RE: AP Yello Jacket .15
As I understand it, high humidity leads to less engine performance because the water vapor displaces the needed air to support combustion. The engine can only take in a set volume of whatever gasses come in through the carb. An increase in water content because of humidity is also a decrease in oxygen. As a result, you have to lean the mixture out on humid days to decrease the flow of fuel to match the available oxygen. And, in fact, if you have an engine's mixture set on a very dry day, then run it again with high humidity, you will find the engine is running rich.
High temps also reduce engine peformance. The air is less dense, so the engine has to work harder to pull in enough oxygen. An increase in temperature has exactly the same effect as an increase in elevation, and the engine produces less useable power.
regardless of how it happens, though, it's clear that engine performance goes down as temp goes up and as humidity goes up and as elevation (altitude) goes up.