Educational - Dewpoint

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The dewpoint is the temperature the air must be cooled to in order for saturation to occur. For example, when the air temperature is 40 degrees and the dewpoint is 35 degrees, the air will get saturated with water vapor and fog will form if the air cools to 35 degrees. When the air temperature is equal to the dewpoint temperature, the relative humidity is 100%.

Dewpoint is a more useful measure of atmospheric moisture than relative humidity for many applications. For example, we know that air with a dewpoint over 70 degrees fahrenheit always feels uncomfortable. In addition, dewpoints over 65 degrees fahrenheit are typically needed for severe thunderstorm formation (plus other factors, like the presence of a strong cold front).

If no fronts are expected to pass, the dewpoint temperature can also give one an idea of what the minimum temperature tonight will be. The low temperature tomorrow morning is usually a few degrees from this afternoon's dewpoint temperature.

Relative humidity can be computed from dewpoint and temperature or dewpoint can be computed from relative humidity and temperature using these formulas:

rh = 100 * exp(5351/T - 5351/Td)
Td = 5351/(5351/T - ln(rh/100))

Where:
rh  = relative humidity
T   = temperature in Kelvin
Td  = dewpoint temperature in Kelvin
exp = exponetial function
ln  = natural (base e) logarithm

Note: Temperature in Centigrade = ( Temperature in Kelvin - 273.15 )