An evaporative or swamp cooler can work in moderate humidity as long as you know what to do and why you're doing it.
The evaporation process that produces the cold air from the unit is the key to getting more cool from your cooler.
If you didn't already know, an evaporative cooling unit can keep you nice and cool during summer yet cost only a small fraction of the high running costs of air conditioning.
That's very attractive to a lot of people who need and want an economical as well as healthy temperature control solution in hot weather, but many are put off getting one because they are afraid the device won't work if the humidity gets too high.