# A summer at home finds a match by the Aegean.

Bucharest’s summers averaged 24.7°C in 2021–2025. Their monthly temperatures resemble those near Aydın, Turkey, in 1971–1990. Include rain and the match changes. Then look up your Romanian locality.

ERA5 · June–August · historical baseline: 1971–1990

2026 is excluded: the APDRC archive used here ends in July.

## A summer has three months.

An average can hide a cool June and a hot August. Compare the months, then see what happens when you include rain.

## How many degrees fit between two places?

The map starts with a question about home: which past summers resemble the ones we experience now? For each selected place, it searches for the closest monthly temperatures across Europe and the Mediterranean region in 1971–1990. The line connects two places and two periods. Each end describes the same three summer months.

The geographical match depends on the comparison. Two regions can share similar temperatures while receiving different amounts of rain. Switch to “Temperature + rain” and see whether the destination stays the same.

## A city is finer than the grid.

Terrain, distance from the sea and built surfaces shape local conditions. ERA5 describes areas tens of kilometres across. Several villages may share one climate cell, while a cold valley can differ from its cell average. The photograph shows Bucharest from orbit; the comparison uses a data grid.

## What does “similar” mean?

### Temperature

We compare the June, July and August means separately, selecting the point with the smallest root mean square temperature difference. The remaining difference, displayed in °C, shows how imperfect the match is.

### Rain can change the match

The combined comparison includes monthly precipitation. Temperature and rainfall are put on comparable scales before matching. Temperature differences are divided by 3.764°C and rainfall differences by 39.284 mm: the spatial standard deviations of the 3,280 reference cells. The two groups have equal weight. These scales describe variation among reference places, rather than observational uncertainty.

### The periods differ in length

Each decade contains ten summers; 2021–2025 contains five. The 1971–1990 reference contains twenty. These windows describe the selected periods; standard climate normals use thirty years. The recent result is more sensitive to a few unusual years.

### A climate neighbour, with limits

The search samples native 0.25° ERA5 cells at 0.5° intervals within the declared domain. Coastal cells can mix land and sea influences. Even the closest result can be a poor match. Mean temperature and rainfall alone do not describe heatwaves, drought, humidity or the climate of the whole year.

## The data behind the map

Two-metre air temperature and precipitation come from ECMWF/Copernicus ERA5 reanalysis, accessed through the University of Hawaii’s APDRC archive. Reanalysis combines observations with a physical model of the atmosphere.

Generated using Copernicus Climate Change Service information 2026. Data provided by APDRC, University of Hawaiʻi at Mānoa.

- [ERA5](https://cds.climate.copernicus.eu/datasets/reanalysis-era5-single-levels-monthly-means)
- [APDRC](https://apdrc.soest.hawaii.edu/datadoc/ecmwf_ERA5.php)
- [GeoNames](https://www.geonames.org/export/)
- [Natural Earth](https://www.naturalearthdata.com/about/terms-of-use/)

[Download the comparison data](https://mariuscomper.uk/clima-ta-s-a-mutat/assets/climate.json)

## Coverage and the opening comparison

15,421 GeoNames records map to 943 ERA5 cells. The 3,280 candidate land cells cover 34–58° N and 10° W–42° E. This includes Europe and the Mediterranean region. GeoNames is a gazetteer rather than an audited official administrative register. Year 2020 falls outside the displayed decades and the separate 2021–2025 window.

Bucharest’s baseline summer mean is 21.3°C and its 2021–2025 mean is 24.7°C: a difference of +3.4°C. Its thermal match is the cell at 38° N, 28° E, 22 km from Aydın, with a monthly temperature mismatch of 0.11°C RMS. Including rain selects 40.5° N, 23.5° E near Chortiátis.

| Month | Bucharest 2021–2025 | Near Aydın 1971–1990 |
|---|---|---|
| June | 22.9°C / 59.8 mm | 22.7°C / 18.7 mm |
| July | 25.9°C / 40.1 mm | 25.8°C / 5.2 mm |
| August | 25.3°C / 41.6 mm | 25.3°C / 4.3 mm |

Photograph: Bucharest from the ISS, 15 August 2003. NASA Johnson Space Center / Earth Science and Remote Sensing Unit, ISS007-E-12761, public domain. [Source and rights](https://commons.wikimedia.org/wiki/File:ISS007-E-12761_-_View_of_Romania.jpg).