It must be understood that not all thermal stress and strain are adverse to humans. Due to the fine line between acceptable and unacceptable levels of heat stress and strain, extreme caution must be exercised to avoid cumulative harm to individuals. When laypersons speak of the prevailing air temperature, determined from a conventional thermometer, they are speaking of the drybulb temperature. Any decrease in the moisture content of the air will result in evaporation from the wetted wick of the wet-bulb thermometer, and in turn, the bulb of the thermometer will be cooled to a temperature which reflects the reduced moisture content of the air. Measurements of Humidity.
Humidity is an expression of the quantity of water vapor mixed with the other atmospheric gases.
Vapor pressure is a measure of water content in the atmosphere under given conditions. Relative humidity is primarily a ratio of partial and saturated vapor pressures, not a measure of water content. Electronic, motorized psychrometers are available to provide direct readout of DB, WB, RH and Td. It is measured by various instruments depending upon the velocities of air movement. In industrial situations any part of the heat radiation spectrum may be present.