On August 28, 1999, about 1845 hours Pacific daylight time, an experimental Richard Wirth Avid Flyer, N625F, registered to and operated by the pilot, encountered wind shear during initial climb from the uncontrolled Sonoma Valley Airport, Schellville, California. Thereafter, the homebuilt airplane descended into terrain and was substantially damaged. Visual meteorological conditions prevailed during the personal flight in the experimental category airplane. The flight was being performed under 14 CFR Part 91. The private pilot received minor injuries. The flight was originating at the time of the accident. According to the pilot, he took off under calm wind conditions. However, he encountered a wind shear upon climbing 50 to 75 feet above the ground. The pilot further reported that the wind was strong enough to yaw his airplane 15 degrees, and it appeared to push him backwards. Control was lost, and the airplane descended until impacting a ditch about 200 feet from the runway. No mechanical problems were experienced with the airplane during the flight.