In mid-summer we're all desperate to get our baps and bellies out at the first glimpse of a sunny day. When we're still in psychological winter mode it takes a while longer. It's about the fourth consecutive day of lovely sunshine where I am, this lunchtime the park was full but everyone was in jumpers and trousers, a few t-shirts and stuff. The same temperature in July will see flesh everywhere I'm sure.
Asking from a purely anthropological perspective of course.