For over a century seasonal fluctuations in suicide have been observed with studies usually showing an increase in people killing themselves in spring and early summer. However, within this seasonal variation it is hard to disentangle the effects of temperature, humidity, hours of sunshine and the length of daylight. A study of daily temperature and suicide rate in the UK between 1993 and 2003 has found that there was no spring or early summer peak in the suicide rate but that above 18C each 1C rise in mean temperature was associated with a 3.8% rise in suicides. Suicides increased by 46.9% during the 1995 heatwave but did not rise at all during the heatwave of 2003.
Page, Lisa A., Hajat, Shakoor and Kovats, R. Sari - Relationship between daily suicide counts and temperature in England and Wales British Journal of Psychiatry August 2007, 191, 106-112