The nights that no longer cool: tropical nights in every Romanian town, 1950–2026

#Where tropical nights multiplied

Each square is about 11 × 8 km and shows the average number of tropical nights a summer there. Play the years and watch the amber spread from the Danube valley and the Black Sea coast into the Bărăgan plain, Oltenia and the west. Hills and mountains stay in the shade: in Brașov, in the Carpathians, the 2016–2025 average is 0; in Cluj-Napoca, 0.3.

Click the map to choose the place there.

The towns, one by one

#How many people now live in the path of warm nights

The count starts from today’s population (the 2021 census) and the summers of each period. On the climate of 1961–1990, 4.5% of them would have lived where a summer brought five or more tropical nights on average. On the summers of 2016–2025, 31%. At ten nights or more: 2.0%, then 20%.

The result is sensitive to one place, Bucharest, home to one Romanian in eleven. Counted in the square on its northern edge (Băneasa), where the weather station is, it is 31%; counted in the city-centre square, which the search also uses, it rises to 40%. The page gives the lower figure.

For 33% of people, a tropical night is still rare in the grid: fewer than one a summer in 2016–2025. Most live in hill and mountain areas, where the grid catches fewer tropical nights than the stations (see below). For them the page also shows the warmest night of the summer: it rose in all 138 towns in this group.

#How far these figures can be trusted

The grid (E-OBS, 0.1°) is a temperature map built from weather stations. It was compared night by night with 25 Romanian stations that feed it. In 1991–2025, of the 7,342 tropical nights measured at those stations, the grid showed 6,510 as tropical (89%). Of the 6,691 nights the grid shows as tropical, 181 were not tropical at the station.

The grid usually counts fewer than the stations, more so in the past (82% in 1961–1990) and far fewer in the hills: at Râmnicu Vâlcea it showed 69 of 162, at Cluj-Napoca 6 of 12. Sometimes it counts more: at Craiova, 382 against 330 at the station. The stations show the same rise as the grid: at the 21 stations with nearly complete records in both periods, 4.2 tropical nights a summer in 1961–1990 and 9.8 in 1991–2025 (the grid, on the same nights: 3.5 and 8.9).

These stations are the ones the grid is built from, so the comparison is not an independent check. Two airport stations are not part of the grid: Timișoara and Suceava, with sparse records. There, in 1991–2025, of 27 tropical nights at the stations, the grid showed 14 (52%); at Suceava, 1 of 8. The sample is small, but it points the same way: the grid’s numbers are more likely too low than too high.

The 1950s are the least certain. The Galați and Constanța series enter the grid only from 1961: for the years before, the grid showed 15 of the station’s 51 tropical nights at Galați, 71 of 127 at Constanța, 36 of 92 at Sulina. Across all 13 stations with data, 63% in 1950–1960. That is why the comparison period starts in 1961.

Station by station, 1991–2025

* station not used to build the grid (independent check).

#Method

Daily minimum temperatures were taken for every 0.1° square (about 11 × 8 km) in Romania, from 1 June to 31 August, 1950–2025. A night is tropical if the day’s minimum is at least 20 °C; in summer the minimum almost always falls at night, and meteorologists count tropical nights this way (the TR20 index). The count, the longest run and the warmest night (the TNx index) are computed for every complete summer.

The comparison periods were fixed before the count: 1961–1990 against 2016–2025. “The last five summers” means 2021–2025. Childhood means the summers at ages 5 to 14. Summer 2026 comes from a preliminary file with fewer stations; it is shown but enters no average.

Each place is read in its own square; places on the sea or the Danube that fall on water take the nearest land square. People are counted with each commune’s and town’s population at the 2021 census, with Bucharest in the Băneasa square.

The page does not publish the measured temperatures, only the class of each night (below 16, 16–18, 18–20, 20–22, 22–24, 24 °C and above) and the warmest night of each summer, rounded to a quarter of a degree.

A reanalysis (ERA5-Land) gave, inland, up to five times as many tropical nights as the station-checked grid (in Craiova, 47.2 a summer in 2021–2025 against 18.6), so it was set aside.

#Sources

• E-OBS v34.0e daily minimum temperature, 0.1° grid (Copernicus C3S / ECA&D)
• Cornes et al. 2018, An Ensemble Version of the E-OBS Temperature and Precipitation Datasets, JGR Atmospheres
• ECA&D and E-OBS data policy
• ECA&D station source list for minimum temperature
• NOAA GHCN-Daily, Romanian stations
• ETCCDI climate indices (TR20, TNx)
• Romanian 2021 population and housing census (INS)
• The night that never cools (explainer)
• The last spring frost, town by town (same grid)
• The climate grandparents knew

Acknowledgement requested by ECA&D: “We acknowledge the E-OBS dataset and the data providers in the ECA&D project (https://www.ecad.eu).” Cornes, R., G. van der Schrier, E.J.M. van den Besselaar, and P.D. Jones, 2018: An Ensemble Version of the E-OBS Temperature and Precipitation Datasets, J. Geophys. Res. Atmos., 123. E-OBS data used for non-commercial education.