**Marine layer influence and typical July climatology position 81°F or below as the dominant outcome at 64.5% implied probability.** San Francisco’s coastal setting produces persistent onshore flow of cool Pacific air, reinforced by the marine layer—a shallow stratus deck that forms overnight and often lingers into the afternoon, capping daytime highs near the seasonal average of 66–70°F. Current forecasts for July 21 show highs in the low-to-mid 60s to low 70s under continued westerly flow and no significant offshore wind shift, keeping temperatures well below the 81°F threshold. The next two buckets (82–83°F at 23.5% and 84–85°F at 7.5%) reflect residual uncertainty around marine-layer clearance timing or a brief warming pulse, but model consensus and historical July records (rarely exceeding 79°F) limit those probabilities. Higher bins remain thin because sustained heat requires strong high-pressure ridging and easterly flow—neither of which is indicated in the latest guidance. Traders are therefore pricing in the high likelihood of another cool, fog-influenced day consistent with long-term patterns.