INST RATED FLT INSTRUCTOR IN A C182 TOOK OFF VFR AND FLEW INTO A FOG BANK AT 450 FT DURING INITIAL CLB.

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WX IN HYA HAD BEEN FOGGY; BUT WAS RPTING VFR THROUGH THE MID MORNING. A LCL 'SEA FOG' LAYER WAS S OF THE ARPT. 3 ACFT (C182; MY C182 AND A CHEROKEE) WERE WAITING FOR VFR DEP. AS THE TWR WAS ALLOWING VFR DEPS; I ASSUMED WE WOULD BE ABLE TO DEPART VFR. THE C182 IN FRONT OF ME DEPARTED AND APPEARED TO EITHER GO INTO OR AROUND A CLOUD. SINCE THE FOG HAD BEEN IN WIDELY SCATTERED CLUMPS; I HAD ASSUMED I WOULD BE ABLE TO CLB THROUGH A HOLE IN A SCATTERED LAYER. I DO NOT RECALL ANY DIRECTIONS FOR A HDG. I DEPARTED AND ALMOST IMMEDIATELY ENTERED THE FOG. AS I WAS STILL CLOSE TO THE GND; I OPTED TO FLY THROUGH THE FOG; RATHER THAN EXECUTE A LOW TURN. AS I WAS WORKING TO KEEP THE WINGS LEVELED; I TURNED TO THE R. I ALSO DISCOVERED THE DIRECTIONAL GYRO HDG WAS A BIT OFF; AND FOUND THAT I HAD TURNED EVEN MORE TO THE R. I CAME OUT OF THE FOG AND EXECUTED A TURN ON COURSE TO AVOID FURTHER CLOUDS AND ADDITIONAL CLASS D AIRSPACE. THE ACFT BEHIND ME WAS CLRED FOR A R TURNOUT. AS SOON AS THE ACFT BEHIND ME WAS CLRED FOR DEP; THE ARPT WAS DECLARED IFR. AFTER I TOOK OFF; HEARD A CONVERSATION BTWN THE TWR AND AN INCOMING ACFT. IT SOUNDED AS IF THE ACFT HAD TO GO AROUND; BUT IT WASN'T CLR IF MY ACFT WAS INVOLVED. I WAS PREPARED TO DEPART IFR; BUT THE TWR WAS VFR; SO I DID NOT EXECUTE AN IFR DEP (THAT IS; FILE AN IFR FLT PLAN). THE TWR MIGHT BE USED TO THESE 'NEAR VFR' DEPS FOR LCL PLTS; BUT IT WAS DISCONCERTING FOR NON LCL PLTS. I MADE THE MISTAKE OR ASSUMING THAT THE FIELD WOULD BE CONSIDERED NOT GOOD FOR VFR DEPS IF A SAFE VFR DEP (1000 FT CEILING AT LEAST) COULD NOT BE MADE. I HAVE LEARNED FROM THIS EXPERIENCE NOT TO MAKE THIS ASSUMPTION. IN THE FUTURE; I WILL FILE TO DEPART AND THEN CANCEL IFR TO GO VFR ONCE CLR OF IFR CONDITIONS. MY RECOMMENDATION TO HYA TWR IS TO BE MORE AWARE OF THE IFR CONDITIONS IMMEDIATELY OFF THE END OF RWYS; EVEN WHEN THE ARPT ITSELF IS VFR. THE REMAINDER OF THE TRIP WAS UNEVENTFUL.