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If PV = nRT, then why is abnormally high pressure caused by cold air?

Asked by: 2163 views Student Pilot

On p. 35 of From the Ground Up, it says that cold air masses can produce barometric pressures higher than 31.00"Hg. This seems very strange and surprising, since the ideal gas law tells us P = (n/V)RT, so we can see that a decrease in temperature should result in a decrease in pressure, not an increase.

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2 Answers

  1. Best Answer

    Michael Stevens on Aug 01, 2013

    The ideal gas law applies to a gas in a closed system. The atmosphere is an open system.

    Imagine a balloon filled with air, you heat it, and the pressure increases. Since the balloon is elastic, it will expand, if you cool it, the balloon will shrink. The gas in the balloon is behaving as you would expect by the ideal gas law, but you will agree that the larger hot balloon and the colder small balloon have the same amount of air and weigh the same.

    Now imagine you have a big cylinder, 10 feet high. You can fit many more cold balloons in that cylinder than hot balloons containing the same mass of air, so the cylinder filled with cold balloons will weigh more.

    The atmospheric pressure is the weight of a column of air due to gravity, it isn’t the pressure of a volume of air exerted against a container.

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  2. Sean McKenzie-Mardelli on Dec 27, 2015

    Thanks for clearing that up. I was thinking of the outer limits of the atmosphere as being the “rubber” of a balloon so I thought the atmosphere as a whole should be considered a closed system.

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