Chapter 31, Problem 2RQ

Why does relative humidity increase at night?

To determine

The reason for a relative humidity increases at night.

Answer to Problem 2RQ

The reason for a relative humidity increases at night is due to the fall in temperature.

Explanation of Solution

The fraction of the quantity of water vapor present in the air to that of the highest amount of the water vapor that can hold in air at a particular temperature is known as relative humidity.

Write the expression for the relative humidity.

$\text{Relative}\text{humidity}=\frac{\text{Absolute}\text{Humidity}}{\text{Maximum}\text{Capacity}\text{of}\text{air}\text{hold}\text{water}\text{vapor}}$

At night, as there is no heat, water vapor stays closer to the ground. As air rises, it cools and expands. The expansion occurs because air moves to a region of lower air pressure. The water molecules in the air condense, and the maximum capacity of the air to hold water vapor gets decreases. As a result, the relative humidity increases at night.

Conclusion:

Therefore, relative humidity increases at night is due to the fall in temperature.