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Suppose you took earth and gave it approximately 3 times the air, with most of the extra being N2 or Ar.

Combustion goes up with the partial pressure of oxygen. So unless we want to burn down the planet around our ears, at 45 psi, we want only 7% O2 or so. This much Nitrogen starts implying nitrogen narcosis, but we will assume that the natives evolved with this pressure, or that visitors can use medication.

Three times the atmosphere will still be fairly transparent. There will be some additional heating in the air from absorption of sunlight, but the bulk will be at the surface.

Tropopause will be much higher.

Three times the air will mean 3 times the water vapour. Days that are now thin cloud would be heavily overcast. Heavy overcast would become dim twilight.

This may reduce sunlight that reaches the surface, resulting in heating that takes place more evenly through the atmosphere.

Stronger greenhouse effect? Move the planet out a few million km?

Rainfall intensity could be brutal.

A rising parcel of air would have much more energy available, and the density differential would be greater. This would mean either a smaller temperature differential would be needed to separate a parcel of air from the surface, and/or updrafts would be incredibly violent.

With more turbulent weather, lightning would be more frequent. Updrafts could support both larger raindrops and larger hail. Being outside in a hail storm could be lethal.

Wind would have 3 times the force at a given velocity. Dust would be picked up at lower velocity, and due to increased drag, would take longer to settle out of the air.

Any other thoughts on the effect of air density on weather?

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Actually, you wouldn't get more water vapor (unless the temperature also rose); the vapor pressure of water is dependent on temperature, not ambient pressure. The boiling point of water would be higher (just as it's lower on high mountains, with less ambient pressure) but evaporation should work as it does on Earth.

The greenhouse effect would not be increased if you only add nitrogen and argon, since these are not greenhouse gases, but if all non-oxygen gases (including CO2) are increased proportionately, it would be. However, three times current CO2 levels is still less than was probably the case in the Jurassic and Cretaceous, so the planet would remain habitable (though the hotter regions might not be suitable for unprotected humans, depending on humidity levels in those areas).

Hail storms... hmm. Even if hailstones were larger, they would fall more slowly (terminal velocity will be less due to the greater air resistance) so they might not be more destructive unless they were a lot larger.

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  • $\begingroup$ Plus one for you catching my error on partial pressure. If PP is the same, then potential latent energy is the same. Momentum is larger though. Implies slower moving air masses. Weather is so confusing. $\endgroup$ – Sherwood Botsford Mar 1 '17 at 3:24
  • $\begingroup$ Greenhouse effect would be larger just due to having a 3 times thicker atmosphere, wouldn't it? $\endgroup$ – Sherwood Botsford Mar 1 '17 at 3:30
  • $\begingroup$ Well your 'classic' greenhouse effect strictly due to atmospheric gases absorbing & re-radiating thermal IR from the planet shouldn't be increased by increasing the quantity of gases that are transparent to thermal IR anyway. There could totally be subtler effects that changed the final temperature though - convective transport of heat from surface to atmosphere might be more effective, denser atmosphere could support clouds better, etc. $\endgroup$ – cometaryorbit Mar 1 '17 at 6:15
  • $\begingroup$ It is though. Thickness of the layer affects the number of times that it is absorbed and re-radiated. Sleeping with 3 blankets. While you don't have a higher percentage of water or CO2, you just have more of it between the surface and space. $\endgroup$ – Sherwood Botsford Mar 1 '17 at 15:11
  • $\begingroup$ Yeah, I was talking about same absolute quantity of greenhouse gases, not same percentage. The nitrogen, argon, and oxygen shouldn't be absorbing meaningful amounts of thermal IR. $\endgroup$ – cometaryorbit Mar 2 '17 at 6:00

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