Page:Handbook of Meteorology.djvu/76

 64 That is, with the relative humidity at 50 per cent, the temperature of comfort is 10 degrees lower than with a very dry or a moist air. These conclusions do not differ from those of Dr. Huntington.

Many manufacturers have installed humidifiers within their factories in order to provide wholesome air to their employees and a correct atmosphere for the economical production of their output. Exhaust ducts carry the air from the work rooms to the humidifier where it is screened, washed, and returned to the various rooms with but little loss of temperature. The saving in fuel very soon pays the cost of a humidifying plant.

Forms of Condensation.—The condensation of the water vapor of the air takes place in many forms—fog, cloud, dew, frost, rain, snow, and hail. The “sweating” of walls, and the film of moisture that forms on the outside of a vessel filled with iced water are also examples of condensation. In any case the cause is the same; the temperature of the air falls below the temperature of saturation and the excess of water vapor is condensed in one or another of the forms noted. The formation of fog and cloud are considered in the following chapter; hail is a feature of thunder-storms.

Dew.—Dew consists of the moisture condensed on such surfaces as radiate their warmth after sundown. If the chilling of the air next to such surfaces carries its temperature below that of saturation—that is, the “dew-point”—the excess is deposited in the form of minute droplets. Not infrequently so much moisture is deposited that foliage and grass become very wet. Vegetation radiates its heat rapidly, and therefore dew is apt to form copiously thereon. At night the temperature of the air two or three inches from the ground may be as much as 5 degrees lower than at a height of 6 feet. Dew therefore may form on grassy surfaces when none occurs on objects materially above ground.

Falling temperature at night is the rule; nevertheless, dew does not always form. The temperature may not go down to the dew-point; the absolute humidity may be very low; wind may keep the air stirring so that the air next the ground may not remain long enough to be cooled to the dew-point; low clouds may prevent the radiation of ground warmth; a “lid”