The fog lay thick on the California coast last Friday as President Reagan arrived on Air Force One for a 10-day Easter vacation at his California ranch northwest of here.

The fog was so dense, in fact, that it appeared Reagan would have to be driven the final leg to Rancho del Cielo by car because helicopters couldn't navigate in the murk.

But the commander-in-chief had other ideas.

Following normal practice, Marine One, the president's helicopter, left Point Mugu Naval Air Station, where Air Force One lands, and set out for the ranch about 50 miles up the coast in the Santa Ynez mountains inland from Santa Barbara. Nancy Reagan was traveling separately from Phoenix and was not aboard.

One White House official traveling with the president said the fog grew very thick, and it appeared that the helicopter would have to land at the Santa Barbara airport.

"I thought we were going back to Santa Barbara," he said, and Reagan would then be driven by motorcade the rest of the way.

The drive is a long one up winding roads and hairpin turns.

But the president was eager to get to the ranch and seemed certain the 688-acre spread could be found from the helicopter, despite the fog, according to the official who was on the helicopter with him.

Reagan insisted that the pilot continue on toward the mountains. Motioning with his hands, Reagan said he could see the outlines of the hills in the distance.

At one point, according to the White House official, Reagan was told the fog was so thick that visibility was only about 100 feet.

But once they were up in the hills, the fog suddenly broke.

As aides looked down in amazement, the landing zone for the helicopter materialized below.

The helicopter set down, dropped off the vacationing president and disappeared back into the fog.