I know very little about humidity - here, the humidity level often drops below 20% during Summer afternoons. Even after a snowy Winter and a cool Spring, it isn't hard for me to remember our searing mid-Summer days, however.
Dawn, the "10 Tips for a Cooler Coop" has good information for cooling a pre-existing coop.
For those folks thinking about construction design it may help to realize that the roof can be a somewhat seperate part of the structure. In other words, the coop can be essentially a box-shaped room and the roof an umbrella above it.
Houses with a well-ventilated attics will have ceilings that are a good deal cooler than they would be without that ventilation above. The sun beating down on a roof would heat the entire building otherwise. You may want to build a ceiling in your coop and open the area under the roof as much as possible to the outdoor air.
Inside the coop - Passive ventilation will work best if the air intake is low on a wall and the outlet is near the ceiling. A larger outlet will help move air while the inlet vent need not be so large since the hot air exiting the room will "pull" outside air thru the inlet. I can open the "chicken door" at ground level to allow cool air in and open a large net-covered window, that reaches all the way to the ceiling, to let hot air out.
Steve