Humidity and temperature changes

"the air humidity also has a great influence on sensitivity to temperature. "

You can read the whole article in the second para of http://www.meteosuisse.ch/web/en/weather/health/heat.html

The human body sheds heat by a combination of evaporation of perspiration, conduction to the surrounding air, and thermal radiation. Under conditions of high humidity, the evaporation of sweat from the skin is decreased and the body's efforts to maintain an acceptable body temperature may be significantly impaired. Also, if the atmosphere is as warm as or warmer than the skin during times of high humidity, blood brought to the body surface cannot shed heat by conduction to the air, and a condition called hyperpyrexia results. With so much blood going to the external surface of the body, relatively less goes to the active muscles, the brain, and other internal organs. Physical strength declines and fatigue occurs sooner than it would otherwise. Alertness and mental capacity also may be affected. This resulting condition is called heat stroke or hyperthermia.

http://en.wikipedia.org/wiki/Humidity#Effects_on_human_body