The market-implied probabilities center on 70-73°F because San Francisco’s August climate is dominated by a persistent marine layer that advects cool, moist Pacific air onshore, capping daytime highs near the long-term average of 69-70°F. Current National Weather Service guidance and model consensus point to upper-60s to low-70s readings under partial morning fog and afternoon sun, with only modest warming possible if the marine layer thins. Recent coastal observations confirm this pattern holds despite inland heat and distant wildfire smoke, keeping probabilities for 74°F+ below 10 percent. Traders are therefore weighting the likelihood of a near-normal maximum today, with resolution hinging on the official San Francisco International Airport observation.