# Figuring out the temperature at the tropopause

Either I am misunderstanding something, or reading conflicting information. Likely the former.

First off, the tropopause is defined as the boundary between the troposphere and the stratosphere. That much seems simple enough. It is also variable in altitude; further up around the equator, than over the poles.

Now as I understand it, the temperature in the stratosphere is a constant -56.6C as per ISA. This seems correct. Every aircraft I've flown on has reported temperatures around that mark at altitude, give or take for ISA deviation.

What confuses me is a question that asks what the height and temperature of the tropical tropopause is. The correct answer is 16Km and -75C. I'm not sure why this is supposed to be correct.

I thought I'd try using ISA and the environmental lapse rate of 1.98C per 1000 ft. 16Km is ~52 000ft, so if ISA temperature on the ground is 15C, then I can easily calculate using:

15 - 1.98 * x = y
x = (52000 / 1000) = 52
15 - 1.98 * 52 = y


However, the result is -88C; a significant difference from the supposedly correct -75C, and even further away from ISA's -56.6C.

As per the (Environmental Lapse Rate)ELR formula, if I wanted to find at what altitude -56.6C should be:

15 - 1.98 * x = -56.6
-1.98 * x = -56.6 - 15 = -71.6
x = -71.6 / -1.98 = 36


So according to this, the average height of the troposphere should be 36 000ft. This makes sense. Yet substituting -75C gives me:

15 - 1.98 * x = -75
-1.98 * x = -75 - 15 = -90
x = -90 / -1.98 = 45


So even here, a -75C tropopause should occur at 45 000ft instead of 52 000ft.

What am I missing?

• universalweather.com/blog/… I think it is definition of ISA (International Standard Atmosphere) what should be tropopause altitude and temperature. – Harish Feb 7 '19 at 9:50
• I don't have enough info to give a full answer, but some of the confusion is from trying to use the constant ISA lapse rate everywhere, when other temperature profiles may be more applicable to the tropics (see e.g. the wikipedia entry en.wikipedia.org/wiki/International_Standard_Atmosphere). Plugging in the constant ISA lapse rate for large altitudes or colder temperatures will give you answers that don't fit well with the exact "tropopause = -56.6C" ISA definition. It'd be useful to know more about the tropical tropopause definition you're using and what assumptions are being made. – dplmmr Feb 7 '19 at 16:30