Question

Which best describes the relationship between humidity and air pressure?

Answer 1

Humid air is lighter, so it has lower pressure.

Answer 2

Answer:Humid air is lighter, so it has lower pressure.

The reason is the molecules of water are H2O, whose molar mass is 18 g/mol.

These molecules displaces molecules of N2 and O2, whose molar masses are:

N2: 2*14g/mol = 28 g/mol, and
O2: 2*16g/mol = 32 g/mol.

Then molecules of 28g/mol and 32 g/mol are being replaced with molecules of 18g/mol, leading to a lower weight of the same volume of air, which results in lower pressure.