I'm from hot and sunny Southern California, OP. I've been in the UK since my early 20s and I'm 52 now. But for sure, even in sunny CA, we always noticed a significant increase in homicides in the summertime. There were several summers spent watching the news with worry, locking our doors and windows at night, living in fear of serial killers, break-ins, rape. I will never forget the heat of that summer when the Night Stalker terrified us. He struck my cousin's neighbours' house on the very night we'd been over for a barbecue. I remember my mom telling her brother and his wife not to sleep with the windows open.
Spike Lee captures the essence summer heat/summer violence in the film Do The Right Thing (simmering racial tensions, simmering summer heat). It wouldn't have hit the same note, that movie, if it were set in the dead of winter.
I think too, in summer, we're rubbing up against each other more, bumping into each other, cutting each other on our sharp edges, socialising way more than we're used to or even comfortable with. I think so much mixing with each other and the stress around enjoying the summer and packing everything in, just squeezes the sanity out of some people. We sleep badly too... rooms are hot and thick, the air is close. I just googled why there are more murders in summer; increased blood pressure and heart rate caused by increased body temperature.
Growing up, we were always aware that things got a little crazier and messier in summertime- something I still totally believe. I always tell my eldest son to take it easy out there in London- drivers drive badly, people drink more, the thick, humid air can act as a depressant, making us sluggish and impatient.