Question

The units of relative humidity are
A) kg-m-3
B) kg
C) kg-m2
D) None of these

Answer 1

Hint:The given question specifically asks about the relative humidity. It does not ask about absolute humidity or even specific humidity.One of the answer options is however provided for absolute humidity just to create confusion. All these terms are interdependent but they also have their own different meaning and significance especially while studying refrigeration and air conditioning.

Complete step by step answer:
The phenomenon of humidity is dominant in coastal regions, and one can easily experience humidity in such places. However, to solve this question we must focus on a term known as relative humidity. Relative humidity may be defined as the ratio of partial pressure of water vapour in the air to the vapour pressure at saturation. Mathematically, it can be written as,
$\phi = \dfrac{{{P_v}}}{{{P_{sat}}}}$...............(1)
where, $\phi $ is relative humidity,

is partial vapour pressure, and

is saturation pressure.
In some places relative humidity is defined as the ratio of amount of moisture actually present in the air at a particular temperature to the maximum moisture air can contain at the same temperature. Or simply it is the ratio of actual vapour density to the saturation vapour density.
Despite variations in defining relative humidity, the definition represented by equation (1) is the most widely accepted one. Now,from equation (1) we can conclude that relative humidity is the ratio of two similar quantities (i.e. pressure), and we also know that a quantity which is the ratio of two similar quantities is itself a dimensionless and unitless quantity. However, for calculation purposes we express relative humidity in percentage.
$\phi = \dfrac{{{P_v}}}{{{P_{sat}}}} \times 100\% $

Hence, option d is the correct answer option.

Note:
-Absolute humidity is a measure of the amount of water vapour present in the given volume of air. It is measured in kg-m^-3 or g-m^-3.
-Specific humidity or humidity ratio is the ratio of mass of water vapour to the mass of dry air. Its unit is kg/ kg of dry air.
-Degree of saturation is the ratio of actual humidity ratio to the humidity ratio at saturation temperature and pressure.