Humidity · Magnus formula
Dew Point Calculator
The dew point is the temperature air would have to cool to for dew or fog to form. Unlike relative humidity, it does not change as the day warms up, which makes it the better measure of how muggy it feels.
Formula
- γ = ln(RH/100) + a·T/(b + T)
- Td = b·γ / (a − γ)
- RH = 100·exp[a·Td/(b + Td) − a·T/(b + T)]
- a = 17.625, b = 243.04°C; T and Td in °C.
Source: Magnus formula with the coefficients of Alduchov and Eskridge (1996), “Improved Magnus Form Approximation of Saturation Vapor Pressure”, Journal of Applied Meteorology.
Valid range
- The coefficients are fitted for -40°C to 50°C (−40°F to 122°F) over liquid water. Outside that range the calculator still answers but flags the result as an extrapolation.
- Relative humidity must be above 0% for a dew point to exist, and a dew point can never be higher than the air temperature: the relative-humidity calculator rejects that combination.
- Below freezing, frost forms at the frost point, which is slightly higher than the dew point over water. This calculator gives the dew point over water, as weather reports do.
Why forecasters watch the dew point
Relative humidity is relative to temperature: the same air reads 90% at dawn and 45% by mid-afternoon without gaining or losing any moisture. The dew point measures the moisture itself, so it stays roughly constant through the day unless a different air mass moves in.
That is why it maps well to comfort. Most people start to notice mugginess once the dew point passes about 60°F (16°C), and dew points in the 70s °F (21°C and up) feel oppressive. Dew points in the 40s °F feel crisp, whatever the relative humidity says.
The gap between temperature and dew point also matters for fog and frost. When the overnight low is forecast to reach the dew point, expect dew, fog or, below freezing, frost.
Dew point by temperature and humidity (°F)
Generated from the same equation as the calculator above.
| Temperature (°F) / Humidity (%) | 20% | 30% | 40% | 50% | 60% | 70% | 80% | 90% | 100% |
|---|---|---|---|---|---|---|---|---|---|
| 30° | −6 | 2 | 9 | 14 | 18 | 21 | 25 | 27 | 30 |
| 40° | 2 | 11 | 18 | 23 | 27 | 31 | 34 | 37 | 40 |
| 50° | 10 | 20 | 27 | 32 | 37 | 41 | 44 | 47 | 50 |
| 60° | 19 | 28 | 36 | 41 | 46 | 50 | 54 | 57 | 60 |
| 70° | 27 | 37 | 45 | 51 | 55 | 60 | 64 | 67 | 70 |
| 80° | 35 | 46 | 54 | 60 | 65 | 69 | 73 | 77 | 80 |
| 90° | 44 | 54 | 62 | 69 | 74 | 79 | 83 | 87 | 90 |
| 100° | 52 | 63 | 71 | 78 | 84 | 88 | 93 | 97 | 100 |
Dew point by temperature and humidity (°C)
Generated from the same equation as the calculator above.
| Temperature (°C) / Humidity (%) | 20% | 30% | 40% | 50% | 60% | 70% | 80% | 90% | 100% |
|---|---|---|---|---|---|---|---|---|---|
| −10° | −29 | −24 | −21 | −18 | −16 | −14 | −13 | −11 | −10 |
| −5° | −25 | −20 | −17 | −14 | −12 | −10 | −8 | −6 | −5 |
| 0° | −20 | −16 | −12 | −9 | −7 | −5 | −3 | −1 | 0 |
| 5° | −16 | −11 | −7 | −5 | −2 | 0 | 2 | 3 | 5 |
| 10° | −12 | −7 | −3 | 0 | 3 | 5 | 7 | 8 | 10 |
| 15° | −8 | −2 | 2 | 5 | 7 | 10 | 12 | 13 | 15 |
| 20° | −4 | 2 | 6 | 9 | 12 | 14 | 16 | 18 | 20 |
| 25° | 0 | 6 | 10 | 14 | 17 | 19 | 21 | 23 | 25 |
| 30° | 5 | 11 | 15 | 18 | 21 | 24 | 26 | 28 | 30 |
| 35° | 9 | 15 | 19 | 23 | 26 | 29 | 31 | 33 | 35 |
| 40° | 13 | 19 | 24 | 28 | 31 | 33 | 36 | 38 | 40 |
Dew point questions
- What is a comfortable dew point?
- There is no official scale, but most people find dew points below about 55°F (13°C) comfortable, start to feel it is sticky around 60°F (16°C), and find 70°F (21°C) and above oppressive. Personal tolerance and what you are used to both matter.
- Can the dew point be higher than the temperature?
- Not in a stable reading. When air cools to its dew point it is saturated (100% relative humidity), and further cooling condenses the extra moisture into dew, fog or cloud. A reading with the dew point above the temperature usually means one of the two sensors is off.
- What is the difference between dew point and relative humidity?
- Dew point is an absolute measure of how much water vapor is in the air. Relative humidity is how close the air is to saturation at its current temperature, so it rises at night and falls in the afternoon even when the moisture stays the same.
- How accurate is the Magnus formula?
- With the Alduchov and Eskridge coefficients used here, saturation vapor pressure is within a fraction of a percent of reference values between −40°C and 50°C, which keeps dew point errors well under the precision weather sensors report.