It's not necessarily warm weather that helps hair grow faster.
Blood circulation in the scalp stimulates hair growth. People tend to be more active in the spring and summer months and the body produces more hormones, having to work harder (sweat) in order to cool down. This increased blood circulation causes the hair and nails to grow faster. Studies have proven this.
When I find one, I'll post it, or you can do some research as well.
Working out regularly and pregnancy both cause the same kind of hair and nail growth spurts because of increased hormone production and blood circulation.
That's why women who aren't pregnant and taking prenatal vitamins for hair growth might as well take a multi vitamin or a hair and nail vitamin. Pregnant women get faster hair growth because of their hormones, not the vitamin.
I'm a Newbie, by the way. I've been lurking for some months and finally decided to post!
ETA: This is also why frequent scalp massages promote hair growth, but ya'll already knew that, lol!!