The formula
How it works
The dew point is the temperature the air must cool to for its moisture to start condensing into dew, fog or droplets. Unlike relative humidity, it is an absolute measure of how muggy the air feels — a higher dew point means stickier air.
FAQ
Dew point or humidity — which tells me how muggy it is?
Dew point is the better guide. Relative humidity changes with temperature, but the dew point directly reflects how much moisture is in the air, so a dew point above about 20 °C (68 °F) feels oppressive regardless of the reading.
Can the dew point be higher than the temperature?
No. The dew point can equal the air temperature — that is 100% humidity, when dew or fog forms — but it can never exceed it.
What dew point counts as comfortable?
Below about 16 °C (60 °F) feels comfortable to most people, 16–20 °C (60–68 °F) starts to feel muggy, and above 20 °C (68 °F) feels oppressive to most.
Why does dew form on grass at night even when it did not rain?
Overnight, surfaces like grass radiate heat and cool below the air temperature; once they reach the dew point, moisture in the air condenses directly onto them.
Does altitude or air pressure affect the dew point?
The Magnus-Tetens formula used here depends only on temperature and relative humidity, so pressure changes have a negligible direct effect on the calculated dew point.
How accurate is the Magnus-Tetens formula?
It is accurate to within a few tenths of a degree for typical temperatures between 0 °C and 50 °C, which is more than precise enough for everyday weather use.
Why does frost form when the dew point is below freezing?
When the dew point itself is at or below 0 °C (32 °F), water vapor deposits directly as ice crystals instead of liquid dew — this is called the frost point.
About the dew point calculator
This calculator finds the dew point from the air temperature and the relative humidity. The dew point is the temperature at which air becomes fully saturated, so any further cooling makes water vapour condense into dew, mist or clouds. Because it does not change when the air simply warms or cools, it is the most reliable single number for how humid and comfortable the air actually is.
How to use it
Pick Celsius or Fahrenheit, then enter the air temperature and the relative humidity. The calculator returns the dew point temperature. For example, air at 25 °C with 50% humidity has a dew point of about 14 °C. A dew point close to the air temperature means the air is nearly saturated and fog or rain is likely; a dew point far below it means dry, comfortable air.
The formula
This uses the Magnus-Tetens approximation. First it computes an intermediate value , then the dew point , where is the temperature in °C, is the relative humidity, and the constants are and . Fahrenheit inputs are converted to Celsius first and the result converted back. The formula is accurate to a fraction of a degree over normal weather conditions.
Where it is used
Meteorologists use the dew point to forecast fog, dew and the chance of rain, and pilots use it to predict cloud base and icing. Air-conditioning and HVAC engineers use it to control condensation and comfort in buildings, and it matters in agriculture, greenhouse management and food storage. Anyone judging how oppressive a summer day will feel gets a truer picture from the dew point than from humidity alone.