I know the common explanation is that the hot air raises due to lower density and creates a low pressure zone and the air then flows from high pressure (cold) to low pressure (warm) areas, but that means that the wind would blow all the time from cold areas to the warm areas, and that is not the case. Many times the wind blows from warm areas to cold areas, the warm wind. Seems to me that this phenomenon has something to do with the enormous weight of water contained in the clouds, exercising a pressure on the air underneath...the pressure would push the air towards the outside of the clouds, like a piston, resulting in a low pressure area transforming into a high pressure and then the wind would be a warm one blowing to some cold areas. Please tell me if you think that my explanation is valid and if not give another.
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$\begingroup$ "With the enormous weight of water contained in the clouds" - that is actually backwards. Water vapor is less dense than dry air. The molecular weight of water = 18, nitrogen = 28, oxygen = 32. The basic reason why winds don't blow "from cold to hot areas" all the time is because the earth is rotating, and moving air (in both the horizontal and vertical directions) is affected by the Coriolis forces which cause hurricanes, cyclones, and smaller weather features as well. $\endgroup$– alephzeroCommented Feb 19, 2017 at 15:01
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1$\begingroup$ A cloud is a huge ensemble of tiny water droplets (less than 100 micron). It is more like a dust cloud. Anyway why should clouds increase pressure beneath them? If a jumbo-jet in the sky flies directly above you, do you feel crushed? $\endgroup$– DeepCommented Feb 20, 2017 at 5:20
1 Answer
A semi-stable low pressure region results from circular wind. The centripetal acceleration of the wind is sustained by the pressure gradient (low pressure in the 'eye' of the hurricane, for example). Such low pressure regions aren't going to shrink like a limp balloon, because the surrounding wind is in a kind of stable orbit.
The circular wind, in turn, may be caused by hot air rising, because the latitudinal momentum of air at ground level remains unchanged when that air mass rises. The unchanged latitudinal momentum, on a rotating planet, means that the rising air achieves some ground speed as it rises, and the amount of ground speed will vary according to the varying causes of the heated air, and with the latitude itself.
Cold air is denser than warm air, and it takes time to mix, so a cold front can carry a high pressure signature.
Then there's moisture, and the various kinds of precipitation which move heat and mass about... it's apparently possible for ice pellets to rise/fall/rise and grow to hailstones as large as melons. There's a lot of air movement on this planet, and it's never simple, but it surely always has some valid physical explanation.
