Article — Dew Point Calculator
Dew Point Calculator: Temperature, Humidity, and Comfort
Dew point is the temperature at which air becomes saturated and condensation begins. The Magnus formula computes it from air temperature and relative humidity with ±0.4 °C accuracy. Unlike relative humidity, dew point is an absolute measure of moisture.
Dew point is the meteorologist's preferred humidity metric because it stays constant as the air warms or cools through the day. A summer afternoon with a 21 °C dew point will still have a 21 °C dew point at dusk, even though the temperature has dropped 10 degrees and the relative humidity has shot up. That stability makes dew point the right number for predicting fog, dew, frost, and human comfort.
What dew point measures
The dew point is the temperature to which air must be cooled, at constant pressure and constant water content, for water vapour to start condensing. If the air around you has a dew point of 15 °C, then cooling any surface (or air pocket) to 15 °C will fog it over. That is why a glass of iced tea sweats on a hot summer day — the glass surface drops below the dew point, and moisture from the air condenses on it.
Dew point depends only on the absolute amount of water vapour in the air. Two cubic metres of air at the same dew point hold exactly the same mass of water, regardless of the temperature. Relative humidity, in contrast, depends on temperature: cool air a bit and the same water content suddenly looks like 100 percent RH.
The Magnus dew point formula
The Magnus equation gives the saturation vapour pressure of water as a function of temperature. Combined with the definition of relative humidity, it produces a closed-form expression for dew point: Td = (b·γ) / (a − γ), where γ = ln(RH/100) + a·T/(b+T). The coefficients a = 17.625 and b = 243.04 °C are the Alduchov-Eskridge values, accurate to ±0.4 °C across –40 to +50 °C.
20 °C, 50 % RH Td = 9.3 °C25 °C, 60 % RH Td = 16.7 °C30 °C, 70 % RH Td = 23.9 °C0 °C, 100 % RH Td = 0 °CDew point vs relative humidity
Relative humidity is a ratio: how much water the air holds compared to how much it could hold at the same temperature. Because the "could hold" rises exponentially with temperature, RH changes with the thermometer even when the actual moisture content is constant. Dew point reports the actual moisture content directly.
For pre-flight checks, climate analyses, agricultural forecasts, and HVAC design, dew point is preferred for exactly this reason. Two air masses with the same dew point have the same water content regardless of temperature. Two air masses with the same RH can carry wildly different amounts of moisture.
The world's highest reliably measured dew point is 35 °C, recorded at Dhahran, Saudi Arabia in July 2003. The air temperature that day was 42 °C, giving a wet bulb temperature near the human survival limit. By contrast, the lowest measured dew point is around –74 °C, recorded in Antarctica.
Dew point comfort scale
People feel comfortable when dew point is below about 13 °C (55 °F). Between 13 and 18 °C (55 to 65 °F), air feels slightly humid but tolerable. Above 21 °C (70 °F) the air feels sticky and physical effort becomes uncomfortable. Above 24 °C (75 °F) the air is oppressive and heat-stress risk climbs steeply, regardless of the actual temperature.
- < 10 °C dry, comfortable air
- 10 to 13 °C ideal comfort range for most people
- 13 to 16 °C slightly humid, still pleasant
- 16 to 18 °C sticky feel
- 18 to 21 °C uncomfortable, sweat doesn't evaporate well
- 21 to 24 °C oppressive, tropical feel
- > 24 °C dangerous heat stress likely
Dew point, fog, and the T–Td spread
The difference between air temperature and dew point — the dew point depression or T–Td spread — is the most useful single number for predicting fog. When the spread is below 2.5 °C, fog, mist, or low cloud is likely. Pilots check the spread before take-off because radiation fog forms during clear nights when the surface temperature drops to the dew point.
Air conditioning engineers use the spread to size dehumidifiers. A 25 °C indoor environment with a target 15 °C dew point requires removing water vapour until the absolute humidity matches that lower dew point. Server rooms typically aim for dew points between 5 and 15 °C to prevent both condensation and static electricity buildup.
Dew point in HVAC, aviation, and growing
In HVAC, dew point determines the latent cooling load. An air conditioner has to remove moisture as well as heat, and the energy needed to condense water from the indoor air is often half of the total cooling demand on a humid summer day.
In aviation, dew point predicts carburettor icing (most likely when air temperature is below 21 °C and humidity is above 80 percent) and cloud-base altitude. The standard rule estimates cloud base as the surface T–Td spread divided by 2.5, multiplied by 1000 feet.
In agriculture and greenhouse management, dew point indicates fungal disease risk. Spores germinate when leaf surfaces stay wet for several hours, which happens when the leaf temperature drops below the dew point at night. Greenhouse heaters and circulation fans manage this by keeping leaf temperature above the air dew point.
Dew point below freezing: the frost point
When dew point falls below 0 °C, water vapour can no longer form liquid droplets. It deposits directly as ice or frost in a process called deposition. The corresponding saturation temperature is the frost point, slightly above the equivalent dew point because the saturation vapour pressure over ice is lower than over supercooled water.
The Magnus formula above uses the water-saturation coefficients, which slightly overestimates dew point at sub-freezing temperatures. For precise work below 0 °C, use the ice-saturation form with a = 21.875 and b = 265.5 °C, or apply a small correction.
Watch the dew point trend, not just the value. A dew point rising by 3 to 5 °C in a few hours signals an incoming warm front or moist air mass, often accompanied by precipitation within 24 hours.
Common dew point mistakes
Dew point and wet bulb temperature both depend on humidity, but they differ. Wet bulb is the temperature an evaporating surface reaches and is always between the dew point and the air temperature. Dew point is colder than wet bulb in every situation except 100 percent humidity, when all three are equal.
Other slips: assuming dew point and humidity move together (they often don't — a sunny morning can have rising temperature and dropping RH while dew point holds steady), reading "dew point" as "the temperature it's going to be tonight" (correct only when air cools to the dew point and stays clear), and ignoring pressure (the Magnus formula assumes standard sea-level pressure; at high altitude, the constants shift slightly).