Demystifying humidity

Humidity is the amount of water vapor in the air. In daily language the term "humidity" is normally taken to mean relative humidity. Relative humidity is defined as the ratio of the partial pressure of water vapor in a parcel of air to the saturated vapor pressure of water vapor at a prescribed temperature.

High humidity makes people feel hotter outside in the summer because it reduces the effectiveness of sweating to cool the body by preventing the evaporation of perspiration from the skin.

The human body sheds heat by a combination of evaporation of perspiration, heat convection in 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.

Source: http://en.wikipedia.org/wiki/Humidity