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

On a hot dry day desert cooler cool better because the surrounding is less humid or the atmosphere has less water particles(moisture), as a result of which even a small force of wind or blow of air(by desert cooler) can move these particles away and empties the atmosphere from those moistures and generates more space for our sweat particles or any other water particles and thus makes the evaporation process more efficient.