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moisture content of the atmosphere, a primary element of climateclimate,
average condition of the atmosphere near the earth's surface over a long period of time, taking into account temperature, precipitation (see rain), humidity, wind, barometric pressure, and other phenomena.
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. 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 hygrometerhygrometer
, instrument used to measure the moisture content of a gas, as in determining the relative humidity of air. The temperature at which dew or frost forms is a measure of the absolute humidity—the weight of water vapor per unit volume of air or other gas at the
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. 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 dewdew,
thin film of water that has condensed on the surface of objects near the ground. Dew forms when radiational cooling of these objects during the nighttime hours also cools the shallow layer of overlying air in contact with them, causing the condensation of some water vapor.
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). 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.



the water-vapor content of the air; one of the most important characteristics of weather and climate. Humidity has great significance in certain technological processes, in treating some diseases, and in preserving books and works of art.

The characteristics of humidity are (1) the vapor pressure (or partial pressure) e of water vapor, expressed in newtons per sq m (N/m2), millimeters of mercury (mm Hg), or millibars (mbar); (2) the absolute humidity a—the amount of water vapor in grams per cu m; (3) the specific humidity q—the amount of water vapor in grams per kilogram of moist air; (4) the mixing ratio w, determined by the amount of water vapor in grams per kilogram of dry air; (5) the relative humidity r—the ratio of the vapor pressure e of water vapor contained in the air to the maximum vapor pressure E of water vapor that saturates the space over a flat surface in pure air (saturation vapor pressure) at a given temperature, expressed in percent; (6) the humidity deficit d—the difference between the maximum and actual vapor pressures of water vapor at a given temperature and pressure; and (7) the dew point τ—the temperature that the air will take on if it is cooled isobarically (at constant pressure) to the saturation point of the water vapor it contains.

The humidity of the earth’s atmosphere varies within wide limits. At the earth’s surface, the average water-vapor content in the air ranges from 0.2 percent by volume in high latitudes to 2.5 percent in the tropics. The vapor pressure e varies accordingly; in polar latitudes in winter it is less than 1 mbar (sometimes only hundredths of a mbar), and in summer it is less than 5 mbar; in the tropics it can be as high as 30 mbar, and even higher. In subtropical deserts e is reduced to 5-10 mbar (1 mbar = 102 N/m2). The relative humidity r is extremely high in the equatorial zone (with annual averages of 85 percent and more), as well as in polar latitudes and, in winter, in the interior of the continents in the middle latitudes, where it results from low air temperatures. In summer, high relative humidity is characteristic of monsoon regions (for example, India, with 75-80 percent). Lower values of r are observed in subtropical and tropical deserts and during the winter in monsoon regions (50 percent and less). The values of r, a, and q decline rapidly with altitude. At altitudes of 1.5-2 km, the water vapor pressure averages only half that at sea level. The troposphere (the lower 10-15 km) contains 99 percent of the atmospheric water vapor. On the average, 28.5 kg of water vapor is found in the air above each square meter of the earth’s surface.

The diurnal variation of vapor pressure above the sea and in coastal areas is parallel to the diurnal variation in air temperature: the water content increases during the day, along with increased evaporation. The same diurnal variation of e is observed in the continental interiors during cold weather. A more complex diurnal variation, with two maximums (morning and evening), occurs during the summer in the continental interior. The diurnal variation of relative humidity r is the opposite of diurnal temperature variation: with the temperature increase during the day and, consequently, with the increase in saturation vapor pressure E, the relative humidity decreases. The annual variation of vapor tension is parallel to the annual temperature variation; relative humidity varies in inverse proportion to annual temperature variations. Humidity is measured by hydrometers and psychrometers.

Since humidity essentially affects the heat exchange of the organism with its environment, it has great significance for human vital activity. At low temperatures and high humidity, heat loss is increased and man is subjected to greater cooling; at high temperature and high humidity heat loss is sharply reduced, leading to overheating of the organism, particularly when doing physical work. High temperatures are more easily endured when humidity is lowered. Thus, work in steel mills with temperatures of 25° C is best done when the relative humidity is 20 percent; this has an optimal effect on heat exchange and comfort. In temperate climatic conditions the most favorable relative humidity for man is 40-60 percent. The elimination of unhealthy effects of humidity in buildings is best accomplished by means of ventilation or air conditioning.


Matveev, L. T. Osnovy obshchei meteorologii. Leningrad, 1965.
Usol’tsev, V. A. Izmerenie vlazhnosti vozdukha. Leningrad, 1959.
Khromov, S. P. Meteorologiia i klimatologiia dlia geograficheskikh fakul’tetov, 2nd ed. Leningrad, 1968.



Atmospheric water vapor content, expressed in any of several measures, such as relative humidity.


Water vapor within a given space or environment.


a measure of the amount of moisture in the air
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