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I’m a little confused here . Studying for ppl exam it is stated that cold air is at higher pressure than warm air rising up, so the flow is from cold to warm. But then again , when flying from warm air to colder air , the pressure in the warmer air will be higher , leading to altimeter error . My question: is the whole “ colder air with higher density is at higher pressure flowing to replace warmer , less dense air has to do with flow? Is that why there is a discrepancy ?

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  • $\begingroup$ The surface pressure is indeed lower over a warm surface, but the pressure lapse rate is also reduced. As a result, in the upper troposphere, the situation reverses and the pressure is higher over a warm surface than over a cold one. Here is a related answer by me on this topic: earthscience.stackexchange.com/a/25491/29450 $\endgroup$ Oct 30, 2023 at 7:48
  • $\begingroup$ If you think of balloons, hot air has the same pressure but lower density. Lower density creates buoyancy. Hot air at the same pressure can rise into cooler air. $\endgroup$ Oct 30, 2023 at 15:39
  • $\begingroup$ Thank you very much . I get it now $\endgroup$
    – Guy
    Oct 31, 2023 at 4:14
  • $\begingroup$ I have another question : with the rising of air mass and expansion in the equator , why is there no cloud formation / storms , etc there ? $\endgroup$
    – Guy
    Nov 1, 2023 at 23:32

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Air flow (wind) is always from higher pressure to lower pressure.

Pressure is dependent on temperature and density.

"Temperature" is essentially how fast air molecules are moving. Colder air will have a higher pressure gradient with altitude because gravity holds more of the "slower" air molecules closer to earth.

A most extreme example of this would be to cool the air down until it liquefies, leaving a puddle with little else above it.

Warm air will have a lower pressure gradient with altitude because more of the "faster" air molecules can populate the higher end of the vertical air column.

These discrepancies are why we must always convert pressure altitude to density altitude, which is most directly related to aircraft performance.

Because the altimeter works on pressure only, corrections may be necessary for both local pressure and, if there is a large deviation from standard temperature, local temperature.

This is especially true for very cold temperatures, which can cause the altimeter to read higher than actual altitude.

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