Humidity

Humidity is the concentration of water vapor in the air. The concentration can be expressed as absolute humidity, specific humidity, or relative humidity.

Absolute humidity

Absolute humidity defines the mass of water vapor in a given volume of moist air or gas, and is usually reported in grams per cubic meter, although grains per cubic foot is more commonly used in the United States.

Specific humidity

Specific humidity is a method of expressing the amount of aqueous vapor in air by using a ratio of water vapor to dry air. Specific humidity is expressed as a ratio of kilograms of water vapor, $m_{w}$, per kilogram of air, $m_{a}$ .

That ratio can be given as:

$x={m_{w} \over m_{a}}$

Partial pressure of water vapor and air can also be used to express the ratio.

Relative humidity

Relative humidity is the ratio of the current vapor pressure of water in any gas (especially air) to the equilibrium vapor pressure, at which the gas is called saturated at the current temperature, expressed as a percentage. Equivalently, it is the ratio of the current mass of water per volume of gas and the mass per volume of a saturated gas.

Effects on human body

Humidity can make the warmth of the surrounding air feel as if it is warmer than the actual temperature would suggest, because the cooling effect of evaporation from the skin is reduced.

This is because as environmental temperatures approach normal skin temperature, cooling of the body becomes more difficult. If the atmosphere is as warm as or warmer than the skin, blood brought to the body surface cannot lose its heat. Under these conditions, the heart continues to pump blood to the body surface, the sweat glands pour liquids containing electrolytes onto the surface of the skin and the evaporation of the sweat becomes the principal effective means of maintaining a constant body temperature. Sweating does not cool the body unless the moisture is removed from the skin by evaporation. 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. 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.

First spoken by Warren Hymer in the 1939 movie Mr. Moto on Danger Island, the expression It's not the heat, it's the humidity refers to unpleasantly muggy weather; that is, the fact that humid air can be significantly less comfortable than drier air at the same temperature.

See Also

Weather forecasting Water vapor Meteorology Concentration

External links

• Glossary definition of specific humidity
• National Science Digital Library - Absolute Humidity