B757 FLT CREW EXPERIENCES WHAT THEY DESCRIBE AS SEVERE TURB AS THEY EXIT CUMULUS CLOUD ON APCH.

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DURING APCH CTL VECTORS TO ATL AT APPROX 5 MI W OF THE ATL ARPT; WE NOTICED SEVERAL SMALL CUMULUS CLOUDS AHEAD. THE CLOUDS WERE APPROX 4000 FT FROM TOP TO BOTTOM. RADAR INDICATED NO PRECIP WITHIN THE CLOUDS. ATL APCH HAD NO PREVIOUS RPTS OF TURB IN THE AREA. WE ENTERED ONE OF THE SMALL CUMULUS CLOUDS AT 12000 FT AND 320 KTS. IT TOOK APPROX 3 SECONDS TO GO THROUGH THE CLOUD. WHILE IN THE CLOUD; WE EXPERIENCED A VERY LIGHT BUMP OF TURB. AS WE EXITED THE CLOUD; WE EXPERIENCED A SEVERE DOWNWARD JOLT -- NEGATIVE G -- SEVERE TURB. THE SEVERE TURB WAS 1 JOLT AND DONE FOLLOWED BY A SMOOTH RIDE ON DOWNWIND TO BASE TO LNDG. THE CAPT COORD PARAMEDICS TO MEET THE ACFT AT THE GATE. AT THE GATE THE PARAMEDICS EXAMINED 2 PAX AND 2 FLT ATTENDANTS. THE 2 PAX CHOSE NO FURTHER TREATMENT AND CONTINUED ON TO THEIR DESTS. 1 FLT ATTENDANT WAS TREATED FOR A MINOR ANKLE SPRAIN AND THE OTHER HAD NO INJURIES; BUT WAS SHOOK UP BY THE INCIDENT. SUPPLEMENTAL INFO FROM ACN 742757: I TRY TO AVOID THOSE CLOUDS IF POSSIBLE; BUT SUMMERTIME IN THE S; THEY ARE SOMETIMES HARD TO AVOID. I THINK THAT FROM NOW ON; IF THERE ARE CUMULUS CLOUDS THAT CAN'T BE AVOIDED; I WILL HAVE THE FLT ATTENDANTS SIT DOWN. SINCE THERE ARE OFTEN THOSE CLOUDS; I WOULD HAVE CONSIDERED THAT 'OVERKILL;' BUT THIS CLOUD SURPRISED THE HELL OUT OF ME; AND I DON'T WANT ANY MORE SURPRISES WITH FLT ATTENDANTS STANDING.