On August 8, 1999, about 1945 mountain daylight time, a Cessna 188B, N9085R, registered to and operated by Sidney Air Service, Incorporated as a 14CFR137 aerial applicator was substantially damaged while landing at Sidney-Richland Airport, Sidney, Montana. Visual meteorological conditions prevailed and no flight plan was filed for the local agricultural flight. The commercial pilot was not injured and there was no report of fire or ELT activation. During a telephone interview and subsequent written statement, the pilot reported that shortly after departure from Sidney Airport, he encountered rain showers and thunderstorms and elected to return to the airport. After touchdown, during the landing roll on runway 01, the aircraft encountered a gust of wind. The pilot stated he was unable to maintain directional control and unintentionally ground-looped the aircraft. The aircraft came to rest approximately 30 feet west of runway 01. The aircraft's landing gear, wings, empennage and fuselage were substantially damaged. The weather at the time of the accident, as reported by the pilot, was winds from 280 degrees at 14 knots with gusts to 25 knots and rain showers. The pilot also reported that rain showers and thunderstorms were forecasted for the area at the time of the accident. The pilot reported no mechanical malfunctions or failures with the aircraft at the time of the accident.