CBSE Class 9 Physics Relative Humidity Notes Set 11

Evaporation and Water Vapour in Air

Air always contains some water vapour. The maximum amount of vapour that air can hold at a given temperature is its saturated state, and the pressure exerted by vapour in a saturated state is the saturation vapour pressure (SVP). SVP increases with temperature and equals the atmospheric pressure (760 mm of mercury) at the boiling point, 100°C for water at normal pressure.

Dew Point and Relative Humidity

1. Dew point is the temperature at which the water vapour present in air just saturates it, so that dew begins to form on cooling. At the dew point the SVP equals the actual vapour pressure.
2. Dew point can be found from the temperatures at which dew forms on cooling and disappears on warming: dew point = average of the two temperatures. It is always below the air temperature unless the air is saturated.
3. Relative humidity (R.H.) = \( \dfrac{\text{actual vapour pressure}}{\text{SVP at the air temperature}} \times 100\% \) = \( \dfrac{\text{SVP at the dew point}}{\text{SVP at the air temperature}} \times 100\% \).
4. Dew cannot be formed if air contains no water vapour at all (completely dry air).
5. When the R.H. is high, less sweat evaporates from our body, so we feel hotter and uncomfortable.

Wet and Dry Bulb Hygrometer

1. It consists of two thermometers, one with its bulb wrapped in wet muslin dipped in water. Evaporation cools the wet bulb, so it reads lower than the dry bulb.
2. The drier the air, the faster the evaporation and the bigger the difference of readings. If the relative humidity is 100% there is no evaporation, so both thermometers read the same.

Worked Example

Example. Air at 20°C forms dew when cooled to 10°C. The SVP at 20°C is 17.54 mm and at 10°C is 9.21 mm of mercury.
R.H. = \( \dfrac{9.21}{17.54} \times 100 = 52.5\% \).