Dew Point Calculator

The dew point is the temperature to which air must be cooled, at constant pressure, for water vapor to condense into liquid. A dew point of 55 F means surfaces at or below 55 F will collect moisture. Unlike relative humidity, the dew point is an absolute measure of atmospheric moisture that does not change as air temperature rises or falls.

The Magnus-Tetens approximation used by the US National Weather Service: alpha = ln(RH/100) + (17.625 x T) / (243.04 + T), then dew point (C) = (243.04 x alpha) / (17.625 - alpha), where T is air temperature in Celsius and RH is relative humidity in percent. The result is accurate to within about 0.4 C for temperatures from 0 C to 60 C and humidity above 1%.

Relative humidity shifts with temperature, so 60% RH at 50 F feels dry while 60% RH at 90 F feels oppressive, which means the same number can describe very different conditions. Dew point is fixed regardless of air temperature, which is why weather forecasters and heat-index charts use it to communicate how humid it actually feels.

The bands are based on dew point in Fahrenheit. Below 50 F is dry and comfortable. From 50 to 59 F is comfortable for most people. From 60 to 64 F starts to feel sticky. From 65 to 69 F is muggy. At 70 F and above the air feels oppressive, and sustained outdoor activity becomes difficult.

Dew point depression is the gap between air temperature and dew point (T minus DP). A small depression of about 3 to 5 F means the air is nearly saturated, making fog, dew, or low clouds likely. A large depression, like 30 F on a dry desert afternoon, means condensation is unlikely and fire weather risk can be elevated.