Don’t get me wrong. Anything this hazardous to a motorist’s health shouldn’t be treated in solely rhapsodic terms. But a confession is in order: I’ve got the foggiest notion.

East County folk are treated to microclimates of fog as varied as Northern California’s microclimates of grape growing. When the tsunami of marine layer batters Mt. Diablo and floods its foothills, when tule reeds adorning the Delta shoreline are veiled in vapor, when wizened sycamores drift wraithlike in and out of tendrils of mist along Marsh Creek, some of us leave our ignition keys on the kitchen counter, bundle up and take a stroll in the neighborhood. Others turn the key and head for the hills.

Last Saturday I awoke to find my Brentwood neighborhood sleeping snugly beneath a blanket of grey. Curious about the blanket’s breadth and depth, I threw together my hiking gear, brewed a thermos of weapons-grade coffee and struck out for Round Valley Regional Preserve. My goal: the park’s summit, 1,220 feet above sea level, where an archipelago of boulders protrudes from a sea of grasses, and the skeletal limbs of a bent blue oak I call Old One frames the high ridges of Morgan Territory and twin peaks of Mt. Diablo.

From the look of the park’s parking lot, I wasn’t the only maven of murk in town. A handful of hikers, cyclists and horseriders were lacing, gearing and saddling up for East County’s Annual Unofficial FogFest.

Long before I reached the summit it was clear that the brand of grey gripping the region was no tule fog rising from the reeds; it was an inversion fog – a big ol’ cloud stooping to our level, pancaking an enormous swath of real estate. When I got to the top, that evocative panorama I’d been banking on was choked in fog harder to see through than lead lingerie on Lois Lane.

Inversion fog can hang around with the persistence of an annoying party guest – especially when above it hang sheets of altostratus preventing the Sun from burning it off. That was the state of the sky the morning of Saturday, Jan. 16.

Normally, the higher you rise into the atmosphere, the more the temperature falls. But in an inversion, the higher you rise, the more the mercury rises with you. Warm air overrides cool. And absent that normally warm air rising from the surface and stirring the lower atmosphere, the inversion’s cool surface air leaves the lower atmosphere unmixed, stable, fogbound – for a long time. Sprinkle the surface with moisture from earlier rain and fog, and voilà! You’ve got yourself all-day grey.

I had perched on Round Valley’s peak many times and knew what to expect: a view 15 miles north to the Solano hills; Mt. Diablo eight miles west; the Ohlone Wilderness beyond Livermore, 25 miles south; and the Sierra Nevada Range 100 miles across Central Valley. But when I arrived at the summit on that misty morning, my first impression was disorientation, as if I’d been teleported into downtown San Francisco on a busy workday but could hear only silence. The vista that morning was the visual equivalent of a soundproof booth. I commandeered the boulders and from my grand altitude saw nothing beyond 60 yards.

As the hours passed and the haze began to dissipate, the spidery veins of oak branches spilling down the summit’s north slope came into focus. Above, the Sun’s faint disc slipped in and out of view like the searchlight of a ship inching its way into a fogbound harbor. The wind freshened and the high ridges of Morgan Territory three miles southwest materialized above the roof of rippling grey. Northwest, Diablo’s deep blue peaks began breaking through the gloom. Ah, finally.

And then, just as abruptly, the vision vanished. Fog flowed over the summit like a tide, drowning the horizon. I had caught sight of an achingly sharp-edged shore beyond a formless ocean, and just as swiftly it had been snatched from me. Was it real or had I dreamed it?

I stayed a while in the silence and stillness, hoping the pale wafer of Sun would reappear, hoping the northwest breeze would scatter the fog from the summit, hoping to reaffirm the existence of that dark and distant shore. But another notion, perhaps a wordless voice from Old One, told me the vision I’d been given was sufficient for a day – for a lifetime. It was more than enough to have seen less than enough. I tipped my hat and turned my back to the tree, disappeared into the fog and began my descent toward the cool and lucid air of earth.