ORIGINAL: pe reivers
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.
To supplement these thoughts, in hot, humid or high altitude environments that require leaner settings, it doesn't mean you will get more power out of the engine. That's especially when compared to same engine running in cooler, drier air or lower altitudes. Why? Less oxygen AND less fuel (stoichiometric) mean less power. The engine must move fuel mass and air mass through to create the necessary torque with the appropriate heat generated by "full" combustion. The more mass per rpm an engine can move through itself at stoichiometric conditions, the greater the power it can generate.
That's why superchargers work. Or other materials that bring in more oxygen (nitrous oxide means more fuel). It's more complex than that but that's the basic concept