A B747 CARGO FLT ENCOUNTERS A TALL; DRY; AND SMALL DIAMETER CUMULO NIMBUS WHILE IN CRUISE FLT AT FL350; AND WAS LIFTED 1000 FT ON AIRWAY A-315 ON FREQ WITH ZMA; FL.

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WE WERE IN STRAIGHT AND LEVEL CRUISE FLT AT FL350. IT WAS A PITCH BLACK NIGHT (NO MOON) AND THE AUTOPLT WAS ENGAGED. WE ENCOUNTERED A VERY SMALL CUMULO NIMBUS -- SMALL IN DIAMETER; BUT OBVIOUSLY VERY HIGH. THIS CUMULO NIMBUS DIDN'T SHOW UP ON RADAR. BOTH OUR SCOPES WERE SET TO 80 MI SCALE; 1 DEG DOWN TILT; AND GAIN IN AUTO POS. EVEN THOUGH THIS CUMULO NIMBUS DIDN'T SHOW ON OUR RADAR; IT HAD ENOUGH VERT WINDSHEAR IN IT TO CAUSE ABOUT 10 SECONDS OF SEVERE TURB. I IMMEDIATELY DISENGAGED THE AUTOPLT. LEAVING THE AUTOPLT ON IN THE 'TURB' MODE IS THE PREFERRED PROC; BUT THE PANEL WAS SHAKING SO BADLY I COULDN'T QUICKLY LOCATE OR MOVE MY HAND TO THE SELECTOR SWITCH. I COULD EASILY PUSH THE AUTOPLT DISCONNECT BUTTON ON THE CTL YOKE. HAND FLYING IS THE SECOND MOST APPROVED METHOD IN TURB. I ATTEMPTED TO MAINTAIN LEVEL ATTITUDE WITH ELEVATOR AND MACH SPD WITH PWR. ATTEMPTING TO MAINTAIN ALT; BESIDES USUALLY BEING IMPOSSIBLE; COULD CAUSE STRUCTURAL FAILURE OF THE ACFT. WHEN THE TURB SUBSIDED; WE HAD GAINED ALMOST 1000 FT. WE DSNDED QUICKLY BACK TO FL350. ZMA QUESTIONED US ABOUT WHAT HAD HAPPENED. THERE WAS NO RAIN OR HAIL IN THE CUMULO NIMBUS WE HIT. IT PROBABLY DIDN'T SHOW UP ON CTR RADAR EITHER. THESE SMALL DIAMETER CUMULO NIMBUS OCCUR OFTEN OVER THE WATER; USUALLY OVER ISLANDS -- EVEN TINY ONES -- DUE TO HEAT CONTENT. THEY'RE USUALLY UNDETECTABLE WITH OLD GERIATRIC MONOCHROMATIC RADARS LIKE THE ONES WE HAD THAT NIGHT. ALSO; WHEN SET EVEN TO THE LOWER SCALE LIKE 30 MI; SOME OF THE CIRCUIT BREAKERS ARE THE SIZE OF A PINHEAD!