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From the question Why are pressure levels raised on warm days?, my understanding is that the air pressure at surface level is not affected by temperature, as the mass of the imagined air column stays the same (even though it's extended higher due to lower density). Then, at any given true altitude, warmer temperatures would correspond to higher pressure. The air column analogy makes a lot of sense to me.

However, I was reading NOAA's explanation of atmospheric circulations, which says "This region would become very hot, with hot air rising into the upper atmosphere. This would create a constant belt of low pressure around the equator". That seems to contradict the concept above.

My guess is, in the air column analogy, when the air column is extended higher, it's "leveled out" with the surrounding air, just like water. So we end up with the column with the same original height but lower density, meaning that the total weight at the bottom is lower than before (which means lower pressure). However, if this is true, then the pressure would be lower at any altitude, not just at the surface.

I'm confused now. Please help, thank you!

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Your guess is correct. As the column of air gets heated, it expands, and that results in a higher pressure in the upper troposphere.

Image α

As a result, the air from this warm column flows outwards along the tropopause. This outflow of air is what causes a reduction in the net air mass within the column, and a subsequent reduction in the surface pressure. Therefore at the surface, the air will flow towards the column; so the air moves in a cyclonic sense:

Image β

However, if this is true, then the pressure would be lower at any altitude, not just at the surface.

The surface pressure has indeed reduced, but so has the pressure lapse rate (since the air has expanded upwards). Therefore, in the upper atmosphere, the pressure will actually be higher above the warm surface (as can be seen in the first image).

(Images source: Meteorology by Oxford)

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  • $\begingroup$ Thank you @Aditya! For the pressure lapse rate part, I'm still confused - it makes sense that the lapse rate is lower under warmer temps, however, if we consider the air columns beginning at the very top, the pressure would be 0 for both, and as we go down, the warmer column would have consistently lower pressure at any altitude (by the reduced lapse rate)? $\endgroup$
    – bigfoot
    Aug 10 at 17:18
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    $\begingroup$ @bigfoot You have to consider that the top of these columns are not at the same height. In case of warmer air, the air has expanded upwards, and so the top of the atmosphere will occur at a higher true altitude. Likewise in case of cold air, since it has contracted, the top of the atmosphere occurs at a lower true altitude. Upon considering the top height difference, it should make sense as to how the surface pressure can be lower despite a lower lapse rate. $\endgroup$
    – Aditya Sharma
    Aug 10 at 23:27

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