Humidity plays a large role in evaporation. I have much less evaporation in the spring/fall when the windows are open because generally the air humidity is higher. When you are running AC or heat, the humidity is generally lower and evaporation increases. I usually have to turn on an extra set of fans when the windows are open to increase evaporation and decrease temp. Evaporation = cooling.

This is exactly why we have central AC here while they can use swamp coolers in lower humidity climates like El Paso. The humidity here is to high for effective evaporative cooling. This is generally not an issue unless you are running MH bulbs.