A May Record 250 Years in the Making

Overview

On 26 May 2026, the Hadley Centre Central England Temperature (HadCET) series recorded a daily mean of 23.2°C — the highest ever measured for any May day in a dataset spanning 254 years. This chart places that reading in context: each dot represents one day in May, from 01 May 1772 to 26 May 2026. Only 3 of those 7,900 days have ever exceeded 21°C as a daily mean. The LOESS trend line traces the century-scale pattern across the full series.

Methodology

Data was downloaded directly from the Met Office HadCET dataset (meantemp_daily_totals.txt), the longest instrumental temperature record in the world. All May readings (days 1–31, years 1772–2026) were extracted using Python. A LOESS smooth was fitted to the 7,900 data points to show the underlying trend. The interactive chart was built in JavaScript using D3.js, with Delaunay triangulation for efficient nearest-point detection on hover and click.

Findings

• 26 May 2026: 23.2°C — the warmest May day in 254 years of record.
• Only 3 of 7,900 May days since 1772 have exceeded 21°C as a daily mean.
• The coldest recorded May day in the series: 08 May 1861 at 3.0°C — a 20.2°C spread across the full record.
• The LOESS trend shows a modest but visible warming signal from around 1980 onward, consistent with the broader Central England temperature trend.

Takeaways

Open climate data — and especially a dataset spanning over two centuries — makes it possible to contextualise a single extreme reading immediately and precisely. The HadCET series is public, machine-readable, and updated in near real-time by the Met Office. A record like 26 May 2026 can be verified, contextualised, and published within hours of its occurrence. That is what open data enables.