What is a Psychrometer (hygrometer)?

A psychrometer is a type of hygrometer (meteorological instrument) used to measure the amount of humidity in the atmosphere.

Principle, Working & Construction

A psychrometer consists of two thermometers i.e. wet-and-dry-bulb thermometer, one that is dry and one that is kept moist with distilled water on a sock or wick. The two thermometers are thus called the dry-bulb and the wet-bulb. At different temperatures , evaporation of water from the wick lowers the temperature, so that the wet-bulb thermometer usually shows a lower temperature than that of the dry-bulb thermometer.
Relative humidity is calculated by comparing the readings using a calculation table that compares the ambient temperature (the temperature given by the dry bulb) to the difference in temperatures between the two thermometers.

hygrometer table

Does depression of hygrometer wet bulb increase at afternoon?

YES, because hygrometer works on evaporation principle which makes reading lower than dry bulb, maximum in hot weather afternoon.