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