The day is hot, the Capulets abroad,
And, if we meet, we shall not scape a brawl;
For now, these hot days, is the mad blood stirring
-Benvolio to Mercutio in Act 3, Scene 1 of Romeo and Juliet by William Shakespeare
The idea that hot weather increase the primitive, aggressive nature of man is well established. The science, however, is a bit iffy. There are some theories about heat's effect on the brain but various scientists have very conflicting opinion.
Wired Magazine has a short, readable article about several studies concerning heat and crime rates. According to one study, up until the temperature reaches 90 degrees (32 Celsius), there is a correlation between temperature and violence. However, after that the correlation is broken. Air conditioning is proposed reason because after temperatures get high many people who would be targets of opportunity are instead now able to stay cool indoors.
Even this science is not settled but is an interesting idea. Humans may have made a minor solution to environmental influences on their psyche. Now if there were only a solution to seasonal affective disorder (SAD).