Relative Humidity
Water vapor as a fraction of the saturation amount.
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Example
100% means saturated air
How it works
Relative humidity compares the actual amount of water vapor in the air to the maximum amount the air could hold at that temperature. The ratio is multiplied by 100 to turn it into a percentage, so it tells you how close the air is to saturation. It works because saturation vapor pressure, e_s, represents the upper limit for water vapor content at a given temperature; the closer e is to e_s, the more humid the air feels.
When to use it
Use this formula in weather, climate, and environmental science problems when you know the actual vapor pressure and the saturation vapor pressure at the same temperature. It is also useful for estimating fog, dew, comfort, and the likelihood of evaporation or condensation.
Worked example, step by step
Suppose the vapor pressure of air is e = 14 hPa and the saturation vapor pressure at that temperature is e_s = 20 hPa. RH = (e / e_s) × 100 RH = (14 / 20) × 100 RH = 0.7 × 100 RH = 70% So the relative humidity is 70%, meaning the air contains 70% of the water vapor needed for saturation at that temperature.
Common mistakes to avoid
- Using temperature by itself instead of comparing vapor pressure to saturation vapor pressure
- Forgetting to multiply the ratio by 100 to express the result as a percentage
- Mixing up e and e_s, which reverses the meaning of the calculation
Did You Know?
At 100% relative humidity the air is saturated, and any further cooling forms dew, fog, or rain.
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