gen-define-relative-humidity-and-dew-point-and-give-one-short-wo-3581

Define relative humidity and dew point and give one short worked example.

# Relative Humidity and Dew Point ## Definitions **Relative Humidity (RH):** The ratio of the actual amount of water vapor in the air to the maximum amount of water vapor the air can hold at that temperature, expressed as a percentage. $$RH = \frac{\text{actual vapor pressure}}{\text{saturation vapor pressure}} \times 100\%$$ **Dew Point:** The temperature to which air must be cooled (at constant pressure) for it to become saturated with water vapor and begin condensing into dew. ## Worked Example **Given:** - Air temperature: 25°C - Actual vapor pressure: 1.59 kPa - Saturation vapor pressure at 25°C: 3.17 kPa **Find:** Relative humidity and dew point **Solution:** **Relative Humidity:** $$RH = \frac{1.59}{3.17} \times 100\% = 50\%$$ **Dew Point:** The dew point is the temperature where 1.59 kPa becomes the saturation vapor pressure. From vapor pressure tables, 1.59 kPa corresponds to approximately **14°C**. **Interpretation:** The air at 25°C is holding 50% of the moisture it could hold. If this air cools to 14°C, water will begin to condense (forming dew, fog, or clouds).