“It’s not the heat, it’s the humidity.” This common refrain heard throughout much of the eastern U.S. in summer refers to how the amount of water vapor in the air affects human comfort. Since the main source of body cooling is evaporation of perspiration, the higher the moisture content of the air, the less evaporation takes place and the warmer we feel. One measure of atmospheric moisture is the dew point temperature.
The dew point, as defined by the NWS, is the temperature to which air must be cooled in order to reach saturation. In other words, the difference between the air temperature and the dew point temperature tells you how much moisture is in the air. The greater the distance between the two, the dryer the air is. Conversely, the closer they are together, the higher the moisture content of the air.
While everyone has a different tolerance for humidity, in summer – when air temperatures tend to be high – a dew point temperature of 50°F is generally considered comfortable. Dew points in the 60s are thought of as muggy and once they reach the 70s or higher, the air can feel down right oppressive. On the opposite end of the spectrum, dew points in the 40s or lower are considered dry. Dry air has its own set of comfort issues, including skin irritations.