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The tropopause is a boundary region in the atmosphere between the troposphere and the stratosphere.

The troposphere is the lowest of the Earth's atmospheric layers and is the layer in which all which we call "weather" occurs. It begins at ground level and ranges in height from an average of 6 km (4 miles) at the poles to 17 km (11 miles) at the equator. The stratosphere begins at roughly 17 km (11 miles) in altitude, and it may reach as high as 50 km (31 miles) from the earth's surface at the equator. This is also the location of the ozone layer.

Measuring the lapse rate through the troposphere and the stratosphere identifies the location of the tropopause. In the troposphere, the lapse rate is, on average, 9.8°C per kilometre. That is to say, for every kilometre in height, the temperature drops by 9.8 degrees. In the stratosphere, however, the temperature increases with altitude. The region of the atmosphere where the lapse rate changes from negative to positive, ie, where the temperature change moves from decreasing to increasing, is defined as the tropopause.

The tropopause is not a "hard" boundary; even though its net temperature change is zero, it undergoes the transition from maximum negative lapse rate in the troposphere to maximum positive lapse rate in the stratosphere over thousands of metres. Vigorous thunderstorms, particularly those of tropical origin, will overshoot into the lower stratosphere and undergo a brief (hour-order) low-frequency vertical oscillation. Such oscillation sets up a low-frequency atmospheric wave train capable of affecting both atmospheric and oceanic currents in the region.