Question

why does a dessert cooler cool better on a hot day?

Answer 1

First up, when liquid evaporates, it carries heat away from its surroundings (which is why we sweat, as the evaporation of the surface liquid removes heat from the body below it). A dessert cooler operates in this fashion, by presenting a surface of water to the air for it to evaporate, and carry heat away from the object it is being used to cool.

Evaporation rates will also depend upon the concentration of moisture in the air. On hot days, air is less dense, meaning that the percentage water vapour per unit volume of air is less, meaning that evaporation rates increase, which will cool the object better than on a colder day. This is why humid days are much more uncomfortable than less humid days, even though the temperature is the same. On humid days, it is harder to evaporate your sweat, thus it is harder to cool you down. Note though that strictly speaking, a desert cooler will cool better on a hot dry day, than on a hot humid day.

Answer 2

Thank you Jonny McA.