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Humidity

Humidity

Humidity is a measure of the quantity of water vapor in the air. There are different methods for determining this quantity and those methods are reflected in a variety of humidity indexes and readings.

The humidity reading in general use by most meteorologists is relative humidity. The relative humidity of air describes the saturation of air with water vapor. Given in terms of percent humidity (e.g., 50% relative humidity), the measurement allows a comparison of the amount of water vapor in the air with the maximum amount water vapor thatat a given temperaturerepresents saturation. Saturation exists when the phase state changes of evaporation and condensation are in equilibrium.

Approximately one percent of Earth's total water content is suspended in the atmosphere as water vapor, precipitation , or clouds . Humidity is a measure only of the vapor content.

Because water vapor exerts a pressure, the presence of water vapor in the air contributes vapor pressure to the over-all atmospheric pressure . Actual vapor pressures are measured in millibars. One atmosphere of pressure (1 atm) equals 1013.25 mbar.

In contrast to the commonly used relative humidity, the absolute humidity is a measure of the actual mass of water vapor in a defined volume of air. Absolute humidity is usually expressed in terms of grams of water per cubic meter.

Specific humidity is a measure of the mass of water vapor in a defined volume of air relative to the total mass of gas in the defined volume.

The amount of water vapor needed to achieve saturation increases with temperature . Correspondingly, as temperature decreases, the amount of water vapor needed to reach saturation decreases. As the temperature of a parcel of air is lowered it will eventual reach saturation without the addition or loss of water mass. At saturation (dew point ) condensation or precipitation. This is the fundamental mechanism for cloud formation as air moving aloft is cooled. The level of cloud formation is an indication of the humidity of the ascending air becausegiven the standard temperature lapse ratea parcel of air with a greater relative humidity will experience condensation (e.g., cloud formation) at a lower altitude than a parcel of air with a lower relative humidity.

The differences in the amount of water vapor in a parcel of air can be dramatic. A parcel of air near saturation may contain 28 grams of water per cubic meter of air at 30°F (1°C), but only 8 grams of water per cubic meter of air at 10°F (12°C).

An increasingly popular measure of comfort, especially in the hotter summer months, is the heat index. The heat index is an integrated measurement of relative humidity and dry air temperature. The measurement is useful because higher humidity levels retard evaporation from the skin (perspiration) and lower the effectiveness of physiological cooling mechanisms.

Absolute humidity may be measured with a sling cyclometer. A hydrometer is used to measure water vapor content. Water vapor content can also be can be expressed as grains/cubic ft. A grain, a unit of weight, equals 1/7000 of a pound.

See also Adiabatic heating; Atmospheric composition and structure; Atmospheric inversion layers; Atmospheric lapse rate; Atmospheric pressure; Hydrologic cycle; Hydrostatic pressure; Weather forecasting methods; Weather forecasting; Wind chill

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humidity

humidity, moisture content of the atmosphere, a primary element of climate. Humidity measurements include absolute humidity, the mass of water vapor per unit volume of natural air; relative humidity (usually meant when the term humidity alone is used), the ratio of the actual water-vapor content of the air to its total capacity at the given temperature; specific humidity, the mass of water vapor per unit mass of natural air; and the mixing ratio, the mass of water vapor per unit mass of dry air. Absolute humidity finds greatest application in ventilation and air-conditioning problems. Humidity is measured by means of a hygrometer. The rate of evaporation decreases as the moisture content of the air increases and approaches saturation. In addition, the saturation point (moisture-holding capacity of the air) increases rapidly as the temperture of the air rises (see dew). Thus cold air, while its moisture content is necessarily quite low (low absolute humidity), may be almost saturated with respect to the maximum amount of water vapor it is capable of holding (high relative humidity). Cold air with high relative humidity "feels" colder than dry air of the same temperature because high humidity in cold weather increases the conduction of heat from the body. Conversely, hot air attended by high relative humidity "feels" warmer than it actually is because of an increased conduction of heat to the body combined with a lessening of the cooling effect afforded by evaporation. On the other hand, a low relative humidity "modifies" the effect of temperature extremes on the human body. Humidity decreases with altitude. Proximity to large bodies of water and the prevalence of moisture-bearing winds favor high humidity. A temperature-humidity index has been developed by the U.S. National Weather Service that gives a single numerical value in the general range of 70 to 80 reflecting the outdoor atmospheric conditions of temperature and humidity as a measure of comfort (or discomfort) during warm weather. The temperature-humidity index, ITH, is defined as follows: ITH=0.4 (dry-bulb thermometer temperature F + wet-bulb thermometer temperature F) + 15. When the index is 70 most people feel comfortable; at 75 about half the population is uncomfortable; at 80 most are uncomfortable.

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humidity

humidity The moistness of air. Weight of water per unit weight of air is absolute or specific humidity. Saturation humidity is the absolute humidity of air that is saturated with water vapour at a given temperature. Relative humidity is the degree of saturation: the ratio of water vapour pressure in the atmosphere to water vapour pressure that would be exerted by pure water at the same temperature.

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humidity

humidity The concentration of water vapour in the atmosphere. The absolute humidity is the mass of water vapour per unit volume of air, usually expressed in kg m–3. A useful measure is the relative humidity, the ratio, expressed as a percentage, of the moisture in the air to the moisture it would contain if it were saturated at the same temperature and pressure.

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humidity

humidity The amount of water vapour present in air, measured as the mass present in a given volume of air (absolute humidity), the mass of water vapour present in a given mass of air including the water vapour (specific humidity), or the amount present as a percentage of the amount needed to saturate the air at the prevailing temperature (relative humidity).

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humidity

hu·mid·i·ty / (h)yoōˈmiditē/ • n. (pl. -ties) the state or quality of being humid. ∎  a quantity representing the amount of water vapor in the atmosphere or a gas: the temperature is seventy-seven, the humidity in the low thirties. ∎  atmospheric moisture.

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humidity

humidity An expression of the moisture content of the atmosphere. Measures of humidity include statements of the total mass of water in 1 m3 of air (absolute humidity), the mass of water vapour in a given mass of air (specific humidity), relative humidity, vapour pressure, and the mixing ratio.

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humidity

humidity An expression of the moisture content of the atmosphere. Measures of humidity include statements of the total mass of water in 1 m3 of air (absolute humidity), the mass of water vapour in a given mass of air (specific humidity), relative humidity, vapour pressure, and the mixing ratio.

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humidity

humidity Expression of the moisture content of the atmosphere. Measures of humidity include statements of the total mass of water in 1 m3 of air (absolute humidity), the mass of water vapour in a given mass of air (specific humidity), relative humidity, vapour pressure, and the mixing ratio.

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humidity

humidity (relative humidity) Measure of the amount of water vapour in air. It is the ratio of the actual vapour pressure to the saturation vapour pressure at which water normally condenses, and is usually expressed as a percentage. Humidity is measured by a hygrometer.

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humidity

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"humidity." Oxford Dictionary of Rhymes. . Encyclopedia.com. 12 Dec. 2017 <http://www.encyclopedia.com>.

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