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The tropopause is a boundary region in the atmosphere between the troposphere and the stratosphere. Here the air ceases to cool at -50°C, and the air becomes almost completely dry.

The troposphere is the lowest of the Earth's atmospheric layers and is the layer in which all of what 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. 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. This is also the location of the ozone layer. It is at its highest level over the equator and the lowest over the geographical north pole and south pole. On account of this, the coolest layer in the atmosphere lies at about 17 km over the equator. There are two types of tropopauses, viz. equatorial tropopause, and polar tropopause.

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, 6.5 °C per kilometre. That is to say, for every kilometre in height, the temperature drops by 6.5 degrees Celsius. In the stratosphere, however, the temperature increases with altitude. The region of the atmosphere where the lapse rate changes from positive (in the troposphere) to negative (in the stratosphere), ie, where the temperature no longer decreases with altitude but rather increases, is defined as the tropopause. The exact definition used by the World Meteorological Organization is:

the lowest level at which the lapse rate decreases to 2 °C/km or less, provided that the average lapse rate between this level and all higher levels within 2 km does not exceed 2 °C/km.

Alternatively, a dynamic definition of the tropopause is used with potential vorticity instead of vertical temperature gradient as the defining variable. There is no universally used threshold: the most common ones are: the tropopause lies at the 2 PVU or 1.5 PVU surface. PVU stands for potential vorticity unit. This threshold will be taken as a positive or negative value (e.g. 2 and -2 PVU), giving surfaces located in the northern and southern hemisphere respectively. To define a global tropopause in this way, the two surfaces arising from the positive and negative thresholds need to be joined near the equator using another type of surface such as a constant potential temperature surface.

It is also possible to define the tropopause in terms of chemical composition. For example, the lower stratosphere has much higher ozone concentrations than the upper troposphere, but much lower water vapor concentrations, so appropriate cutoffs can be used.

The tropopause is not a "hard" boundary. Vigorous thunderstorms, for example, 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.