If you had driven on a rainy day and encountered better engine performance, this is most likely due to the moisture in the air going into the intake and aiding the combustion in the engine.
So how to prolong or make the engine performance a permanent effect without waiting for a downpour?
Water injection is the key!
Water injection is not a new discovery. It has been commonly used for racing engines, especially for turbocharged engines. According to Wikipedia entry on the subject, water injection cools the combustion chamber and creates greater compression ratios and also eliminates engine pinging. It is also said to be able to increase the octane rating of the air fuel mixture.