Just to clarify, Jim is correct. Air is LESS dense with high humidity, high temp, high altitude and lower barometric pressure. Changes in air density effect the quantity of O2 that is pulled into the chamber during each cycle, so fuel mixture needs to be adjusted accordingly. I.E, you will need to lean your mixture on hot humid days and you will need to richen your mixture on cool dry days. To a certain extent, this is why engines make less power on hot humid days, i.e. you had to lean the mixture so you are buring less fuel per cycle so you are making less power. I don't know drag racing, but what Phread said makes sense. After a TStorm, humidity typically drops and the rain probably cooled the air too. This would make for cool and dry air (i.e. dense air) after the storm, which means richer mixtures and more power.

Densisty Altitude Calculator
http://www.wahiduddin.net/calc/calc_da_rh.htm