Question

why is air passing through a desert cooler become cool?

Answer 1

Desert Coolers:  Desert coolers are based on the simple principle that when unsaturated air comes in contact with water, the water evaporates. In the process, the moisture content of air increases, while its temperature decreases. The resulting cold but moist air is used for providing cooling. Thus a desert cooler is a simple device, which consists of an arrangement for blowing dry and hot air over a wet surface and an arrangement for keeping the surface wet continuously. The cooler normally consists of a blower and a pump. Desert coolers are economical (both initial and running costs are low) and are effective in hot and dry areas. They are not effective in humid areas.

Answer 2

The particles of liquid in desert cooler absorb energy from the surrounding to regain the energy lost during evaporation. In summer this is fast. Hence desert cooler cool better in a hot dry day.