What is Tropopause height, 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&nbsp;˚C/km or less, provided that the average lapse rate between this level and all higher levels within 2&nbsp;km does not exceed 2&nbsp;˚C/km. Occasionally, a second tropopause may be found if the lapse rate above the first tropopause exceeds 3&nbsp;˚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.