THE HELICOPTER CRASHED IN AN OPEN FIELD SHORTLY AFTER TAKEOFF IN ADVERSE WEATHER. THE PILOT WAS NOT INSTRUMENT RATED ANDHAD RECEIVED WEATHER BRIEFINGS BY TELEPHONE FROM A FLIGHT SERVICE STATION. THE FORECAST WAS NOT FAVORABLE, BUT THE PILOTSTATED 'WELL I THINK I CAN GET OUT FROM UNDERNEATH IT WITH A HELICOPTER, NO PROBLEM.' THE PILOT TOOK OFF WHILE SEVERAL WITNESSES WATCHED. ONE WITNESS OBSERVED HIM WIPE CONDENSATION OFF THE INSIDE OF THE PLEXIGLASS TWICE PRIOR TO DEPARTURE.AFTER TAKEOFF,WHICH THE AIRPORT MANAGER THOUGHT WOULD BE POSTPONED FOR WEATHER, THE HELICOPTER DISAPPEARED IN THE FOG ATA DISTANCE OF ABOUT ONE HALF MILE AND ALTITUDE OF 200 FT. THE WITNESSES HEARD THE AIRCRAFT NOISE CHANGE FROM NORTHWEST TO NORTHEAST WHICH PUZZLED THEM. NO ONE HEARD THE SOUNDS OF THE ACFT IMPACT. THE WRECKAGE WAS FOUND ABOUT ONE AND ONE- HALF MILES NORTHEAST OF THE AIRPORT BY A FARMER, APRX TWO HOURS AFTER DEPARTURE.