Mark Cane sends us this:
ROMEO and JULIET ACT 3, SCENE 1a
[A street. MERCUTIO, BENVOLIO & Servants]
I pray thee, good Mercutio, let's retire.
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.
[And later they do meet the Capulets and Tybalt kills Mercutio, then Romeo kills Tybalt and the rest is tragedy.]For more fun evidence on the psychological effect of heat on aggression, see evidence from road rage and MLB. Also this.