Hover the chart for any day's high and low. Data: ECA&D official daily series; 2026 tops up from Heathrow METAR to 8 Jul.

Each line is a decade's average daily high by week of the year (complete years only, 1960–2025), which smooths out single-day outliers. The summer hump lifts and broadens decade on decade. Hover for every decade's value in a given week.