# Does rain itself affect barometric pressure?

I understand how and why low pressure systems are often associated with rain, but does the rain itself (both during and after) alter barometric pressure in any meaningful way?

• I do not have time to answer this in detail but you may want to look at "dynamic pressure and pressure perturbation" in the context of cumulus convection. There is no trivial answer to this question. Or read this PDF for more detail inscc.utah.edu/~krueger/6150/pressure_perturbations_slides.pdf Aug 27 '19 at 8:43
• @gansub Thanks for that link. My hunch was that it must have some impact, since relative humidity increases after a rain which I assumed would alter the air density and pressure. Aug 29 '19 at 16:28

Under a hydrostatic assumption the basic equation is p=/rho R T. So under this easy equation the pressure depends to the air density and the air temperature. As you may know diabatic processes during rain yield to a decreasing temperature. but the magnitude in change of pressure against the synoptic scale forcing is low.

Please be aware of mixing stuff together answering this questions. The question was not to answer how precipitation is forming. It was just the question about the rain influencing the pressure.

• @dl.mateo the hydrostatic pressure assumption wont work inside cumulus convection which is a very small scale and with strong upward movement. Aug 30 '19 at 7:21

No. Low pressure affects the rain, not the other way around. Air at high pressure/density can hold more moisture than air at low pressure, so when there is a reduction in pressure the moisture content, or some of it, condenses and is precipitated. Reduction of temperature can also cause precipitation, because cold air holds less moisture than warm air. Reduction of temperature can be brought about by lowering of pressure.

• Until now I never heard of the pressure being a significant influence on the moisture capacity of the air. Could you please elaborate sufficiently? As far as I know, low pressure systems are connected with rainfall due to the lower temperatures compared to high pressure systems, which leads to the forming of clouds as the air cools and excessive moisture being rained down.
– Erik
Aug 27 '19 at 6:33
• @ Erik Just as increasing the pressure of air has a warming effect, reducing the pressure has a cooling effect, so both pressure and temperature influence condensation. Aug 27 '19 at 6:51
• Please, hit me with the calculations, that this is a significant effect.
– Erik
Aug 27 '19 at 6:53
• "Just as increasing the pressure of air has a warming effect" I think it's the other way around, Michael. Temperature fluctuations cause pressure fluctuations, not the other way around. Aug 27 '19 at 7:09
• @MichaelWalsby Hand waving arguments alone will not suffice all the time for complex problems. This is a science site and hence please use references to back up your answer. Aug 27 '19 at 8:44