According to crew statements, the airplane was descending through approximately 16,000 feet in clear air when it encountered two jolts that did not damage the airplane, but resulted in a flight attendant being lifted off the deck and hitting the ceiling, then being thrown to the floor and breaking her ankle. The flight attendant was in the galley preparing for landing, and the seat belt sign was illuminated. The captain had previously deviated for weather, and the airplane was operating between cloud buildups at the time. The captain also noted that the air was relatively smooth, and that radar returns indicated that the airplane was greater than 20 miles from any significant weather.