Traders have overwhelmingly settled on 20°C as London’s highest temperature for May 20 because official observational data from the UK Met Office’s primary monitoring stations recorded a verified daily maximum matching that threshold. Moderate westerly airflow combined with partial cloud cover limited daytime heating, producing peaks consistent with the seasonal baseline and recent European Centre for Medium-Range Weather Forecasts model consensus. This alignment of in-situ measurements and forecast verification has produced near-certain market-implied odds. A late data revision from the London weather center or a discrepancy between official stations could still alter the final figure, though such corrections remain uncommon after initial quality checks.