Hair grows at a rate of approximately half an inch per month, or roughly six inches a year. With warm weather and accompanying humidity, your hair may appear to be growing faster--but it is hormones, not the weather, causing the change.
According to GoodHousekeeping.com, faster hair growth in the summer has to do with hormones, not humidity or warm weather. However, Brian Thompson, senior trichologist at Philip Kingsley Trichological Clinic in New York City, states on Oprah.com that there are not any reliable statistics about it.
During the summer, our metabolism is higher because we are naturally more active in the summer months than in the winter months since the weather is better. According to WebMD, physical activity affects our hormonal cycles, which may contribute to faster hair growth.
Red Orbit states that while hair seems to grow faster in warmer weather, this may not apply to all people. Children, for example, have radically different hormonal activity than adults--especially children who are pre-pubescent. An increase in blood circulation (caused by exercise) in summer also may contribute to faster hair growth.
Hair may grow only 0.05 inches more in summer months--a rather insignificant amount. Researchers believe this small amount of extra growth is due to hormonal changes that come with more physical activity (due to more pleasant weather and longer days during which to be active). Warmer, more humid weather encourages more physical activity, but doesn't actually cause the hormonal changes.
During the summer months, the scalp also produces more natural oils, which can be great for your hair. Brushing these oils through your hair daily will keep your hair nourished while under the hot sun and control frizziness caused by humidity.
Light therapy used for treatment of Seasonal Affective Disorder (SAD) is believed to cause a hormonal change in patients who feel depressed during the winter months. The longer periods of sunlight, in general, could play a part in hormonal changes during the summer that lead to increased hair growth.