The effects of humidity on air density are around 1%, The effects of humidity on engine performance can be as much as 10%.
Quote: Alright, so if you have a cool temperature of 20 degrees you have a MP of say 20". Now you move into air which is 100 degrees. Why wouldnt the MP do the same thing? The air density is the contributing factor to the MP. So if a rise in temp has a decrease in density, and a rise in altitude has a decrease in density, why are their opposite effects on the MP limit chart?
The air density is not a contributing factor to MP. Only the air pressure is. If the ambiant pressure is the same, the MP will be the the same regardless of temperature. However, the higher the temp, the lower the power because the air density is lower. As you climb, you lose available horsepower because of the reduction of air pressure available to give you the same MP. But for a given MP, the horsepower increases as you climb because of the reducton of back pressure.
You might want to study a private pilots book meteorology section.