American Airlines flight 2548 encountered turbulence while descending through 18,000 ft to land at the Miami International Airport (MIA), Miami, FL, and a flight attendant (FA) fractured her spine. The flight crew reported that during descent, with the seatbelt sign illuminated and after notifying the FAs to prepare for landing, air traffic control (ATC) issued a vector for the approach at MIA with a flight path that intersected a thin cloud layer. After the airplane passed through this cloud layer about 18,000 ft, the pilot’s noticed a towering cumulus cloud directly ahead that had not been displayed on the airplane’s weather radar display. Unable to maneuver around the cloud build up, the airplane passed through the tops of the cumulus clouds, and the flight encountered moderate turbulence for about 3 seconds. The flight crew then received a call from the cabin advising them that a FA had been injured. At the time of the turbulence event, the number 4 FA was in the aft galley and was thrown to the floor. She came to rest with her back against the aft jumpseat. The injured FA was provided oxygen, helped to her jumpseat, and secured with her safety belt. Upon being notified of the injury, the flight crew requested to have paramedics meet the airplane at the gate. The injured FA was transported to the hospital where she was diagnosed with a fractured spine. Eddy dissipation rate (EDR) is a universal measure of turbulence rate. Recorded data from the airplane revealed that about 1900 to 1901 EST the EDR indicated 0.156 to 0.175 which equates to light turbulence. About 5 minutes later the EDR indicated 0.367 which equates to moderate turbulence. At the time of the flight’s arrival into MIA there were no active significant meteorological information (SIGMETs) for the area where the turbulence occurred.