Relative humidity is the ratio of two vapour pressures
Relative humidity is how much water vapour the air holds compared with the most it could hold at that temperature. You divide the actual vapour pressure (set by the dew point) by the saturation vapour pressure (set by the temperature).
Both pressures come from the same Magnus formula, evaluated at the dew point Td for e and at the air temperature T for es:
How the gap between temperature and dew point reads
The closer the dew point is to the air temperature, the higher the humidity. A spread of a few degrees already means the air is near saturation.
| Air temp − dew point | Relative humidity (near 20 °C) |
|---|---|
| 0 °C spread | 100% (saturated) |
| ~2 °C | ~88% |
| ~5 °C | ~72% |
| ~10 °C | ~52% |
| ~15 °C | ~38% |
Values shift slightly with temperature because the saturation curve is not linear. The dew point can never exceed the air temperature.
What the numbers mean indoors
- Comfort band. 40–50% RH is the sweet spot; below 30% dries skin and airways, above 60% invites dust mites and mould.
- Mould threshold. Growth starts once surface humidity holds above about 60%, and accelerates past 70%.
- Dew point, not RH, for weather. Meteorologists track dew point because it stays put as the day warms and cools, while RH rises and falls purely from the temperature change.
Common questions
Can I get relative humidity from temperature alone?
No. Temperature only sets how much water vapour the air could hold. You need a second reading of how much is actually there, which is what the dew point gives you.
What does it mean when the dew point equals the air temperature?
The air is fully saturated, so relative humidity is 100 percent. Any further cooling makes water condense out as dew, fog or mist.
Why is dew point a better comfort guide than relative humidity?
Dew point measures the actual moisture in the air and does not swing with temperature. A dew point above about 16 C (60 F) feels muggy regardless of what the relative humidity reads.
