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Madrid's summer nights are getting hotter

Every night in one city from 1 June to 30 September, 1920 to now, drawn as one ridge per summer — how many of that summer's nights fell to each temperature. Press play to run down the century. The pile slides steadily right, from a middle of — in the first thirty summers to — in the last thirty, while the 20 °C line stays where it is. Above that line a night never cools down at all, and over those same two stretches Madrid went from about — a summer to —. Tap or hover any summer.

Nights that fell below 20 °C
Tropical nights — the low never reached 20 °C
The 20 °C line, which does not move
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Data & source

Data: NOAA GHCN-Daily, station SP000003195 — Madrid Retiro, — m, run by AEMET in the Retiro park in central Madrid · public domain, freely redistributable · readings — to — · fetched —.

Scope and caveats, plainly: this is one station in one city, not Spain and not Europe. The gauge sits in the Retiro park in central Madrid and the city has grown around it, so part of the warming drawn here is the urban heat island rather than the climate — a real effect, worth naming rather than hiding. A "tropical night" is one whose minimum never falls below 20 °C, the threshold AEMET and the WMO both use. GHCN-Daily records one minimum per calendar date; observing conventions vary by station and era, so a reading can sit on the date either side of the night it belongs to. Each ridge is a kernel density of that summer's nightly lows (Gaussian, 0.9 °C bandwidth) scaled as a share of the summer's nights per degree, so every ridge is on the same vertical scale and a short summer is still comparable. Readings that failed NCEI's quality checks are dropped, never interpolated; a summer missing more than a tenth of its 122 nights is dropped whole. The running summer is drawn as an unfilled outline and labelled "so far" — it has not met September yet, which is the coolest month of the four, so its middle sits warmer than a finished summer's would. Days warmed by about the same amount as nights over this record; what makes the tropical-night count explode is that a fixed threshold sits in the middle of a moving distribution.