why fountains are more useful for cooling in a hot dry climate than in a hot humid climate?

Answer 1

If the air is humid, then it is more difficult for water to evaporate. In fact, if the air is already holding all the water it can hold, no water can evaporate. (Actually some water will evaporate and an equal amount of water vapor will condense, so there is no net increase in the amount of water vapor in the air. (This situation is called 100% humidity. It doesn't mean that the atmosphere consists entirely of water. It just means that the atmosphere is holding as much water VAPOR as it is capable of holding AT THE CURRENT TEMPERATURE without causing some of the vapor to condense into clouds (which we call fog when it is at ground level).

So that's the situation if NO more water can evaporate. If the humidity is high but less than 100%, then some water can evaporate, but it will not evaporate as rapidly as it would at lower humidity.

So what difference does evaporation make? It takes HEAT to cause the water to evaporate. When water evaporates it absorbs heat from its surroundings (from the air, or from the body of water from which it evaporates, or from the evaporative cooler that is used to cool a building, or from a person's body in the case of sweat).

If the humidity is low, a lot of water can evaporate and a lot of cooling takes place. If the humidity is high, very little water can evaporate into the atmosphere, and very little cooling takes place.

Answer 2

Evaporation is an endothermic reaction, meaning that heat is absorbed by the process. If the air is "dry", evaporation of the water on your body occurs faster, and more cooling results. If you're IN the fountain, you wouldn't feel much difference, but once you exit, you will feel cooler if there is less humidity in the air, assuming equal ambient air temperatures.