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

A desert cooler causes cooling by the process of evaporation. When a desert cooler works, its inner walls are constantly sprinkled with water. Due to hot dry conditions, this water evaporates and causes the cooling of the inside air of the cooler. This cool air is sent inside the room with the help of fan. On a hot dry day, humidity is very low so the cooler works efficiently. If the amount of humidity in air is already high, the rate of evaporation decreases and desert cooler will not cool properly.