Why does a desert cooler cool better on a hot dry day ?

Explanation

A cooler cools better on a hot dry day because the rate of evaporation is directly proportional to the temperature and inversely proportional to hutoidity. Air around us cannot hold more than a definite amount of water vapour at a given temperature. On a hot dry day he rate of evaporation increases (air can hold more water vapour) thus causing a cooling effect.