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enter image description here

See this picture from the book Hartmann, Dennis L - Global Physical Climatology.
There is a minimum temperature point of 200K at 15km height above the equator, in June/July/August.

I don't know why this happens. Can anyone explain this?

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That's because the tropopause, the boundary between the troposphere and stratosphere, is highest in equatorial regions. The high amount energy from the Sun in equatorial regions makes for a lot of convection, which in turn pushes the tropopause to about 16 km above the surface (and sometimes to 18 km) in those equatorial regions. Near the poles, sinking air and reduced solar intensity makes the tropopause much closer to the surface, where it is only 8 km or so high.

The lapse rate in the upper troposphere is roughly the same as the dry adiabatic lapse rate, 9.8 °C/km. The 8 km difference between the tropopause height between equatorial and polar regions results in about 80 degrees more cooling in equatorial regions than polar ones.

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  • $\begingroup$ It may be worth noting that the stratosphere is warmer than the tropopause. $\endgroup$ – farrenthorpe Mar 14 '17 at 22:07
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    $\begingroup$ Thank you for this comment. You mean that the height of tropopause varies from the convection. That's interesting for me. $\endgroup$ – kayak Mar 15 '17 at 1:19
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    $\begingroup$ There also be a minimum point at height 25 km in polar regions. This is higher than the tropopause of the equatorial regions. This is opposite to your answer. $\endgroup$ – kayak Mar 15 '17 at 1:46
  • $\begingroup$ @kayak -- The nature of the stratosphere, troposphere, and the boundary between them in polar winter is rather complex. I didn't want to get into that in this answer because you asked a simple question for which there is a simple answer. The complex answer would be the subject of another question. $\endgroup$ – David Hammen Mar 15 '17 at 17:03
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    $\begingroup$ @kayak -- No, not by any definition of the tropopause (temperature, dynamic, or ozone). The temperature-based definition is where the lapse rate falls below 2 °K/km rather than where the temperature reaches a minimum. That concept not very useful in Antarctic winter as the tropopause inversion layer is hard to find or non-existent. The engine that drives the tropopause inversion layer is the ozone cycle, which is inactive during polar winter. This is particularly so in the Antarctic, whose winter atmosphere is isolated from the rest of the world by a very long-lived polar vortex. $\endgroup$ – David Hammen Mar 16 '17 at 12:06

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