Question

The dry bulb temperature (DBT) of atmospheric humid air is \(30^{\circ}\text{C}\). The dew point temperature (DPT) is also \(30^{\circ}\text{C}\) and the humidity ratio is \(0.025\ \dfrac{\text{kg of water vapor}}{\text{kg of dry air}}\).

The relative humidity (RH) of the air is ______ %.

• A. 100
• B. 25
• C. 50
• D. 0

Hint:

Recall that air at its dew point temperature is, by definition, saturated air.

Answer 1

The Correct Option is A

This question checks the link between dew point temperature and relative humidity, not a calculation with the given humidity ratio.

1. (A) 100: correct. The dew point is the temperature at which air of a given moisture content becomes saturated. The air here is already at its dew point, since DPT equals DBT, so it is saturated right now, and saturated air has relative humidity of 100 percent.
2. (B) 25: wrong. This would apply only if the actual vapor pressure were a quarter of the saturation vapor pressure, which would show up as a dew point well below the dry bulb temperature, not equal to it.
3. (C) 50: wrong, for the same reason. A dew point below the dry bulb temperature gives a relative humidity below 100 percent, but here the two temperatures coincide.
4. (D) 0: wrong. Zero relative humidity means completely dry air with no water vapor at all, which could never have a dew point equal to the dry bulb temperature.

Since the dew point equals the dry bulb temperature, the air is saturated, and option A, 100 percent, is correct.