Humidity does have some affect, but primarily it is heat which causes the Air Density to lessen. Thus, the air that is flowing into your induction system is less dense and draws less fuel up through the jets. Less fuel + Less Air = Less Power. This is a phenomenon that all pilots have to deal with as low air density means less available power from the engine and more speed is necesary to produce lift from the wings in the thinner air. Hot Rodders and Drag Racers also contend with "charge air density", especially guys running Supercharged/Turbocharged engines. The idea is always to keep the air flowing into the engine as cold as possible. Drag Cars always turn better times in colder whether. So, that is what you're experiencing. Come Fall, you will notice better acceleration and top speed.