Question

how does water vapour affect air pressure?
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Answer 1

As humidity increases pressure decreases. Since water vapor is less dense than dry air if both have the same temperature, the addition of water vapor decreases the overall density of the air and lowers its pressure.

Answer 2

Answer:

The vibration of vapour molecules is a phenomena induced by thermal energy and the energy of vibration depends on the temperature of the vapour. At higher temperatures, the kinetic energy of water vapour molecules increases, so increasing the pressure.