Dew Point Calculator

Dew point: when air becomes saturated

The dew point is the temperature to which air must be cooled at constant pressure for relative humidity to reach 100% — the moment water vapor begins to condense into droplets. The Magnus-Tetens approximation is Td = (b·γ)/(a − γ) with γ = (a·T)/(b+T) + ln(RH/100), where a = 17.625 and b = 243.04 °C. When the dew point sits close to the air temperature, the environment feels muggy: Td above 20 °C is uncomfortable, and above 24 °C is oppressive. The gap T − Td is the dew point depression — small values mean saturated air, large values mean dry air. Example: T = 25 °C with RH = 60% gives Td ≈ 16.7 °C.

Applications

Fog and dew forecasting (formed whenever surface air cools to its dew point), air conditioning sizing (condensation control on coils), agriculture (frost risk and crop irrigation timing), aviation (METAR reports always include Td next to T), and building physics (condensation inside walls and windows when surface drops below Td).

FAQ

Is dew point the same as relative humidity? No. RH is a percentage relative to the current temperature; dew point is an absolute temperature, so it's a more reliable measure of how much moisture is actually in the air.

Why can dew point never exceed the air temperature? Because it would imply RH above 100%, which is supersaturation — physically the excess vapor immediately condenses, capping Td at T.

What does a high dew point feel like? Sticky and stuffy — sweat evaporates slowly, so the body struggles to dissipate heat. Hence the heat index rises sharply when Td is high, even if the actual temperature is moderate.