FLC OF B727 ENCOUNTERS SEVERE TURB AT ALT IN CRUISE IN VMC. SOME MINOR INJURIES; DRINKS AND BOOKS ALL OVER THE COCKPIT.

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WHILE CRUISING AT FL330 WITH SEAT BELT SIGN OFF; I/WE ENCOUNTERED SEVERE TURB FOR A PERIOD OF 5-6 SECONDS. THE ACFT WAS BEING FLOWN ON AUTOPLT. THE TURB THREW COFFEE; DRINKS; ICE; AND BOOKS ALL OVER THE COCKPIT. SEVERAL PAX WERE INJURED SLIGHTLY. THERE WAS NO NEED TO DIVERT TO AN ARPT OTHER THAN OUR ORIGINAL DEST. THERE HAD BEEN NO RPTS OF LIGHT OR MODERATE TURB. THERE HAD BEEN TURB AND TSTMS RPTED BY COMPANY METEOROLOGY ABOUT 4 HRS EARLIER. HOWEVER; THE STORMS WERE DISSIPATING AND THERE WAS NO TURB/LIGHTNING OR ECHO RETURN BY OUR RADAR. THE CITIES' AND TOWNS' LIGHTS WERE VISIBLE AS WERE THE STARS. IT WAS A BEAUTIFUL NIGHT. THEN ALL OF A SUDDEN THE ACFT WAS THROWN ALL OVER THE SKY WITH A LOSS OF ABOUT 200 FT BY THE SEVERE TURB. THERE WAS PRECIP THAT SOUNDED LIKE HAIL AND ST ELMO'S FIRE. AS I SAID BEFORE; THE DURATION WAS ABOUT 5-6 SECONDS THEN ALL OF A SUDDEN IT WAS SMOOTH AGAIN. I FEEL THAT WE EITHER FLEW THROUGH THE TOP OF A SMALL CELL THAT COULD NOT BE SEEN ON RADAR OR WE FLEW THROUGH THE PRECIP OF A DISSIPATING STORM THAT WAS UPWIND AND DID NOT HAVE ENOUGH RAIN TO GIVE US A RETURN ON OUR RADAR. IF LATTER IS TRUE; HOW DO YOU EXPLAIN THE VISIBILITY? IT MIGHT HAVE BEEN CLR AIR TURB; BUT WE DID HAVE THE PRECIP. I DON'T KNOW HOW YOU AVOID A WX CONDITION LIKE THIS.