As I climbed higher in elevation above Putah Creek, there was an abundance of B-letter wildlife: blue jays, bumblebees, butterflies. At one point, the bushes along the trail rustled, and a brown bunny hopped across my path. (Though I spotted a sign announcing the presence of mountain lions, I didn’t see any.) At 800 feet, I stopped for a long moment, moved to do so by the view of mountains and fog seen through a scrim of rustling wheat-gold grass.

As the switchbacks began piling up, the charred black skeletons of trees below made for a striking overlay on the otherwise blazingly bright native ground vegetation. During warmer months, this is a hike to start in the early morning cool air, since the 2015 fires cleared away much of the tree cover above the valley floor. A handful of mature trees were left standing, a California blue oak here, a manzanita there.

“If you’re a hiker out there, as you’re walking, there are those places you naturally want to stop and rest because it’s cooler; there are trees, some shade,” Mr. Clary said, describing the many microclimates along the hike. “In the same way, you see species moving, not very far, but to microclimates that are a little cooler than where they came from. With a fire, those kinds of shifts can happen overnight. Most climate change models for California suggest more frequent and more intense fires. And that’s happening all over the world.”

As I rounded the northern part of the loop, sprouts of poison oak extended onto the trail to catch my hair and clothes. Lizards skittered across the trail, throwing up puffs of dust. Just as I reached the exposed ridgeline, short of breath, I was rewarded with my first glimpse of Lake Berryessa. Viewed from this vantage point, the water shone a glorious, thirst-quenching blue.

Thirst-quenching in a psychic way, that is; there is no actual potable water on this hike. I paused to swig some of the liter-plus water I’d brought, and to appreciate the breeze, while turkey buzzards the color of char rode the cool updraft over the lake.

Though it was quickly getting hot, the astounding blue of the water below was a balm. Even as I focused on carefully bouldering up a narrow cliffside passage, I kept turning to take sips of that view. At one point, I passed a group of hikers going the other way, clockwise on the loop, exclaiming about the lake as they came up upon it. “Kudos to you for doing this alone,” one young woman chirped as we scrambled past each other. I smiled weakly, blinking back the sweat as it poured over my brow.

In the end, I was happy I’d gone counterclockwise, getting the bulk of elevation gain under my belt early on so that I could circle back on the Homestead Trail section of the loop; it has a more gradual slope, with wooden stairs in sections and some shade. (Still, I admit the descent was murderous on my legs.)