Humidity
the condition of the atmosphere with respect to its water-vapor content, may be absolute or relative, the former being measured by the actual amount of vapor in the air per unit volume, while the latter is the ratio between such actual amount and the maximum amount which the air would contain at the same temperature. Relative humidity may thus be expressed as a percentage, as it is in the daily weather reports. The higher the temperature of the air the more water vapor it will absorb, whence the drying power of heated air and the dryness commonly found in furnace-heated houses where cold air is drawn in from outside and highly heated without any new supply of moisture. On the other hand, high relative humidity means lessened drying power, so that a combination of high temperature with a high degree of humidity produces feelings of great physical discomfort, the humidity retarding the evaporation of perspiration by which the body heat is largely regulated. Extremely high temperatures in arid regions are on this account found to be more endurable than considerably lower ones where the atmosphere is moist. While vapor baths of over 80 degrees are insupportable, a temperature of 200 degrees Fahrenheit may be borne for several minutes in dry air. Humidity is now recognized as one of the most important factors in ventilation, and the installation of humidifying and dehumidifying systems has become an engineering specialty. Ac- of Nations, September 18, 1922.