FOLLOWING TAKEOFF, THE PILOT NOTICED THE UPPER DOOR LATCH ON THE CABIN ENTRY DOOR WAS NOT SECURE. HE DECIDED TO RETURN TO THE AIRPORT FOR LANDING AND SECURE THE DOOR BEFORE CONTINUING HIS FLIGHT. ACCORDING TO THE TOWER CONTROLLER, THE PILOT BECAME GEOGRAPHICALLY DISORIENTED FLYING BENEATH THE CLOUDS AND SHE ASSISTED HIM IN FINDING THE AIRPORT. SHE THEN OBSERVED THE AIRPLANE LAND SHORT OF THE RUNWAY.