Can anyone explain what humidity has to do with it?
Sure.
Adding water vapor to air makes air less dense.
More humidity, more water vapor.
(I studied meteorology in school)
Water molecules weigh less than O2 (oxygen molecule) and N2 (nitrogen molecule)
Now, can I help you out on why Tiger is a phony, or how remebering a dead one at ceremony is hot being hyprocritical if one misses a funeral, or how wishing Lebron James getting cancer is not funny?