The damp earth is more conductive than dry soil, allowing more objects to register on a detector. Even a popular area often visited by metal detecting enthusiasts can reveal new buried treasures and finds after a heavy rain, so wait for the drops to subside and head out early. Also, during spring thaw, the ground shifts from being frozen solid in the cold winter months. Objects that were once buried too deeply to make a beep on a detector may rise, allowing them to be discovered. I have had good luck with the wet ground finding coins deeper and more of them.