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Science

Dew Point Calculator

Enter air temperature and relative humidity to get the dew point — the temperature the air would have to cool to before water vapour starts to condense (fog, dew, foggy glasses and air-conditioner drip are all this mechanism). The calculator also shows the temperature–dew-point spread (how close the air is to saturation) and a 7-level human comfort verdict, useful for muggy Hong Kong summers, HVAC sanity checks and gym ventilation alike.

Dew point

Temp − dew point (spread)

The smaller the spread, the closer the air is to saturation. A spread of 2 °C or less usually means fog or condensation.

Human comfort (by dew point)

Formula (Magnus-Tetens, Alduchov & Eskridge 1996)

γ = ln(RH/100) + (17.625 T) / (243.04 + T) ; T_d = 243.04 γ / (17.625 − γ)

T is in °C and RH is a percentage. When RH = 100 % the dew point equals the air temperature; the lower the RH, the wider the gap.

Formula

γ(T, RH) = ln(RH/100) + (17.625 · T) / (243.04 + T) T_d = (243.04 · γ) / (17.625 − γ) // T, T_d in °C, RH as a percent // Constants from Alduchov & Eskridge (1996) Spread = T − T_d // smaller spread = closer to saturation; ≤ 2 °C usually means fog

Frequently asked

Which feels more muggy — a dew point of 22 °C, or 30 °C air at 70 % humidity?

They're very close. 30 °C / 70 % RH plugs into Magnus and gives a dew point of ≈ 24 °C (slightly muggier than 22 °C). A 22 °C dew point corresponds to roughly 30 °C / 62 % RH — already in the "oppressive" zone. The lesson: judge mugginess by dew point, not RH. 90 % RH on a 10 °C winter morning (dew point ≈ 8 °C) actually feels dry; 60 % RH at 30 °C (dew point ≈ 21 °C) is sticky.

Why do my glasses fog up when I move between an air-conditioned room and the street?

When a lens is colder than the surrounding dew point, water vapour condenses on it. In summer, moving from a 20 °C air-conditioned room (your lenses at 20 °C) into outdoor air with a 24 °C dew point puts the lens 4 °C below the dew point — instant fog. Fixes: warm the lenses with your hands or warm air before going out, use an anti-fog coating, or pre-warm them in your pocket. The reverse (cold lenses entering a warm room) works the same way in winter.

What is the ideal indoor dew point, and how dry should the air-con dehumidify the room?

For most people, an indoor dew point of 10–13 °C — corresponding to 22–25 °C room temperature at roughly 45–60 % RH — feels best. This is also ASHRAE 55's recommended comfort envelope. Hong Kong outdoor air with a 24 °C dew point has to be cooled to ~10 °C at the coil for the AC to wring out half its moisture. Energy tip: rather than blasting the temperature low, use the dehumidify mode (continuous low-speed condensation) — it lowers the indoor dew point without turning the room into a fridge.

At what point do walls and floors start "sweating" during a Hong Kong south-wind day?

When warm moist outdoor air (south-wind days typically have a dew point of 18–22 °C) meets a colder indoor surface — say a 16 °C floor that has cooled overnight — the surface is below the air's dew point and vapour condenses on it. Fix: run a dehumidifier to push the indoor dew point below ~12 °C (≥ 4 °C lower than the floor), or seal windows so outdoor moisture stays outside. The "Temp − dew point" spread shown above is the simplest gauge — once it drops to ≤ 2 °C, condensation is likely.

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