On March 27, 2006, about 1330 Atlantic standard time, a Cessna 402C, N223PB, registered to and operated by Hyannis Air Service, Inc., dba Cape Air, as flight 36, experienced in-flight clear air turbulence while flying over Dorado, Puerto Rico. Instrument meteorological conditions prevailed in the area and an instrument flight rules flight plan was filed for the 14 CFR Part 135 scheduled domestic passenger flight from Eugenio Maria de Hostos Airport, Mayaguez, Puerto Rico, to Luis Munoz Marin International Airport, San Juan, Puerto Rico. The airplane was not damaged and the pilot and four passengers were not injured. One passenger was seriously injured, and two passengers sustained minor injuries. The flight originated about 1317, from Mayaguez. The pilot stated that while flying at 5,000 feet in a broken layer with some light rain and turbulence, the flight had, "...just broken out for a few miles. Every one was at ease a few passengers were sleeping, that's when we hit a severe [turbulence]. This only lasted about 1 second, Everyone was thrown in [their] seats. It was so severe that even though I was strapped in I was floating for a few seconds until my seatbelt caught me. I looked down at the altimeter and it was still showing 5,000 [feet] and on course. The autopilot stayed on. Our [weather] radar was not picking up anything ahead." The person in the right front seat and several passengers hit their heads on the ceiling. She looked back and the passengers appeared, "... stunned but otherwise OK." She asked air traffic control for a lower altitude and weather deviation, and was advised to deviate to the north only and to descend to 4,000 feet. She "... avoided all clouds after that and she pulled the power back." She notified operations for medical assistance, and proceeded to the destination airport where she landed uneventfully. The incident occurred a few miles west of the Dorado NDB. The passenger who sustained the serious injury reported that the pilot advised them that the flight would be "bumpy" and they needed to keep their seatbelts fastened. She further reported that her seatbelt was tightly fastened at the time of the turbulence encounter, and at that time, she could hear "...something like small pebbles dropping onto the plane. There was mist on the windshield that streamed from the bottom/middle of the windshield to the top." She further stated that after assessing herself, "maybe a minute or two, the pilot turned on a screen that had been blank up to that point." She also reported seeing "...green blobs that came on and off. It reminded me of the radar screen that I have seen on TV." The pilot landed without any more incidents. NTSB review of weather radar images from the reported area and time of occurrence revealed that intensities of approximately 20-45 DBZ were noted west of Dorado, Puerto Rico. These correlate to National Weather Service (NWS) Video Integrated Processor (VIP) Level 1 (Very Light) to Level 4 (very strong) echo intensities.