What is Dew point temperature and What is Dry bulb Temperature ?

 If a mixture of air and superheated water vapor is cooled at constant pressure, the partial pressure of each constituent remains constant until the water vapor reaches the saturated state. Further cooling causes condensation. The temperature at which water vapor starts condensing is called the dew point temperature tdp of the mixture. It is equal to the saturation temperature at the partial pressure pw, of the water vapor in the mixture. 

Dry bulb temperature (dbt) is the temperature recorded by a thermometer with a dry bulb. 

Wet bulb temperature (wbt) is the temperature recorded by a thermometer when the bulb is enveloped by the a cotton wick saturated with water. As the air stream flows past it , some water evaporates taking the latent heat from the water -soaked wick, thus decreasing its temperature. Energy is then transferred  to wick from the air. When equilibrium condition is reached , there is a balance between energy removed from the water film by evaporation and energy supplied to the wick by heat transfer, and the temperature recorded is the wet bulb temperature. 

A psychrometer is an instrument which measures both the dry bulb and the wet bulb temperature of air. A sling thermometer has mounted on a frame with a handle. The handle is rotated so that there is good air motion. The wet bulb temperature is the lowest temperature recorded by the moistened bulb.