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

(N/A) desert cooler functions on the principle of evaporative cooling.
The rate of evaporation is directly proportional to the temperature and inversely proportional to the humidity of the air.
On a hot and dry day,the temperature is high and the humidity is low,which significantly increases the rate of evaporation.
Since evaporation causes a cooling effect,the increased rate of evaporation leads to better cooling by the desert cooler.