Weather data is fetched from Open-Meteo, which serves both live forecasts and the Copernicus ERA5 reanalysis archive (1940–present). Upcoming competition days use the forecast API; completed ones use the ERA5 archive — and so does every 50-year baseline, so both sides of an anomaly always come from the same source.
For each competition day's location, we pull ERA5 daily maximum temperature for every year 1974–2023, within ±7 days of that day's calendar date. The anomaly is today's observed or forecast max minus that 50-year mean, at the same location. Decade means and the "then vs now" day-count comparisons on edition pages are drawn from the same 50-year window.
Baselines are rebuilt per location, not reused across an edition or between editions — a Grand Tour's route changes every year, so a stage finishing in a given town this year needs its own baseline for that exact spot, not last year's. The same method works unchanged for a fixed single-venue event (a baseline computed once for that venue) and a moving multi-day route (a fresh baseline per stage) — the location is just an input, not a special case.
Both sides of every anomaly must come from the same ERA5 grid cell. Mixing an observed reading from one source with a baseline from another would bake in whatever systematic offset exists between them — we verified our own pipeline never does this (see "Validating the pipeline itself," below).
We estimate Wet Bulb Globe Temperature using the Minard (1961) combination — WBGT = 0.7·Tnwb + 0.2·Tg + 0.1·Ta — the same combination used by Cvijanovic et al. (2026, via ECMWF's thermofeel library). Our natural-wet-bulb and black-globe sub-terms are a documented simplification, not a full port of the iterative Liljegren et al. (2008) model, so treat WBGT values as estimates, not certified measurements. WBGT is always shown in °C — it's a regulatory index (the UCI High Temperature Protocol publishes its thresholds in °C), so it doesn't convert with the °C/°F toggle.
| WBGT | Tier | Action |
|---|---|---|
| < 15°C | Very low-risk | No specific measures. |
| 15–17.9°C | Low-risk | Warm-up in the shade with fans. |
| 18–22.9°C | Moderate low-risk | Ice vests, wet towels, individualized hydration plans, ice socks. |
| 23–27.9°C | Moderate high-risk | Shaded start areas, more drink/ice motorbikes, adapted hydration rules. |
| ≥ 28°C | High-risk | Modified start/finish times or route, or cancellation. |
The UCI's Extreme Weather Protocol (2015) was extended with a dedicated High Temperature Protocol in 2023–24, using these tiers. At the red tier, the UCI can modify a stage's start or finish time, shorten or reroute it, or cancel it outright.
Western Europe had its hottest June–July on record in 2026. Copernicus put the two-month average at 21.62°C — about 2.79°C above the 1991–2020 normal — with five separate heatwaves from May onward. That's the real backdrop behind almost every anomaly on this site, not an artifact of how we compute them.
Don't read a typical day's anomaly here as +5°C to +10°C— that's what an exceptional heatwave year looks like against a 50-year average, not this site's normal range.
Seeing every stage read +5°C to +10°C hot is the kind of pattern that should raise suspicion on its own — so before trusting it, we ran a control: the identical baseline + observed methodology, at the same real locations, on the same calendar dates, but for 2021 and 2024 instead of 2026. If the pipeline had a systematic bug (for instance, comparing a daily maximum against a baseline built from daily means), those control years would show the same uniform, one-sided offset. Instead, six real locations across both live editions came back scattered near zero with mixed signs — roughly −7°C to +4°C, some years reading below the 50-year mean, some above. Same code, same 50-year baseline, only the year changed. That confirms 2026's pattern reflects real weather, not a computation bug.
The check is a real script in the codebase (scripts/check-anomaly-control.ts), not a one-off calculation — it runs against any edition's real stage and baseline data, so the same control is available for every future edition too.
ERA5 is a reanalysis— a physically-consistent gridded reconstruction of past weather, not a network of individual weather stations. Each grid cell (~28km for ERA5, ~11km for the finer ERA5-Land product) represents an average over that whole area, which can differ from a single station's reading — especially in coastal cities, where a grid cell partially averages in the adjacent sea.
We checked this directly: for Barcelona, the official 1991–2020 climate normal (AEMET) puts the July mean daily max around 28–29°C. Our ERA5 figure for the exact same period and month comes out to 26.8°C — about 2°C cooler. We also tried the finer-resolution ERA5-Land product (26.75°C) and an explicitly-pinned ERA5 model (27.0°C); neither closed the gap. This is a known, documented characteristic of gridded reanalysis data for coastal locations, not a bug in our pipeline.
Why the anomaly is still trustworthy:the anomaly compares today's value to ERA5's own50-year average at that exact grid cell — the same offset applies to both sides of the subtraction, so it cancels out. What it means is: absolute baseline numbers on this site describe "ERA5's climatology for this location," not an official government climate normal for the named town — we phrase them as "the 50-year average at this location (ERA5)" for that reason, never as "the average temperature in [town]."
Cycling editions show a real-elevation route band per stage, built from ~200 samples per stage via the Open-Meteo Elevation API (Copernicus DEM 2021, GLO-90) along a route interpolated from open data and real intermediate towns, never traced from an organiser's own profile graphics. These profiles won't match a race organiser's published elevation chart exactly — that's a deliberate choice about what we can license and verify, not an error, and each edition's own page discloses its specific known gaps.
Cvijanovic, I., Begg, J.D., Mistry, M.N., Petrova, D., Brimicombe, C., Sultan, B. (2026). "The future of European outdoor summer sports through the lens of 50 years of the Tour de France." Scientific Reports, 16, 2644. Open access.
UCI High Temperature Protocol thresholds: uci.org.