thought of as the height at which temperature stops decreasing.
On average, the tropopause is 10-12 km (6.2-7.5 miles) above Earth's surface, but it can range from 7 km to 18 km (4.3 - 11.1 mi). It is higher during the summer than in the winter and higher at the equator than at the poles. The temperature at the top of the troposphere is often around -50C (-58F) but it can get as cold as -80C (-112F) over Antarctica in the winter.
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