Hint: Try to figure out the phenomenon of a desert cooler. Compare how it works on a hot and cold day with low humid conditions and high humid conditions.
Complete step by step answer: We know that a hot day has high temperature and the dry day has low or no humidity. On a hot dry day, water vapor will be less and the temperature will be high. As a result, water will readily evaporate on the supply of the little amount of heat.
Evaporation is a cooling process that cools the surroundings. Desert coolers transfer the heat energy from circulating air to water. Water evaporates resulting in the cooling of the surrounding air and this air is circulated.
On a hot dry day, the humidity will be low and the temperature will be high. The temperature of the air will be relatively higher and energy transferred will be more. Since the same volume of air will be circulated energy transferred per given volume of air will be more on a hot day. Evaporation will be faster resulting in better cooling. On a dry day, water vapor present in the air will be less, and the circulation of this air will provide faster cooling compared to a humid day. Humid air contains more water vapor content so energy transferred will be less and the rate of cooling will be less. Therefore desert cooler cools better on a hot dry day.
Note: We may think humid air which has a lot of water vapor which contains more energy and can transfer a lot of energy but heat cannot be absorbed from water vapor resulting in the circulation of a lot of water vapor which does not cause a cooling effect.