Weather

Dew Point Calculator

Find the dew point — the temperature at which air becomes saturated so moisture can condense as dew, frost, or fog.

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°C
%

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Result

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The formula

Dew point is computed from air temperature and relative humidity using the Magnus–Tetens approximation, a standard engineering fit for vapor pressure over water. When RH approaches 100%, dew point approaches the air temperature.

Td = (b·α)/(a−α) where α = (a·T)/(b+T) + ln(RH/100)

Worked example

  1. Air temperature 20 °C
  2. Relative humidity 60%
  3. Dew point ≈ 12 °C

Result: ≈ 12 °C

How dew point is calculated

Dew point tracks absolute moisture in the air better than relative humidity alone.

Magnus approximation

We invert a Magnus-type saturation vapor-pressure formula. Constants a and b are empirical; results are accurate enough for everyday weather and HVAC use, not for laboratory humidity metrology.

RH vs dew point

Relative humidity rises when air cools even if moisture content is unchanged. Dew point stays roughly steady when moisture is steady — which is why meteorologists prefer it for “how muggy is it?”

Comfort and fog

Many people find dew points above ~18 °C (64 °F) muggy and above ~21 °C (70 °F) oppressive. When temperature nears dew point, fog or dew becomes more likely.

Interesting facts

Comfort cue

Many people find dew points above ~18 °C (64 °F) muggy, and above ~21 °C (70 °F) oppressive.

Fog clue

When air temperature nears the dew point, relative humidity approaches 100% and fog or dew becomes more likely.

Not the same as RH

Relative humidity depends on temperature. A cool morning can show high RH while the dew point (absolute moisture) stays modest.

Heat-index cousin

High dew points make hot days feel worse because sweat evaporates slowly — the same moisture that raises heat index.

Aviation and frost

Pilots watch temperature–dew-point spreads for fog risk; gardeners watch them for overnight frost potential.

Frequently asked questions

Relative humidity depends on temperature; dew point is an absolute moisture measure expressed as a temperature. High dew point means more moisture in the air.

No. At 100% RH they are equal; otherwise dew point is lower than air temperature for moist air over liquid water.

From temperature and RH using a Magnus vapor-pressure approximation, then solving for the temperature that would saturate the air at that moisture content.

Many people find dew points above about 18 °C (64 °F) muggy and above about 21 °C (70 °F) oppressive — personal comfort still varies.

References

  1. Dew point vs humidity — U.S. National Weather Service Official explanation of dew point as a moisture measure.
  2. Magnus formula background — National Oceanic and Atmospheric Administration (related humidity education) Worked meteorological relationships among T, RH, and dew point.