Fog in Ontario

When I checked the forecast for Ontario this morning in Fort Worth, the weather was predicted to be no lower than 3 miles visibility for our ETA. I noted that the temperature/dew point spread was a mere 2 degrees, and it seemed likely that the temperature would continue to drop. When these two numbers come together, fog is the result. With no wind to mitigate the situation, the final landing of this duty day would probably be challenging, in spite of the not-so-bad forecast. Enroute, as it always seems to, the visibility began to drop. By the time we were in range and ready to start down from our cruise altitude of 38,000', the touchdown zone on runway 26 left was reporting 1200 RVR, or less than 1/4 mile. We switched to a monitored approach mentality in the cockpit. This means that the First Officer is now in charge of flying the approach down to (in this case) 300' above the ground, freeing the Captain to monitor the whole situation without having to actually fly the airplane. As we came down, the lights on the ground were very visible except where the airport was supposed to be. Very dense fog covered the runway. The runway lead-in lights became visible (barely) through the fog at 500'. At 300', I announced "approaching alert height, I have the airplane" and physically displaced the FO's hand from the throttles. At 100', the runway edge lights had just begun to peek out at us. With this type of approach "Cat 3 with rollout" I do not have to see to land as long as the tower is reporting visibility of at least 600 RVR, and the airplane must autoland. There are about 100 things that have to be working perfectly for all this to come together, and we practice it quite often, but when it's for real, you tend to sit up a little straighter in your seat. This ain't a video game! In the final phase of a monitored approach, my job is to initiate a go around if anything short of perfection occurs. This close to the ground, it's better to get some altitude first and ask "what happened" later. The three autopilots did a great job and we touched down right on centerline, ending another routine night of flying the line.