Which is heavier: moist air or dry air?

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Dear Tom,
Which is heavier: moist air or dry air? 

Thomas Duncin,

Dear Thomas, 
A given volume of moist air weighs less than an equivalent volume of dry air at any given temperature. This is so because water molecules are lighter than the proportionally averaged molecular weight of the gases that constitute dry air. Adding water vapor to dry air causes the air’s density — its weight per volume — to decrease. 

It is a widely held misconception that moist air is heavier than dry air. The confusion probably results from the fact that the moisture-holding capacity of air approximately doubles with every 20-degree (Fahrenheit) increase in temperature. Warm air can hold much more moisture than cold air, but that does not change the fact that moist air weighs less than dry air.


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