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.
S. P. KHROMOV