like truckracer said. The dry air caused your engine to lean out. This can cause a very noticeable power increase if your engine was tuned a couple 100 rpm rich off peak power.
General rule:
More humidity makes your engine run richer, reduces power (less oxygen)
More heat makes your engine run richer, reduces power (thinner air, less oxygen to burn)
more altitude makes your engine run richer. (thinner air, less oxygen)

and vice versa.