Question

Why cooler is quite effective on a hot and dry day???​

Answer 1

Evaporation causes colling and evaporation depend upon factors like Temperature,Humidity etc...So in hot day temperature is high and humidity is low so the rate evaporation of increases hence causes cooling

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.