Most every golfer believes that their golf ball flies much farther in the heat of the summer than in the colder months of winter (or even fall or spring), and, well, it does. However, the effect heat has on the distance the golf ball travels isn't as much as most golfers, including many professionals, think.
So how much does heat affect how far the golf ball travels? According to the folks at TrackMan, for every 10 degrees of increase in temperature, the golf ball flies approximately 1.33 to 1.66 yards farther, depending on the club being used. This is because the temperature changes air density, which is ultimately the factor determining how well the golf ball can fly through the air with minimal drag and ideal lift.
Humidity has a significant effect on golf ball distance, too. The higher the humidity, the longer the ball flies. You can feel humid air more, but water weighs less than the nitrogen and oxygen molecules that make up dry air. So, you're hitting into thinner air, which means the ball goes farther as though a golfer were playing at a higher altitude. Ultimately, however, going from basically no humidity to full humidity has the effect of a single yard.
All told, these kinds of common weather factors will never have more effect than a whole club. Altitude and wind have the greatest and most apparent effect on golf ball distance.