INFLT ENCOUNTER; TURB. CLBING IN AREA OF BUILDING CELLS; FLC WAS UNABLE TO OUT CLB A CUMULUS CLOUD; ENCOUNTERED MODERATE TURB AND A PAX WAS INJURED. THE PAX WAS IN THE BATHROOM IN SPITE OF THE SEAT BELT SIGN ON.

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WE WERE CLBING BTWN WIDELY SPACED CELLS. THE SEAT BELT SIGN WAS ON SINCE TKOF. 1 FLT ATTENDANT TOLD A 70 YR OLD MAN ON HIS WAY TO THE BATHROOM THAT HE SHOULD STAY SEATED. ABOUT 2-3 MINS LATER; WITH HIM STILL IN THE BATHROOM AND ATTENDANTS WITH BEVERAGE CARTS IN THE AISLES; THE CAPT ANNOUNCED FOR THE ATTENDANTS TO TAKE SEATS. SHORTLY AFTER; WE ENCOUNTERED 10-20 SECONDS OF MODERATE TURB AS I CLIPPED THE TOP OF A CUMULUS CLOUD BUILDING FASTER THAN WE WERE CLBING. 2 ATTENDANTS FELL DOWN IN THE AISLE WHERE NO SEATS WERE IMMEDIATELY AVAILABLE. THE OLDER MAN FELL INSIDE THE BATHROOM. HE WAS LATER HELPED TO HIS SEAT BY AN ATTENDANT AND EXAMINED BY A NURSE PAX. ON THE NURSE'S ADVICE; WE HAD A MEDIC MEET THE FLT ON ARR. THE MEDICS SPLINTED HIS L ANKLE BECAUSE OF A SUSPECTED SPRAIN OR FRACTURE. PLT LESSONS LEARNED: BEWARE OF BUILDING CUMULUS CLOUDS; THEY CAN EXCEED YOUR CLB ABILITY. GIVE AS MUCH WARNING POSSIBLE TO THE CABIN WHEN TURB APPEARS PROBABLE OR UNAVOIDABLE. CABIN LESSON LEARNED: RESPECT THE SEAT BELT SIGN. WHEN THE ATTENDANTS OR PLTS SAY SIT DOWN; DO IT.