WET Bulb Temperature:

A psychrometer includes a wet bulb thermometer and dry bulb thermometer. The dry bulb thermometer is exposed directly to the air and measures the real temperature of air and is known as dry bulb temperature. While the thermometer bulb is surrounded through a wet cloth exposed to the air. The temperature that is measured through the wick-covered bulb of such a thermometer shows the temperature of liquid water in the wick and is known as the wet bulb temperature. It is mentioned by the symbol t'.

The difference among the wet bulb and dry bulb temperatures is known as wet bulb depression (WBD).

                                WBD = (t - t' )

If the ambient air is saturated, that means the RH is 100 %, then there shall be no evaporation of water on the bulb and therefore WBT & DBT shall be equivalent. The WBT is an indirect measure of the dryness of air.