THE PILOT STATED THAT HE ATTEMPTED TO TAKEOFF ON RUNWAY 13, BUT MIDFIELD HE ABORTED THAT TAKEOFF. HE STATED THAT HE NOTICED THE WIND FAVORED RUNWAY 31, SO HE SWITCHED RUNWAYS. HE REPORTED THAT THE TAKEOFF WAS NORMAL, HIS RATE OF CLIMB 300 FEET PER MINUTE AND AIRSPEED ABOUT 80 MPH. HE STATED THAT DURING THE CLIMB HE WAS ABOUT TO MAKE A RIGHT TURN INTO THE VALLEY WHEN HE FELT A DOWNDRAFT TO THE RT SIDE OF THE AIRPLANE. ACCORDING TO THE FAA THE AIRPORT IS SITUATED IN A MOUNTAIN VALLEY WITH HILLS ON BOTH SIDES AND A 400 FOOT HILL LOCATED ABOUT 1500 FEET BEYOND THE DEPARTURE END OF RUNWAY 31. THE LOCAL PILOTS WHEN DEPARTING USUALLY TURN 60 DEGREES LEFT OR RIGHT TO AVOID THE HILL. A WITNESS REPORTED SEEING THE AIRPLANE APPROACHING THE HILL, AND MAKING A STEEP RIGHT TURN BEFORE IT DESCENDED INTO THE TREES. THE PILOT REPORTED THAT THERE WAS NO MECHANICAL MALFUNCTION.