At altitudes from 12-20 km (7-12.5 mi) above the midlatitudes the temperature remains constant with altitude (minus 56.5°C in the ISA). This part is usually considered to be in the stratosphere, but is there a special reason for that, or wouldn't it matter if the tropopause was set at the upper boundary of the isothermal layer rather than the lower one? There's another isothermal layer, the stratopause at 48-52 km (29.8-32.3 mi), here we set the stratopause simply at its center, to 50 km. Why isn't the tropopause set to the center of its isothermal layer as well, but on its lower boundary?
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$\begingroup$ Guess (without any official background): traditionally the focus is on the troposphere for weather... so where a sudden change occurs, that's the boundary. The EL, the rough upper limit for convective storms, is around the tropopause. So traditional focus kind of comes through, as it does in many phrases\geography\etc in the world... surely there was tons of attention given to the troposphere before it even became widely known there was a stratosphere? So where it ended was what mattered most when setting the boundary? $\endgroup$– JeopardyTempestCommented Nov 24, 2021 at 16:34
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$\begingroup$ Could also argue that temperature decrease is thermodynamically the expectation, and so all the regions it doesn't is attributed to the alternate cause (ozone layer)... and the isothermal mixed layer belongs to that as it's still a standout. As to why the stratopause would be defined in the middle instead, #shrugshoulders#, but I'd say up there gets less focus. Could well just be though that the troposphere has the exception because it's how we defined it before we ever set about laying out conceptual layer boundaries, and otherwise the middle roughly makes sense more. $\endgroup$– JeopardyTempestCommented Nov 24, 2021 at 16:36
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$\begingroup$ Above the tropopause there's still the jetstream and Cumulonimbus clouds can go higher than 60,000 ft above sea level, but I too think it's just traditional convention. $\endgroup$– GiovanniCommented Nov 24, 2021 at 16:49
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$\begingroup$ Maybe I should've noted... the tropopause height varies with location (higher in warm regions). The jet stream is in the troposphere beneath the tropopause (see this Wikipedia\NWS image), and as far as I know cumulus also roughly stay in the troposphere, perhaps managing just a bit above the tropopause, except in the case of overshooting tops... the level where updrafts begin to decelerate rapidly is the equilibrium level (EL), which is in the troposphere closely related to the tropopause $\endgroup$– JeopardyTempestCommented Nov 24, 2021 at 23:13
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$\begingroup$ @JeopardyTempest Over the equator the tropopause is around 56,000 ft (17 km). Cumulonimbi have reportedly been as high as 70,000 ft. $\endgroup$– GiovanniCommented Nov 25, 2021 at 6:18
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This is simply because of the way that the troposphere is defined and the fact that isothermal layers are of constant temperature. The troposphere has one defining characteristic; the air temperature drops with increased altitude. The isothermal layer is where that stops happening and temperatures remain stable with increased altitude, so the tropopause is at the base of it.
The stratopause is different because it is defined as the point of maximum temperature of both the stratosphere and the mesosphere. It is traditionally represnted as a line placed in the middle of the upper stratospheric isothermal layer as the halfway mark between the two layers but in reality the entire isothermal layer is, by definition, the stratopause.
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$\begingroup$ Right, the entire isothermal layer should be considered the tropopause, it's the coldest point of both the troposphere and the stratosphere we could say. $\endgroup$– GiovanniCommented Nov 25, 2021 at 8:20
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$\begingroup$ This chart calls it the tropopause: lh3.googleusercontent.com/proxy/… $\endgroup$– GiovanniCommented Nov 26, 2021 at 9:05
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$\begingroup$ @Giovanni Shouldn't be, not under the textbook definition of the troposphere. $\endgroup$– AshCommented Nov 26, 2021 at 19:09
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$\begingroup$ It's called the tropopause, not -sphere. Isn't the textbook definition of the stratosphere that temperature rises with altitude? That's neither the case in the tropopause nor in the stratopause. One may consider the tropo-, strato- and mesopause a kind of seperate layers as they're isothermal with altitude. $\endgroup$– GiovanniCommented Nov 27, 2021 at 8:38
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$\begingroup$ @Giovanni The tropopause marks the top of the troposphere it should be at the bottom of the isothermal layer because the troposphere is only defined by temperature behaviour; the stratosphere is defined with reference to the layers either side of it (the troposphere and mesosphere) as well as by temperature behaviour. $\endgroup$– AshCommented Nov 28, 2021 at 6:35