Southwest Airlines flight 311 encountered unanticipated severe turbulence while descending through a thin cloud layer. The flight originated from Dallas Love Field Airport (DAL), Dallas, Texas, with a planned destination of Northwest Florida Beaches International Airport (ECP), Panama City, Florida. Prior to departure, the captain briefed the flight attendants (FAs) to remain seated during departure, due to cumulous cloud buildups and anticipated turbulence. While climbing to 37,000 ft (FL370), the flight encountered occasional light chop. After leveling off, the flight crew informed the FAs that they could perform their duties as assigned and turned the fasten seat belt sign off. According to the flight crew, during initial descent, at approximately FL340, the first officer made a public address (PA) announcement indicating that they had started their initial descent, to return to their seats and fasten their seatbelts for the remainder of the flight. The captain observed scattered cumulous clouds ahead of their flight path at approximately FL200 and told the FAs to secure the cabin and be seated early. The airplane descended through FL290 in a hazy cloud layer and encountered approximately 10-15 seconds of turbulence. During the turbulence encounter the autopilot disconnected, the airspeed fluctuated, and the airplane shook. The flight crew leveled off at FL270 and contacted the FAs. The flight crew were informed that several passengers and FAs were injured during the event. The flight crew notified company dispatch and air traffic control (ATC) of the turbulence encounter and the injuries. The remainder of the flight was uneventful. After parking at the gate, emergency medical services (EMS) met the airplane and transported three FAs and four passengers to the hospital for examination. One FA was diagnosed with a right ankle fracture and underwent surgery. Ten minor injuries were also reported and there was no damage to the airplane.