This is an interesting fact that I probably never knew. I've always believed that humid air, being muggier, was heavier than dry air. Water, when vaporized into dry air, causes the combined mixture to become lighter. There is a law in the physics of gases that any volume of gas, under the same pressure, always contains the same number of gas molecules, no matter what the gas, or mixture of gases might be. As a water vapor molecule weights less than either a nitrogen or an O2 molecule, when water is vaporized into dry air, the water molecule displaces either a nitrogen or an O2 molecule and the new mixture per unit volume and pressure becomes lighter. I wonder if this is in anyway related to the creation of oceanic lows that form hurricanes?