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I have seen the term "Tropopause height" used in a flight navigation plan. What is it and how is it determined?

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    $\begingroup$ It is not calculated. It is forecast. $\endgroup$ – Jan Hudec Dec 1 '15 at 12:44
  • $\begingroup$ Maybe it refers to the pressure altitude which is 30 thousand feet MSL. ? $\endgroup$ – Gürkan Çetin Dec 1 '15 at 19:31
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    $\begingroup$ @GürkanÇetin, no, it does not. See the answers, they are correct. $\endgroup$ – Jan Hudec Dec 1 '15 at 20:57
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According to Wikipedia, the World Meteorological Organization defines tropopause as

The boundary between the troposphere and the stratosphere, where an abrupt change in lapse rate usually occurs. It is defined as 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. Occasionally, a second tropopause may be found if the lapse rate above the first tropopause exceeds 3 ˚C/km.

The tropopause height varies with latitude and season. The variation with latitude, based on annual mean conditions, is shown below:

Troposphere height

Image from das.uwyo.edu

Troposphere can be identified only if the lapse rates (the rate at which atmospheric temperature decreases with increase in altitude) in the stratosphere and troposphere are known. Also, weather phenomena like thunderstorms shift the tropopause. As a result, the height of the tropopause is forecast based on localized weather conditions, and is not a fixed boundary.

The International Standard atmosphere, however, defines tropopause as lying between ~11,000 and 20,000 meters (geopotential altitude) above MSL.

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  • $\begingroup$ It is not strictly true that thunderstorms shift the tropopause. They can, in severe cases, burst through it: But they do not move it. $\endgroup$ – os1 Jan 11 '16 at 9:31
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The tropopause height varies with latitude and season. Its height can be determined by looking at an atmospheric sounding as temperatures in the stratosphere increase with height. This also means there is a strong static stability gradient at the tropopause. The height can also be defined by a specific value of potential vorticity units (e.g. the 1.5 PVU surface).

Here is a current sounding from a balloon launched this morning at 12Z: enter image description here

Here you can see a sharp transition in temperature lapse rate at about 200 mb between 11 and 12 km height. Where the temperatue begins to rise with height (the kink around 200mb) is where the tropopause is located.

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    $\begingroup$ It might be interesting to add what is it useful for (I know it has something to do with optimal cruise level, but not details). $\endgroup$ – Jan Hudec Dec 1 '15 at 20:58
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    $\begingroup$ @JanHudec: This answer explains why the tropopause is an optimum altitude. $\endgroup$ – mins Dec 1 '15 at 22:21

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