Dew Point Calculator
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Enter any two of air temperature, relative humidity and dew point and leave the third box blank. The dew point calculator uses the Magnus formula and rates how muggy the air will feel.
At 70 °F and 65% relative humidity, the default example, the dew point is about 57.7 °F (14.3 °C): comfortable, but noticeable on a warm afternoon.
What is humidity?
Humidity is the amount of water vapor in the air. It is invisible; clouds, fog and steam are tiny liquid droplets that form once vapor condenses. The amount of vapor air can hold before it condenses depends strongly on temperature, roughly doubling for every 11 °C (20 °F) of warming, which is why warm summer air can be far more humid than cold winter air.
Absolute humidity is the mass of water vapor in a given volume of air, such as grams per cubic meter. Specific humidity is the mass of vapor per kilogram of air, a measure meteorologists like because it does not change when air expands or contracts.
Relative humidity, the figure in most forecasts, is the amount of vapor present as a percentage of the maximum the air could hold at its current temperature. Because that maximum depends on temperature, relative humidity changes during the day even when the amount of moisture does not: cool mornings often show high relative humidity, which drops as the afternoon warms. That makes relative humidity a poor guide to how muggy it feels.
Indoors, relative humidity between about 30% and 50% is generally recommended. Air that is too dry dries skin and airways and causes static, while persistently high humidity encourages mold, dust mites and condensation on windows and cold walls.
What is dew point?
The dew point is the temperature to which air would have to cool, at constant pressure and moisture content, to become saturated. Cool it any further and water condenses: dew forms on grass, fog forms in valleys, and a cold drink glass sweats. When the dew point is below freezing it is called the frost point.
Unlike relative humidity, the dew point measures how much moisture is actually in the air, so it is the better guide to comfort. The closer the air temperature is to the dew point, the higher the relative humidity; when they are equal it is 100%. Forecasters watch the dew point to predict overnight lows, fog and thunderstorms.
| Dew point | How it feels |
|---|---|
| Below 50 °F (10 °C) | Dry, comfortable |
| 50 – 54 °F (10 – 12 °C) | Very comfortable |
| 55 – 59 °F (13 – 15 °C) | Comfortable |
| 60 – 64 °F (16 – 18 °C) | Getting sticky |
| 65 – 69 °F (18 – 21 °C) | Humid, uncomfortable |
| 70 – 74 °F (21 – 24 °C) | Very humid, oppressive |
| 75 °F (24 °C) and above | Extremely oppressive |
Td = c·γ ÷ (b − γ), γ = ln(RH ÷ 100) + b·T ÷ (c + T)
- T
- air temperature in °C
- RH
- relative humidity in percent
- b, c
- Magnus constants 17.625 and 243.04 °C
Accurate to within about 0.1 °C for typical weather temperatures.
Frequently asked questions
What is a comfortable dew point?
Most people find dew points of 55 °F (13 °C) or lower comfortable. Above 65 °F (18 °C) it feels sticky, and above 70 °F (21 °C) oppressive.
Can the dew point be higher than the temperature?
No. The air temperature can fall to the dew point, at which point the air is saturated, but extra moisture condenses rather than raising the dew point further.
What is the difference between dew point and humidity?
Relative humidity is a percentage that depends on temperature. Dew point is a temperature that reflects the actual amount of moisture in the air.
Why does dew form at night?
Grass and other surfaces cool by radiating heat to a clear sky. When they fall below the dew point, water vapor condenses on them.
How do I calculate dew point in Excel?
With the temperature in °C in A2 and relative humidity in percent in B2, enter =243.04*(LN(B2/100)+17.625*A2/(243.04+A2))/(17.625-LN(B2/100)-17.625*A2/(243.04+A2)). This is the Magnus formula the calculator uses. For 21 °C and 65% humidity it returns about 14.2 °C.
Is there a quick way to estimate dew point?
When relative humidity is above about 50%, the dew point in °C is roughly the temperature minus (100 − RH) ÷ 5. At 25 °C and 70% humidity that gives 25 − 6 = 19 °C, close to the 19.1 °C from the Magnus formula. Use the calculator when you need better than a degree or two.
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