Yeah, a "mole" of gaseous molecules takes up the same volume (ideally), so a lighter gas (molecular weight) will be less dense than a heavier gas.
Humid air "feels" heavy, but it isn't. This is a small correction factor in flying, usually ignored.
The temperature of air cools off about 4.5°F for every thousand feet of altitude.
If you know the dew point and the current temperature, you can approximate the bottoms of cumulus clouds in the air.
The term "knots" is used because it is related closely to degrees of longitude and latitude.