On March 3, 2012, about 1720 eastern standard time, Air Wisconsin 3733, a Bombardier CL-600-2B19, N453AW, experienced turbulence during descent into Philadelphia International Airport (PHL), Philadelphia, Pennsylvania. Of the 50 passengers and crew onboard the airplane, one flight attendant received serious injuries and the airplane was not damaged. The flight was operating under 14 CFR Part 121 as a regularly scheduled passenger flight from Cleveland Hopkins International Airport (CLE), Cleveland, Ohio to PHL. According to the flight crew, as the airplane was descending through about 17,000 feet, the flight initially experienced light turbulence so the flight crew turned on the seatbelt sign. Shortly after, the airplane experienced severe clear air turbulence that caused the airplane to drop at least 300 feet. At the time of the turbulence, the flight attendant (FA) was in the galley securing the beverage cart and was thrown to the ceiling and onto the floor injuring her back. The FA was assisted into her jumpseat by a passenger. The flight crew declared an emergency with ATC and requested emergency personnel meet the airplane at the gate. After landing, the FA was transported to the hospital and diagnosed with a fracture of her L-1 vertebrae.