BINGO!
The basic idea of an engine is to ignite an air/fuel mixture and produce power. The more air we can get into a motor, the more fuel we can add to it, the more air/fuel mixture the more power we make. When temperatures rise air becomes less dense (less air in any given space), obiviously when temp decreases air becomes more dense (more air in a given space). Thats why on cold nights (when the car effectivly get more air) we get more power, and on hot days (car effectivly gets less air) we lose power.