The single pilot of a small jet reported a taxiway excursion in SBA while attempting to cross Runway 7 from Taxiway C to Taxiway F at night in low visibility conditions. Lack of a taxiway centerline from C to F across the runway is cited as a contributing factor.

Narrative

I was cleared for taxi by SBA ground. The ceiling was 5000 FT; but there was mist in the air; and patchy ground fog at night. My aircraft was parked at the FBO. I was cleared to taxi on Taxiway C; cleared to cross Runways 15 Left and Right and instructed to hold short of Runway 25 at C. Visibility out of the cockpit was poor; but with my landing lights and recognition lights; I could clearly see the yellow taxiway striping for the assigned taxi route. Throughout the taxi; I maintained a slower taxi speed than normal consistent with the reduced visibility conditions. Subsequently; I was cleared to cross Runway 25 at Taxiway C and proceed on Taxiway F; then execute a right turn and taxi down Taxiway A for a full length takeoff on Runway 7. As I proceeded across Runway 25 I noticed that there was no longer a yellow taxi center way marking so I used the blue light taxi edge markers on the opposite side of the runway for reference. After crossing Runway 7 my nose wheel tire left the prepared surface of the taxiway. Because of the reduced visibility; my taxi speed was slower than it would normally be and I stopped the aircraft with both main gear on the prepared surface; but my nose gear had left the prepared surface.I radioed ground and advised that I needed help to reposition my aircraft on the taxiway. Flight support from an FBO repositioned the aircraft on the prepared surface by connecting a tow bar to the nose gear; rotating the nose gear close to 90 degrees at which angle the tow bar extended onto the prepared surface and could be connected to the tow equipment. During the repositioning I walked off the prepared surface next to the nose gear; so that I could closely monitor both the main and nose gear throughout the process of repositioning. I requested the aircraft to be towed to FBO so that I could do a close inspection of both the main gear and the nose gear under good lighting conditions. I did a close inspection of the gear and then tested and confirmed the brakes. I filed a return flight plan and returned to my home base.There was no damage done to the aircraft or to airport property as a result of the incident. As pilot in command I am fully responsible for the aircraft at all times. Reflecting upon this incident; I realize that the incident would have be en avoided if: 1) I had asked for permission to back taxi on Runway 7 rather than accepting the clearance to cross Runway 7. The runway edge lighting would have been very easy for me to see had I chosen that option. 2) The incident would also have been avoided if a yellow taxi line had been painted from the Taxiway C hold short line; across Runway 7 to the Taxiway F hold short line. In SBA aircraft are frequently given the clearance to cross Runway 7 from Taxiway C to Taxiway F. In low visibility conditions the yellow taxi centerline is a very helpful point of reference.