The New International Encyclopædia/Humidity

HUMIDITY (from Lat. humiditas, moisture, from humidus, umidus, moist, from humere, umere, to be moist; connected ultimately with Gk. ἱγρός, hygros, Icel. vökr, moist). Atmospheric. The moisture or aqueous vapor in the atmosphere. This vapor is really an invisible gas, and is the most important component of the atmosphere as to quantity next after nitrogen and oxygen. When this invisible vapor becomes visible it is called dew, fog, mist, haze, cloud, rain, hail, snow, frostwork, or frost, according to the size of the drops of water or the method in which the vapor condenses. Water and ice are not included under the term ‘atmospheric humidity,’ that term being strictly confined to the invisible vapor.

The aqueous vapor in the air is perpetually falling to the earth as rain or snow, and is renewed by evaporation from the ocean, the lakes and rivers, and the soil itself. The quantity of aqueous vapor in a cubic foot of air varies greatly with temperature and locality on the earth's surface. Thus in the air of Arizona and New Mexico there is oftentimes only from 3 to 10 per cent. of the amount that could be held in case the air was saturated. The quantity that can be held in a saturated space varies greatly with the temperature, but does not depend upon the barometric pressure, and is also quite independent of the presence of air in that same space, since the aqueous vapor and the dry air co-exist side by side. The elastic pressure of the dry air and the elastic pressure of the aqueous vapor added together produce what is ordinarily called barometric or atmospheric pressure. The weight of the moisture and the weight of the dry air added together determine the density of a cubic foot of atmosphere at any time. When a cubic foot of space contains as much aqueous vapor as it can possibly hold at any given temperature, it is said to be saturated with moisture. The following table shows that there may be as much in an extreme case as twenty grains of invisible aqueous vapor in a cubic foot if the space is saturated at the temperature of 100° F., and this vapor will exert an elastic pressure of about one pound to the square inch, such as would be counterbalanced by a column of mercury of about 1.9 inches of the barometer. If, now, dry air is added to this cubic foot of space until a pressure of thirty inches is exerted in all directions, then the weight of the dry air at a temperature of 100° F. will be 465 grains per cubic foot, and the combined weight of the vapor and the air, or the so-called saturated air, will be 485 grains. The following table gives these relations for saturation at a pressure of thirty inches and for temperatures between zero and a hundred, computed according to the data adopted in the Psychrometric Tables of Prof. C. F. Marvin (United States Weather Bureau, 1900):

Pressure and Weight of Vapor and Air in a Cubic Foot of Atmosphere at a Pressure of 30 Inches when Saturated at the Respective Temperatures

TEMPERATURE	Vapor  pressure	Dry air  pressure	Vapor weight	Dry air weight	Total weight saturated air
DEGREES FAHR.	Inches	Inches	Grs. per cu. ft.	Grs. per cu. ft.	Grs. per cu. ft.
0	0.038	29.962	0.48	605.32	605.80
10	0.063	29.937	0.78	591.93	592.71
20	0.103	29.897	1.24	578.79	580.03
30	0.164	29.836	1.94	565.79	567.73
40	0.247	29.753	2.85	552.89	555.74
50	0.360	29.640	4.08	539.93	544.01
60	0.517	29.483	5.74	526.75	532.49
70	0.732	29.268	7.98	513.00	520.98
80	1.022	28.978	10.93	498.48	509.41
90	1.408	28.592	14.79	482.65	497.64
100	1.916	28.084	19.77	465.77	485.54

This table shows that the capacity of the unit volume of space or air for aqueous vapor increases very rapidly with rising temperature. When the space is not saturated, the atmosphere is said to have a relative humidity expressed as a given percentage of complete saturation. Thus if the air has a temperature of 100 degrees and contains only 10 grains of vapor per cubic foot, it contains only 50 per cent. of the maximum amount possible at that temperature.

The humidity of the atmosphere is usually determined by either the dew-point apparatus or the psychrometric apparatus. In the former the air is cooled down without altering the quantity of moisture that it contains, and the temperature at which that moisture saturates the air is determined. This dew-point temperature is always lower than the temperature of the free air. In the psychrometric method the temperature of a thin layer of water that is evaporating under standard conditions in the open atmosphere is determined. From this temperature of evaporation the psychrometric formula gives us the vapor-tension, the temperature of the dew-point, and the quantity of moisture in the air. In general the temperature of the dew-point is about as far below the temperature of evaporation as the latter is below the temperature of the air.

The rate of evaporation from a moist surface diminishes with increasing humidity of the air, so that the total evaporation under a given wind in any given unit of time indicates the average dryness of the air during that time.

Nearly all animal and vegetable substances, by reason of their cellular structure, absorb moisture from moist air, but give it up to very dry air. They are, therefore, perpetually expanding and contracting, curling and uncurling, and their changes may be utilized in the construction of hygrometers. When moist air cools by radiation at night to temperatures below the dew-point, the vapor is precipitated, forming cloud, mist, fog, or dew. On the other hand, when air rises, thereby coming imder less pressure, it expands, does work, and is cooled dynamically by reason of the work done. Ordinary air cools at the rate of one degree Fahrenheit for 183 feet of ascent. In this way air that is forced over a mountain may be cooled below the dew-point and form cloud or rain. The moisture in ordinary dry air is easily absorbed by many substances, such as sugar, flour, salt, and in very moist weather objects may become so damp that fungus germs floating in the air and settling on them take root and cover them with mold, or set up fermentation within them.

The humidity of the atmosphere, although invisible, has a special and strong influence in absorbing radiant energy, whether from the sun or from the earth. It therefore converts the dry air into a powerful obstructor to the passage of radiant heat, and by this means the humid atmosphere is made to act as a blanket, keeping the earth and the lower air much warmer than it would otherwise be. See Hygrometer.

COPYRIGHT, 1902, BY DODD, MEAD & COMPANY.