On July 10, 2009, at 1608 eastern daylight time, an Airbus A320-232, N606JB, operated by JetBlue Airways Corporation as flight 133, encountered turbulence while on approach to Southwest Florida International Airport (RSW), Fort Myers, Florida. The 2 certificated airline transport pilots, 3 flight attendants, and 144 passengers were not injured; while 2 passengers received serious injuries and 2 passengers received minor injuries. The airplane was not damaged. Visual meteorological conditions prevailed at RSW and an instrument flight rules flight plan was filed for the air carrier flight conducted under the provisions of 14 Code of Federal Regulations Part 121. The flight originated from John F Kennedy International Airport (JFK), New York, New York, at 1403. According to the captain's written statement, the airplane was jolted as it descended through 12,000 feet and went through a small cumulus cloud (top approximately 14,000 feet). Specifically, the airplane dropped about 20 feet instantaneously. The seatbelt sign had been illuminated since 19,000 feet (about 5 minutes prior) and there were no returns on the weather radar within 20 miles of the airplane's position. The captain had instructed the flight attendants via intercom to sit down a few minutes prior to the turbulence encounter. Within 15 seconds of encountering the jolt, the airplane was in clear air and the captain called the flight attendants to tell them it was safe to get up. He was then notified that a female passenger in seat 24C did not have her seatbelt fastened, and had fallen forward into the stowed tray table in front of her. Shortly thereafter, the captain was notified that another female passenger (seat 22C) was in an aft lavatory during the turbulence and was complaining of back pain. The flight landed uneventfully and emergency medical services (EMS) personnel, who had been requested by the flightcrew, boarded the airplane at the gate. At that time, two additional passengers (seats 21E and 24E) informed EMS that they were injured. All four passengers were transported to local hospitals. The first officer and flight attendants reported that the captain had made a passenger announcement during initial descent and prior to the turbulence encounter, emphasizing the need for passengers to take their seats and fasten their seatbelts when the seatbelt sign was illuminated. Additionally, a flight attendant made a public announcement when the seatbelt sign was illuminated. According to a Senior Air Safety Investigator at JetBlue, the passengers in seats 21E and 24E were released from the hospital with no fractures reported. The passenger in the lavatory suffered two spinal fractures and the passenger in 24C suffered fractures of two ribs. The Senior Air Safety Investigator at JetBlue further stated that review of the airplane's quick access recorder (QAR) data revealed that approximately 4 minutes elapsed from the time the airplane descended through 19,000 feet, until the time of the turbulence encounter. He added that the company's standard operating procedure is for the seatbelt sign to be activated when descending through 18,000 feet. Further review of the QAR revealed that at 1608, the airplane experienced a positive g-load of 1.98, followed by a negative g-load of -0.43 less than 1 second later. At that time, it was at an altitude of approximately 12,500 feet, near the MAZZY waypoint. Review of Nexrad and Doppler weather radar revealed light to moderate intensity echoes east of the airplane's position, which the captain noted in his written statement. There were no Significant Meteorological Information (SIGMET)s or convective SIGMETs in effect at the time and location of the turbulence encounter. The recorded weather at RSW, at 1553, was: wind from 090 degrees at 6 know; sky clear; visibility 10 miles; temperature 32 degrees Celsius; dew point 21 degrees Celsius; altimeter 30.10 inches of mercury.