Before the flight, the instructor saw a thunderstorm approximately 20 miles north of the field, moving slowly to the south-southeast. He and the student elected to make a short flight. After taking off, they made a practice forced landing in an open field about 1 mile northwest of the departure point. After the forced landing, they started another takeoff run. At the same time the tail was raised during the roll, a sudden gust of wind was encountered. The instructor said the air was dead calm, then 1 second later, it gusted to 40 mph. He said that the ultralight went straight up in a flat climb, much like a helicopter. After reaching an estimated 200 feet above ground level, the rough air reportedly rolled the ultralight into a 90-degree left bank and it began a fast descent. Subsequently, the vehicle impacted in a shallow pond while traveling about 50 mph. The student was wearing a motorcycle helmet and stated that it saved him from having a serious head injury.