Humidity

Humidity, moisture content of the atmosphere. The atmosphere always contains some moisture in the form of water vapour, the maximum amount depending on the temperature. The amount of vapour that will saturate the air increases with a rise in temperature: at 4.4° C (40° F), 454 kg (1,000 lb) of moist air contains a maximum of 2 kg (5 lb) of water vapour; at 37.8° (100° F), 1,000 lb of moist air contains a maximum of 18 kg (41 lb) of water vapour. When the atmosphere is saturated with water, the level of discomfort is high because the evaporation of moisture from the body as perspiration, with its attendant cooling effect, is impossible.

The weight of water vapour contained in a volume of air is known as the absolute humidity and is expressed in pounds of water per pound of dry air. Scientists express absolute-humidity measurements in grams of water vapour per cubic metre. Relative humidity, given in weather reports, is the ratio between the actual vapour content of the atmosphere and the vapour content of air at the same temperature saturated with water vapour.

If the temperature of the atmosphere rises and no change occurs in the vapour content of the atmosphere, the absolute humidity remains the same but the relative humidity is lowered. A fall in temperature increases the relative humidity, producing dew.

Humidity is measured with a hygrometer. The temperature-humidity-index (THI—also referred to as “discomfort index” or “humisery index”) expresses in numerical values the relationship between temperature and humidity as a measure of comfort or discomfort. It is calculated as 40 per cent of the sum of the dry-bulb temperature plus the wet-bulb temperature in degrees Fahrenheit, plus the number 15. If the dry-bulb temperature is 90° F and the wet-bulb temperature is 70° F, the THI is 79. When the index is 70, most people are usually comfortable; an index of 75 means that half will be uncomfortable.