What Is Relative Humidity?
Relative humidity (RH) is a measure of how moist the air is. It expresses the actual amount of water vapor present in the air as a percentage of the maximum amount the air can hold at a given temperature. In other words, relative humidity tells us how close the air is to being fully saturated with moisture.
RH = 100 × (actual vapor pressure / saturation vapor pressure)
When relative humidity reaches 100%, the air is fully saturated and dew — or fog — starts to form.
How to Calculate Relative Humidity
The most accurate method uses the Revised Magnus formula (Alduchov & Eskridge, 1996), which relates relative humidity to air temperature (T) and dew point temperature (Dₚ):
RH = 100 × exp(β·Dₚ / (λ + Dₚ)) / exp(β·T / (λ + T))
where β = 17.625
λ = 243.04 °C (Revised Magnus coefficients)
The same formula can be inverted to find the dew point if you already know temperature and RH:
Dₚ = λ × (ln(RH/100) + β·T/(λ+T)) / (β − (ln(RH/100) + β·T/(λ+T)))
How to Use the Relative Humidity Calculator
- Select your unit system — Metric (°C) or Imperial (°F).
- Enter the air temperature (dry-bulb temperature measured in shade).
- Enter the dew point temperature — the temperature at which air becomes saturated. It must be ≤ the air temperature.
- Click Calculate. The calculator instantly displays relative humidity, absolute humidity, vapor pressures, and a comfort level assessment.
Example: Air temperature 25 °C, dew point 14 °C → RH ≈ 50.7% (comfortable).
How Do We Measure Relative Humidity?
Relative humidity is measured with a hygrometer. Common types include:
- Psychrometer (wet-and-dry-bulb thermometer) — uses two thermometers; one has a wet wick. Evaporative cooling on the wet bulb gives the dew point.
- Capacitive sensor — a polymer film changes capacitance with moisture; used in digital weather stations and smartphones.
- Hair hygrometer — human hair lengthens when humid; inexpensive but less accurate.
- Chilled mirror hygrometer — cools a mirror until dew condenses; extremely precise, used in laboratories.
What Is an Ideal Level of Relative Humidity?
| RH Range | Comfort Level | Notes |
|---|---|---|
| < 25% | Very Dry | Skin irritation, static electricity, increased dust |
| 25–35% | Dry | May cause dry skin and respiratory discomfort |
| 35–60% | Comfortable | Ideal range for human health and indoor comfort |
| 60–70% | Humid | Feels muggy; mold risk increases |
| 70–80% | Very Humid | Sweat evaporation slows; perceived temperature rises |
| > 80% | Oppressive | Heat stress risk; rapid mold and bacterial growth |
Absolute Humidity vs Relative Humidity
Absolute humidity (AH) is the actual mass of water vapor in a unit volume of air, expressed in g/m³. It does not depend on temperature — unlike relative humidity, which changes as temperature changes even if the moisture content stays the same.
AH (g/m³) = 216.7 × Pw / (273.15 + T)
where Pw is the actual vapor pressure in hPa and T is temperature in °C.
FAQs
Why does relative humidity feel higher at the same temperature in summer?
Warm air can hold more water vapor. The same dew point — say 20 °C — produces a higher RH at 25 °C than at 35 °C, because the saturation vapor pressure is lower at the cooler temperature.
Can relative humidity exceed 100%?
In clean air, supersaturation (RH > 100%) is briefly possible, but in practice water quickly condenses onto aerosol particles, so RH stays at or below 100%.
What is the dew point spread?
The dew point spread (T − Dₚ) indicates how far the air is from saturation. A spread of less than 2–3 °C means fog or dew is imminent; a spread greater than 20 °C indicates very dry air.
How does humidity affect perceived temperature?
High humidity slows the evaporation of sweat, reducing the body's ability to cool itself. This is why 35 °C at 80% RH feels far hotter than 35 °C at 20% RH. The heat index (apparent temperature) accounts for both temperature and humidity.