The answer the regulars give is layered. "Cool air, ocean breeze, sun-kissed hillsides, moist eucalyptus, moss, and dirt," one commenter wrote, racking up 160 upvotes — which is about as close to a civic consensus as the internet allows. The eucalyptus part is doing real work there. The trees are everywhere on the city's western and southern edges: along the shoulder of Twin Peaks, through the Sunset's park strips, in thick groves at the top of Corona Heights. On a morning when the marine layer is burning off slowly, they release something sharp and camphor-adjacent that mixes with the damp soil underneath and the particular brine of a bay that's open to the Pacific in a way most urban waterfronts aren't.

"Depending on your route, could have been eucalyptus," another commenter noted, more simply. Both answers are right. The smell changes block by block, hill by hill — diesel near the freeway stacks, sourdough if you're idling near the Embarcadero at the right hour, something unmistakably mulchy if you've cut through Golden Gate Park after rain.

None of this is secret knowledge. It's what the city smells like when the windows are down and you're paying attention, which is the only way to catch it. Someone driving through tomorrow on the 101 south, just past Cesar Chavez, will either notice it or they won't — but it'll be there, coming in off the hills the same as always.