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The tropopause is the atmospheric boundary between the troposphere and the stratosphere. Going upward from the surface, it is the point where air ceases to cool with height, and becomes almost completely dry. More formally, it is the region of the atmosphere where the environmental lapse rate changes from positive (in the troposphere) to negative (in the stratosphere). The exact definition used by the World Meteorological Organization is:

The troposphere is the lowest of the Earth's atmospheric layers and is the layer in which most weather occurs. The troposphere begins at ground level and ranges in height from an average of 11 km (6.8 miles/36,080 feet at the International Standard Atmosphere) at the poles to 17 km (11 miles/58,080 feet) at the equator. 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. Due to the variation in starting height, the tropopause extremes are referred to as the equatorial tropopause and the 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 in the absence of inversions. In the stratosphere, however, the temperature increases with altitude. 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 (PVU). 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.[citation needed]

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.[citation needed]

The tropopause is not a "hard" boundary. Such oscillation sets up a low-frequency atmospheric gravity wave capable of affecting both atmospheric and oceanic currents in the region.[clarification needed]

Most commercial aircraft are flown below the tropopause or "trop"[citation needed] if at all possible to take advantage of the troposphere's temperature lapse rate. Jet engines are more efficient at lower temperatures.[citation needed]

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