~ $ cd open-data/hot-nights && cat README.md

Hot nights, counted

A night whose lowest temperature stays at 20 °C or above is called a tropical night: the house does not cool down and nobody sleeps well. The Meteocat publishes the daily minimum of every automatic station it runs. This counts them, year by year, and cuts each record in two to see whether it has moved:

The threshold slides, because what the site keeps is not the count but a histogram of each month's days. Move it to 25 °C and the nights are torrid; choose another kind of day and the same page counts hot afternoons, frost, or rain.

What is in it

• Every one of the nine stations has more tropical nights in the second half of its record than in the first. At Badalona, 75.1 a year became 86.1. At the Raval, in the middle of Barcelona, 93.2 became 103.0.
• The nights at 25 °C or more are where the Raval changes most: 6.1 a year before 2016, 19.8 since.
• 2022 is the year with most tropical nights at four of the nine, and with most days at 30 °C or more at eight of them.
• The exception is worth as much as the rule. The Observatori Fabra, on its hill above Barcelona, has fewer days at 30 °C or more in its second half than in its first, 33.9 against 42.3. Its first half holds the summer of 2003.
• Rain does not move the same way. Days with 1 mm or more go slightly up at some stations and slightly down at others, and the largest difference between two halves is five days a year.

What it is not

Each station is cut in half over its own years, so two stations' halves are different periods: a record that starts in 2009 has no flat 1990s in it, and will look steeper for that alone. Compare a station with itself.

The series are not homogenised. A sensor replaced, a screen moved, a car park built next door: each can make a step that has nothing to do with the climate, and nothing here corrects for it. A station is never joined to another, even in the same town. A year missing more than one day in twenty is drawn as an outline and kept out of every mean. The year still running is not shown.

The names — tropical night, torrid night — are common usage, not official definitions. Frost is "below 0 °C" rather than "0 °C or below" because only the first can be counted exactly from the histograms. The difference between two halves is a description of what a record did. It is not a forecast, and it does not say why.

Where it comes from

This is the small version of an explorer I built in the summer of 2026, which has every station of the network that is still reporting, humidity and a map: Nits de calor a Catalunya. Here there are nine stations and four variables, the daily series stays at its source, and the site keeps only what it derives from it, a finished year at a time.