A cubic foot of dry air at standard pressure and temperature is weighed on a scale. Then a cubic foot of damp air at the same pressure and temperature is weighed. Which will weigh more?
Answer
The damp air weighs less because water molecules (molecular weight of 18) have replaced more massive molecules of air (nitrogen: molecular weight of 28, oxygen: molecular weight of 32). Both volumes have the same number of molecules. Therefore, increase in humidity lowers the barometer reading. The barometer pressure value gets lower as a storm approaches.