NAVING THROUGH A LINE OF CUMULUS CLOUDS PLT DEVIATES FROM ASSIGNED ALT WHILE HAND FLYING ACFT.

Narrative

FLT WAS SCHEDULED TO FLY ATL-PNS. ATL HAD NUMEROUS TSTMS ALL QUADRANTS; WITH A FAIRLY EXTENSIVE LINE JUST S OF THE ARPT. SOUTHERLY DEPS HAD BEEN HALTED FOR A BRIEF TIME; BUT HAD RESUMED. WE WERE THE 4TH FLT TO DEPART TO THE S. TWR WAS ASSIGNING A HDG OF 255 DEGS; 10000 FT FOR DEPS. WE HAD THE RADAR/TCASII ON WHILE HOLDING AS #1 AND OBSERVED THE PREVIOUS FLT GOING THROUGH A SMALL CORRIDOR BTWN CELLS APPROX 10-15 MI SW OF THE ARPT. AFTER TKOF; DEP CTL ISSUED A TURN TOWARD THE SAME CORRIDOR. WE ASKED WHAT THE RIDE WAS LIKE FOR THE PREVIOUS FLT; AND THEY RPTED ONLY LIGHT TURB. I (CAPT) WAS HAND FLYING USING VNAV AND FLT DIRECTOR IN THE HDG SELECT MODE. PASSING 8300 FT; DEP CTL TOLD US TO STOP OUR CLB AT 9000 FT BECAUSE AN INBOUND FLT WAS DEVIATING THROUGH THE SAME CORRIDOR WE WERE USING AND THEY HAD DSNDED TO 10000 FT. (WE NEVER PICKED UP THE INBOUND FLT VISUALLY -- WE WERE IMC -- OR ON TCASII.) THE ALT ALERT WINDOW WAS RESET TO 9000 FT AND BOTH THE FO AND MYSELF VERIFIED (DOUBLE POINT). WE ENTERED THE CORRIDOR BTWN CELLS AND LEVELED AT 9000 FT AT ABOUT THE SAME TIME. SINCE THE CORRIDOR WAS ONLY 5 MI WIDE AND CURVED ALMOST 90 DEGS; I WAS NAVING BY REF TO THE RADAR DISPLAY ON MY NAV DISPLAY AND STILL HAND FLYING. WITHIN A VERY SHORT TIME PERIOD (LESS THAN 15 SECONDS); I NOTICED WE WERE CLBING PAST 9250 FT. BY THE TIME I REACTED AND LEVELED OFF WE HAD REACHED A MAX ALT OF 9400 FT. I BEGAN AN IMMEDIATE DSCNT TO 9000 FT. DEP CTL DID NOT QUESTION OUR ALT NOR DID WE RECEIVE A TCASII TA OR RA. WE CONTINUED THROUGH THE CORRIDOR AND EXPERIENCED ONLY LIGHT TURB. I AM NOT 100% SURE WHAT CAUSED THE ALTDEV. A (POSSIBLE) UPDRAFT DUE TO THE CONVECTIVE ACTIVITY WE WERE IN CLOSE PROX TO; OR (MOST LIKELY); A CLB CAUSED BY MY INADVERTENT INPUT WHILE HAND FLYING AND TRYING TO NAV LOOKING AT THE NAV DISPLAY. I AM SURE I LEVELED OFF AT 9000 FT. I AM ALSO SURE I NEVER REALLY STABILIZED AT 9000 FT -- WE STARTED CLBING TOO QUICKLY. IF I HAD BEEN USING THE AUTOPLT; THEN I COULD AT LEAST BE SURE I DIDN'T CAUSE THE DEV! I HAVE SAID IT BEFORE; I'LL SAY IT AGAIN; AND I'LL ALWAYS SAY: USE THE AUTOPLT IN HIGH STRESS OR DISTRACTING SITS!