As far as I'm concerned, humidity below 25% is bearable.
I've been out West when it was 110° at dawn and the humidity was around 5%. That was in Yuma, AZ one summer. The A/c in the truck I was driving took a dump and that was a MISERABLE trip in a cab over.
This was taken last summer.
and I was running about 40 mph when I snapped the pic.
http://i658.photobucket.com/albums/u...1/Pic017-2.jpg