At what temperature will your windows fog up?
Find the dew point from temperature and humidity, or the humidity from the dew point.
What you enter stays on your device while you use this tool. Opening a shared link sends the settings in its address to HowToolbox to prepare the page.
More options
Dew point
53.6 °F
Absolute humidity 10.3 g/m³
T_d = c·γ / (b − γ), γ = ln(RH/100) + b·T / (c + T), b = 17.62, c = 243.12 °C
Sponsored Links
Find the dew point from temperature and humidity, or the humidity from the dew point.
How to use
- Choose what to find: the dew point, the relative humidity or the air temperature.
- Type the other two, for example the room temperature and the humidity from a hygrometer. Switch to °F under More options.
- Read the answer and the absolute humidity, the grams of water in each cubic metre of air.
Dew point at an air temperature of 20 °C
How it works
Warm air can hold more water vapour than cold air. Relative humidity says how full the air is compared with what it could hold at its temperature. Cool that air and it fills up: the dew point is the temperature at which it is full and water starts to condense, on a cold window, a bottle from the fridge or the grass at night.
The calculator uses the Magnus formula for the pressure of saturated water vapour, eₛ = 6.112 hPa × exp(17.62 T / (243.12 + T)), with T in °C and the coefficients of Sonntag (1990). The dew point is where the vapour in the air would saturate: with γ = ln(RH/100) + 17.62 T / (243.12 + T), T_d = 243.12 γ / (17.62 − γ). For example, a room at 20 °C and 60 % humidity has its dew point at 12.0 °C, so a window colder than that mists up. Absolute humidity follows from the ideal gas law: about 10.3 g of water per m³ here.
The formula is accurate to about 0.35 °C between −45 °C and 60 °C, over liquid water. Below freezing, frost forms at a slightly higher temperature than the dew point shown.
Sponsored Links
Frequently asked questions
What dew point feels comfortable?
Most people find a dew point below about 13 °C (55 °F) comfortable, while above 18 °C (65 °F) the air feels muggy, whatever the temperature.
Why do my windows fog up in winter?
The glass is colder than the room air's dew point. Lowering the humidity, airing the room or warmer glass (better insulation) stops it.
What is the difference between relative and absolute humidity?
Absolute humidity is the amount of water in the air, in grams per cubic metre. Relative humidity is that amount as a share of the most the air could hold at its temperature, so it changes when the air warms or cools even if the water stays the same.
Can the dew point be above the air temperature?
Not for the inputs accepted here. At 100% relative humidity, dew point equals air temperature.
Is dew point the same as frost point?
No. This calculation is for liquid water; below freezing, frost point is slightly higher.
Next you might need
- Density altitudePressure and density altitude of your airfield from elevation, QNH and temperature, with what the thin air means for take-off., private, runs on your deviceAviationCalculatorFlyingWeatherTemperature
- Laundry symbolsEvery care-label symbol decoded: wash tub, triangle, square, iron and circle, with temperatures, dots and bars explained in plain words., private, runs on your deviceEverydayReferenceTemperature
- METAR and TAF decoderPaste an aviation weather report and read every group in plain words, with the flight category, humidity and density altitude., private, runs on your deviceAviationReferenceWeatherFlyingEncoding
- Beaufort wind scaleTurn a wind speed in m/s, km/h, knots or mph into its Beaufort force and name., private, runs on your deviceUnit conversionConverterWeatherSpeed
- Altimeter settingsAltitude on QNH, flight level and QFE side by side, with hPa ↔ inHg ↔ mmHg, the transition level, cruising levels and the cold-temperature correction., private, runs on your deviceAviationCalculatorFlyingConverterTemperature
- Crosswind calculatorCrosswind and headwind or tailwind on any runway, with gusts, a runway diagram and your aircraft's crosswind limit., private, runs on your deviceAviationCalculatorFlyingWeatherSpeed
Popular now
Sponsored Links
Sponsored Links
Sponsored Links