One of Yosemite’s most underrated winter treats is the radiation fog that hugs the valley floor on cold, clear, still mornings. Unlike the advection fog that drapes the San Francisco Bay Area (among other places) when (relatively) warm, saturated air passes over the colder ocean and blows inland, radiation fog forms in place when plummeting overnight temperatures cause airborne water vapor to condense.
A sheltered valley with a cold river, soggy meadows, and a spongy forest floor, Yosemite Valley is ideal for the formation of radiation fog. Each winter, storms fill the Merced River and soak the meadows and forest. On nights when there’s no wind to mix the atmosphere, cold air sinks until it meets the water-laden air near the ground. Because cold air can’t hold as much water vapor as warmer air, and the air on the valley floor is completely saturated, the airborne water vapor condenses as soon as the air chills even slightly: Fog.
Often no more than a thin, gray veneer, in radiation fog that’s dense enough to obscure trees across a meadow, it’s sometimes possible to see stars or blue sky overhead. Viewed from a distance (for example, from Tunnel View), Yosemite’s radiation fog appears to be in constant motion, alternately engulfing and revealing treetops, sometimes rising hundreds of feet and completely disappearing in a matter of minutes. With no wind to move the fog, the reality is that what appears to be motion is primarily fog forming and dissipating in place. Yosemite’s radiation fog persists until the air heats enough to hold the available airborne moisture, or the wind picks up and mixes warmer air above with the colder, saturated air below.
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Last Friday morning I went out to scout a new route to one of my favorite Yosemite spots, a bend in the Merced River upstream from Sentinel Bridge with view and reflection of Half Dome. My Yosemite Winter Moon workshop started that afternoon, but the original access here had been obliterated by major roadwork underway in Yosemite Valley, so I needed to make sure I could still get my group out here.
With very little time to spare, I originally left my camera bag in the car, but didn’t get too far before second thoughts sent me back for it. Good thing—after a few minutes of traipsing across crunchy snow, I made it out of the woods and to the river just in time to catch sunlight illuminating a diaphanous radiation fog. Shadows cast by sunlight passing through evergreen branches created a beam effect in the illuminated mist, while upstream a blanket of fog basked in golden sunlight.
The sun was about to disappear behind the granite ridge beneath Glacier Point and I new I only had a few more minutes of this spectacularly illuminated fog. At first I tried to position the sun behind the trees, but at this distance the treetops were so thin that very little blocking occurred and I ended up with a white blob of blown highlights. So I hustled over to the intersection of the ridge’s shadow with the sunlit ground and prepared for a sunstar. Every lens creates a different sunstar effect, some much better than others, and my Sony/Zeiss 16-35 f/4 is my favorite. Stopping down to f/16, I went to work. Moving with the sun so I was always straddling the shadow line, I was able to shoot for about five minutes before the entire beach was in shade and I was finished.