A HOT AIR BALLOON PLT GOT CLOSER TO A CLOUD THAN THE REGS ALLOW.

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MY SINGLE PAX AND I WERE ON THE SECOND FLT OF THE MORNING. I WAS CLBING AT 300 FPM IN ORDER TO CATCH A SOUTHERLY FLOW. IT WAS SUNNY EXCEPT TO THE N WHERE SOME HAZE WAS VISIBLE. MY PAX SAID 'GEE; LOOK AT THAT CLOUD.' I TURNED AROUND TO SEE A CLOUD OR FOG ONLY 100 FT AWAY AT OUR ALT (800 FT). IT WAS LIKE IT HAD JUST FORMED THERE IN LESS THAN A MIN. THE BALLOON WAS NOT MOVING; SO THERE WAS VERY LITTLE AIR MOVEMENT. I IMMEDIATELY VENTED AND BEGAN A DSCNT AS QUICKLY AS WAS SAFE (600 FPM). IN LESS THAN 3 MINS I HAD AN UNEVENTFUL LNDG AND WAS ON THE GND AFTER DECIDING TO END THE DAY'S FLYING. I HAD FLOWN JUST OVER 2 HRS WHICH IS TYPICAL FOR THIS AREA. I MAINTAINED VISUAL CONTACT WITH THE GND AT ALL TIMES AND OUR CHASE CREW THOUGHT WE WERE IN SUNSHINE ALL THE TIME. THERE WERE NO OTHER ACFT IN THE AREA. 2 OTHER BALLOONS HAD JUST LANDED. I BELIEVE THAT A BUBBLE OF WARM MOIST AIR FROM A FIELD BELOW ME MOVED UP INTO COOLER AIR AND FORMED VISIBLE MOISTURE. THIS TOOK LESS THAN A FEW MINS AND COULD NOT HAVE BEEN ANTICIPATED. SINCE THE CENTRAL VALLEY IS SO FOG PRONE; BETTER OR MORE TEMP DEW POINT READINGS OR INFO COULD BE OBTAINED. THIS WAS A VERY LCL OCCURRENCE.