1995 Other Subaru Models starting problem

My son has a problem starting his vehicle whenever it is below freezing or if it has just rained and is very damp outside. When trying to start it , it backfires inside the engine compartment but will not start. Otherwise when it is above freezing and/or dry outside it starts and runs good. Could you give us some possibilities so when we take it to the garage,we have some idea what they are talking about. Also do you think it could be a easy fix or something serious. P.S. This is a Leagacy Brighton Wagon. Thanking You Very Much!!!!!!!!