There have been many discussions about this on this board, probably archived by now. I do believe hair growth is faster in warm weather and there is a science behind it.
ETA:
Here are some links about this:
http://www.oleda.com/oleda_tips/index.cfm?ID=26&Ref=Hair-Loss
Did you know? Hair is a “miracle fiber.” It is composed of nitrogen, sulfur, water, amino acids and iron traces. A person has ±100,000 hairs on their scalp. Women’s hair lasts about 25% longer than men’s before falling out and being replaced (2-6 years). Hair grows faster in summer and, strangely, faster at night.
http://health.allrefer.com/news/index.php?ID=3959
And if you are worried about hair growth, according to Tobin, a 1991 study suggested that androgen-dependent hair growth is faster during the spring and summer months and slower during the winter months in temperate regions such the US. Although this study was conducted in men, the results may also apply to women. The results however may not apply to children as their hormonal activity is very different from that in adults, at least before puberty.
http://www.afraidtoask.com/hair/hairlifecycle.html
A person normally sheds up to 100 hairs per day.
Hair growth occurs at about an inch per month, faster when it's hot (summer) and slower when it's cold (winter). This rate slows down with age, and shuts off in more and more hair follicles as time marches on. Things that influence hair growth include not only hormones, but nutrition, vitamins, emotional states, and many unknown factors.