# Why does air rising at constant velocity cause low pressure in the atmosphere if pressure is the weight of the overlying air column?

According to wiki: https://en.wikipedia.org/wiki/Pressure_system#cite_note-3, "Diverging winds aloft ahead of these troughs cause atmospheric lift within the troposphere below, which lowers surface pressures as upward motion partially counteracts the force of gravity."

It does not say whether the air is accelerating upward or at a constant velocity, but that seems unlikely to matter because there is also high surface pressure at the subtropical highs year-round, and I do not see how there could always be air accelerating within the entire column there. At a constant velocity, a simple force balance still gives the surface pressure as the weight of the entire column, and I do not understand why that would change under constant-velocity upward motion.

• Use bernoulli's principle – Shreyansh Pathak Feb 15 '20 at 6:58

## 1 Answer

The simple relation of "pressure is equal to the weight of the fluid above" is only valid for static fluids. As soon as fluids are moving, there is also a dynamic pressure component.

but since in this case the pressure is decreasing while the velocity is increasing, I am assuming the upward motion somehow cancels the gravitational force more than it increases the dynamic pressure?

Dynamic pressure depends on the direction of movement (it is not always an increase). Because we are describing a situation where the air above the surface is lifting, the fluid is moving away from the surface and the dynamic pressure in this case is negative. It reduces the total pressure below what you would expect from static pressure alone.

https://www.grc.nasa.gov/www/k-12/airplane/dynpress.html

• That page says, "dynamic pressure is the increase in a moving fluid's pressure over its static value due to motion," but since in this case the pressure is decreasing while the velocity is increasing, I am assuming the upward motion somehow cancels the gravitational force more than it increases the dynamic pressure? – Philip Meyer Feb 15 '20 at 19:12
• I would guess weather models use static pressure because they rely on constant-pressure surfaces. It makes more sense to me that the combination of heating and rising air does cause acceleration which counteracts gravity. So my answer may have been phrased incorrectly or asked in the wrong place. I think this is a better answer: physics.stackexchange.com/questions/287491/… – Philip Meyer Feb 16 '20 at 21:54