**Trader sentiment for the lowest temperature in São Paulo on August 21, 2026, reflects genuine forecast uncertainty around radiational cooling and cold-air advection in the Southern Hemisphere winter.** Leading market-implied odds favor 9°C or below (39%), with nearby bins clustered near 27–32.5%, indicating traders weigh the balance between clear-sky nocturnal cooling and any moderating effects from cloud cover or a passing front. Official forecasts from sources such as BBC Weather and timeanddate project overnight lows near 10–12°C under partly cloudy or overcast conditions, while other model outputs suggest potential for 6–8°C if skies clear and a stronger southerly flow arrives. Key differentiating factors include the strength and timing of the cold front (frente fria) noted in regional guidance, which can deliver cooler, drier air masses, versus local urban heat-island effects that often keep official station readings (e.g., Congonhas or INMET sites) several degrees warmer than rural surroundings. Historical August minima typically hover around 12–14°C, so current model spread and any late-day shower activity introduce the observed clustering around the 9–13°C thresholds. Traders are closely monitoring the next model runs and official INMET or CPTEC updates for confirmation of whether radiational cooling will push readings below 9°C or remain in the low double digits.