Various references describe specific uses for a humidity sensor in controlling an engine in certain specific conditions.  See for example, U.S. Pat.  No.  6,062,204. 
The inventor of the present application has recognized a disadvantage, however, with prior approaches.  In particular, prior art approaches have tended to focus on a specific, or narrow, application in which a humidity sensor is utilized.  This results in piecemeal systems with no overall structure taking full advantage of humidity sensors for controlling an engine in a vehicle.