Air temperatures adjusted to represent human comfort or discomfort under prevailing conditions of temperature, humidity, radiation, and wind. Theoretical formulas attempt to compare the rate of heat loss to surroundings with rate of heat production by work and metabolism. Most modern empirical relations, based on relative comfort expressed by human subjects under differing atmospheric combinations, attempt to indicate the temperature at which air at some standard humidity, air motion, and radiation load would be just as uncomfortable (or comfortable). Many former indices, however, had arbitrary scales.