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Dew Point Calculator

With air temperature 25 °C, relative humidity 60 percent, dew point comes to 16.70 °C — dew point. It is reached in 5 steps, the last of which is 243.04 * 1.1330522 / (17.625 - 1.1330522), and each one is printed on the page with its numbers filled in. The formula is the one published by NOAA National Weather Service, not an approximation fitted to it.

Dew point from temperature and humidity by the Magnus formula, with what that dew point actually feels like.

Formula and sources checked · How we check

Air temperature 25 °C, Relative humidity 60

16.70 °C

Dew point for the example below. Editing a field recomputes the calculator below; this figure holds the answer the page was loaded with.

It is written into the HTML rather than drawn by a script, so a search engine reading this page without running JavaScript still finds an answer.

Dew point
16.70 °C
Dew point
243.04 * 1.1330522 / (17.625 - 1.1330522)16.698 °C
Dew point in Fahrenheit
16.697664 * 9 / 5 + 3262.056 °F
Spread between air and dew point
25 - 16.6976648.302 °C
Comfort band, 0 dry to 4 oppressive
2
Frost forms if the surface reaches
min(16.697664, 0)0 °C

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Worked example

25 °C at 60% humidity has a dew point of 16.7 °C — the temperature at which the air would be saturated. That is where "sticky" begins, and it is the number meteorologists watch rather than the humidity percentage.

How to work it out yourself

  1. 1.Enter the air temperature and the relative humidity. The dew point is always at or below the air temperature, and equal to it only at 100%.
  2. 2.Read the spread. A small gap means fog and condensation are close; a large one means dry air whatever the humidity reading says.
  3. 3.Use the dew point rather than the humidity to judge how it will feel. 60% humidity at 5 °C is dry; 60% at 30 °C is oppressive.

The formula

  1. Dew point243.04 * 1.1330522 / (17.625 - 1.1330522)
  2. Dew point in Fahrenheit16.697664 * 9 / 5 + 32
  3. Spread between air and dew point25 - 16.697664
  4. Comfort band, 0 dry to 4 oppressive
  5. Frost forms if the surface reachesmin(16.697664, 0)

Source: NOAA National Weather Service — humidity and dew point calculations

Questions people actually ask

What is the dew point?
The temperature at which the air would have to cool for its water vapour to start condensing. It is an absolute measure of how much moisture the air holds, unlike relative humidity, which is a ratio against what that air could hold at its current temperature.
Why is dew point better than humidity for comfort?
Because relative humidity moves with temperature even when the moisture does not. Air at 60% humidity is dry at 5 °C and oppressive at 30 °C. Dew point does not shift like that: below 10 °C is comfortable, 16 to 18 is noticeably humid, above 21 is uncomfortable and above 24 is oppressive, wherever you are.
Why does my window fog up?
Because its surface is below the dew point of the air inside. Condensation forms whenever a surface is colder than the dew point, which is why a cold drink sweats and why insulating a wall stops damp appearing on it. The fix is either warmer surfaces or drier air, and the dew point tells you how much of either you need.
How accurate is the Magnus formula?
Within about 0.4 °C from −40 to +50 °C, using the Alduchov and Eskridge coefficients this page uses. That is better than the instruments most people have, and the error grows only at temperature extremes well outside ordinary weather.

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