The dew point is the temperature to which air must cool, without changing its moisture or pressure, for it to become saturated and for water vapor to start condensing into droplets. When the temperature falls to the dew point, dew, fog or condensation on windows appears; if the dew point is below 0 °C (32 °F), you get frost instead. It is the best single measure of how much water the air really holds, and so of how muggy a day feels.
Key takeaways
- The dew point is the temperature at which air reaches 100 % relative humidity and vapor condenses.
- The National Weather Service describes dew points of 55 °F (13 °C) or lower as dry and comfortable, 55–65 °F as sticky, and 65 °F (18 °C) or higher as oppressive.
- It is calculated from temperature and relative humidity with the Magnus formula; the WMO recommends the constants 17.62 and 243.12 °C.
- When the evening temperature is within about 2–3 °C (4–5 °F) of the dew point and the night will be clear and calm, fog or dew is likely.
- The highest dew point on record is 35 °C (95 °F), measured at Dhahran, Saudi Arabia, on 8 July 2003, according to NOAA.
What is dew point in simple terms?
In simple terms, the dew point is the temperature at which the air would be completely full of water vapor. Unlike relative humidity, which changes whenever the temperature rises or falls, the dew point only changes when moisture is added to or removed from the air.
An everyday example: take a can out of the fridge at 5 °C (41 °F) onto a patio at 25 °C (77 °F) with 50 % humidity, and it is covered in droplets within seconds. The dew point of that air is about 14 °C (57 °F): the air touching the can cools below that and its vapor condenses.
Dew point vs humidity
Dew point and relative humidity are linked: the closer the temperature is to the dew point, the higher the relative humidity, and when they are equal it is 100 %. The difference is what they tell you. Relative humidity says how close the air is to saturation at the current temperature; the dew point says how much water is actually in it. Our guide to what relative humidity is explains the percentage in detail.
That is why 90 % humidity on a 10 °C (50 °F) morning feels fresh, while 50 % at 35 °C (95 °F) feels heavy: the first has a dew point near 8 °C (47 °F), the second near 23 °C (73 °F).
Dew point comfort chart
The dew point comfort chart below shows how most people feel at each dew point. It is not an official scale, but it is widely used by meteorologists because the dew point directly reflects how hard it is for your body to evaporate sweat.
| Dew point | How it feels |
|---|---|
| Below 50 °F (10 °C) | Dry, pleasant |
| 50–55 °F (10–13 °C) | Comfortable |
| 55–60 °F (13–16 °C) | Pleasant for most; humidity starts to show |
| 60–65 °F (16–18 °C) | Somewhat humid, sticky evenings |
| 65–70 °F (18–21 °C) | Humid and muggy for many people |
| 70–75 °F (21–24 °C) | Oppressive |
| Above 75 °F (24 °C) | Very oppressive, typical of tropical climates |
The National Weather Service uses a simpler three-step version: 55 °F or lower is dry and comfortable, 55 to 65 °F becomes sticky with muggy evenings, and 65 °F or higher means lots of moisture and oppressive air.
Dew points of 70 °F (21 °C) and above are common in summer along the US Gulf Coast, in Florida and on the Mediterranean coast of Spain, which explains the sticky nights in Miami or Valencia. In the Caribbean, as in Cancún, dew points above 75 °F (24 °C) are normal. Runners often watch the dew point rather than the temperature, because the higher it is, the more slowly sweat evaporates and the less it cools the body. On hot, humid days, know the heat stroke symptoms.
Dew point chart by temperature and humidity
This dew point chart is calculated with the Magnus formula and the WMO constants. Find the air temperature in the first column and the relative humidity along the top. Values are in °F:
| Temperature | RH 30 % | RH 40 % | RH 50 % | RH 60 % | RH 70 % | RH 80 % | RH 90 % |
|---|---|---|---|---|---|---|---|
| 40 °F (4 °C) | 11 °F | 18 °F | 23 °F | 27 °F | 31 °F | 34 °F | 37 °F |
| 50 °F (10 °C) | 20 °F | 27 °F | 32 °F | 37 °F | 41 °F | 44 °F | 47 °F |
| 60 °F (16 °C) | 28 °F | 36 °F | 41 °F | 46 °F | 50 °F | 54 °F | 57 °F |
| 70 °F (21 °C) | 37 °F | 45 °F | 50 °F | 55 °F | 60 °F | 64 °F | 67 °F |
| 80 °F (27 °C) | 46 °F | 54 °F | 60 °F | 65 °F | 69 °F | 73 °F | 77 °F |
| 90 °F (32 °C) | 54 °F | 62 °F | 69 °F | 74 °F | 79 °F | 83 °F | 87 °F |
| 100 °F (38 °C) | 63 °F | 71 °F | 78 °F | 84 °F | 88 °F | 93 °F | 97 °F |
Notice one detail: 100 °F at 30 % humidity (dew point 63 °F) holds less water than 80 °F at 70 % (69 °F). That is why a dry afternoon in Phoenix or Madrid is easier to bear than a humid evening in New Orleans, even though the thermometer reads higher.
How do you calculate dew point?
The most common way to calculate the dew point is the Magnus approximation, which uses the air temperature (T, in °C) and relative humidity (RH, in %). Every online dew point calculator uses some version of it:
- First calculate an intermediate value: γ = ln(RH/100) + (a × T) / (b + T)
- Then the dew point: Td = (b × γ) / (a − γ)
With the constants a = 17.62 and b = 243.12 °C, which the WMO lists in its guide to meteorological instruments, the result is very accurate between -45 °C and 60 °C (-49 °F and 140 °F). Other versions use slightly different constants (17.27 and 237.7) and differ by tenths of a degree.
Worked example
At 21 °C (70 °F) and 60 % relative humidity:
- γ = ln(0.60) + (17.62 × 21) / (243.12 + 21) = -0.511 + 1.401 = 0.890
- Td = (243.12 × 0.890) / (17.62 − 0.890) = 12.9 °C (55 °F)
Without a calculator, a quick rule works well when humidity is above 50 %: Td ≈ T − (100 − RH) / 5, with temperatures in °C. At 21 °C and 60 %, it gives 13 °C, almost the same as the full formula.
Below freezing, when vapor deposits directly as ice, meteorologists speak of the frost point. The WMO uses the constants 22.46 and 272.62 °C in the same formula, and the result is slightly above the dew point.
What is dew point used for?
The dew point is used to forecast fog, dew and frost, to prevent condensation at home and to judge mugginess. These are the most common uses.
Forecasting fog and dew
On clear, calm nights the ground and the air cool down. If the temperature drops to the dew point, dew forms on grass and cars; if a whole layer of air saturates, fog forms. Forecasters watch the gap between temperature and dew point, called the dew point spread: when it is only 2–3 °C (4–5 °F) in the evening and the night will be calm, fog is likely. The guide on fog vs mist explains the different types.
Forecasting frost
If the dew point is below 0 °C (32 °F) and the temperature reaches it, vapor deposits directly as ice: that is frost. If the air is so dry that its dew point is far below the night's minimum, it can freeze without any visible ice.
Preventing condensation at home
Any surface colder than the dew point of the indoor air gets wet. At 21 °C (70 °F) and 60 % humidity, the dew point is 12.9 °C (55 °F): if a single-glazed window is at 10 °C (50 °F) on a winter morning, it will stream with water. Lower the humidity to 45 % and the dew point falls to 8.6 °C (47 °F), and the condensation disappears. That is why ventilating and using extractor fans works better than wiping windows every day.
How to check the dew point with Clima de Hoy
Clima de Hoy shows the current dew point for each city next to humidity and visibility, calculated with the same Magnus formula and WMO constants described above. Compare the weather in Miami with the weather in Madrid on a summer afternoon: with similar temperatures, Miami's dew point is usually far higher.
To anticipate fog or frost, look at the forecast minimum in the hourly view, such as the hourly forecast for New York, and compare it with the current dew point. If the minimum gets close to it and the night will be clear and calm, expect fog, dew or frost by morning.
Frequently asked questions
What is a comfortable dew point?
A dew point below about 55 °F (13 °C) feels comfortable to most people. Between 55 and 65 °F (13–18 °C) the air starts to feel sticky, and above 65 °F (18 °C) the National Weather Service describes it as oppressive. Above 75 °F (24 °C), conditions are typical of the tropics.
Can the dew point be higher than the temperature?
Not for long. The dew point can at most equal the air temperature, when relative humidity is 100 %. If the air cools further, the extra vapor condenses into fog, dew or rain, and the dew point falls along with the temperature.
What is the dew point when it rains?
When it rains, the dew point is usually very close to the air temperature, because falling rain evaporates and moistens the air near the ground. Relative humidity often rises above 90 %. After a cold front passes, though, the dew point can drop sharply even while showers continue.
Why do car windows fog up?
Car windows fog up because the glass is colder than the dew point of the air inside, which is loaded with vapor from breathing and wet clothes. The air conditioning fixes it because it dries the air and lowers its dew point. Blowing warm air onto the windshield also helps, because it warms the glass above that value.
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