=== versions === python 3.12.0 | xarray 2026.9.0 | zarr 3.4.1 | rioxarray 0.23.0 | rasterio 1.5.2 | geopandas 1.2.0 | numpy 2.5.4 | pandas 3.0.6 === Step 1: open ARCO-ERA5 === opened in 1.9 s: 273 variables, dims {'time': 1323648, 'latitude': 721, 'longitude': 1440, 'level': 37} valid ERA5 data: 1940-01-01 to 2026-06-30 (ERA5T, preliminary, to 2026-10-15) mx2t: Maximum temperature at 2 metres since previous post-processing [K], chunks (1, 721, 1440), 5.5 TB if loaded === Step 2: July 2022 daily maximum temperature from ARCO-ERA5 === downloaded 744 hourly UK grids (744, 46, 44) in 51 s daily Tmax grid: {'time': 31, 'latitude': 46, 'longitude': 44}; -8.75..2.0 E, 49.75..61.0 N hottest ERA5 cell on 19 Jul 2022: 39.0 °C at 53.00 N, -0.75 E === Step 3: 1991-2020 July climatology from the Planetary Computer === era5-pds-1991-07-fc: chunks {'time': 372, 'lat': 150, 'lon': 150} downloaded 30 Julys in 72 s climatology: 930 July days, 1991-2020 ARCO vs Planetary Computer, 15 Jul 2020 daily Tmax: max |diff| 0.032 °C === Step 4: clip to the UK polygon === grid transform: (0.25, 0.0, -8.875, 0.0, -0.25, 61.125) cells with centre inside the UK: 535; touching the UK: 762; land-sea mask > 0.5 in box: 650 of the centre-inside cells, 503 are also land (lsm > 0.5) === Step 5: area-weighted UK means === cos(lat) weight: 0.485 at 61.00 N, 0.646 at 49.75 N cell area: 499 km² in the south, 375 km² in the north; UK cells total 241,786 km² UK mean July daily Tmax 1991-2020: 18.07 °C (unweighted 17.97) UK mean July daily Tmax 2022: 19.61 °C, anomaly +1.54 °C 19 Jul 2022: UK-mean Tmax 29.0 °C, 1991-2020 mean for the day 19.0, highest of the 30 years 26.8 18 Jul 2022: UK-mean Tmax 28.3 °C, 1991-2020 mean for the day 18.5, highest of the 30 years 25.8 17 Jul 2022: UK-mean Tmax 24.4 °C, 1991-2020 mean for the day 18.2, highest of the 30 years 24.1 warmest UK-mean July day in 1991-2020: 26.8 °C on 19 Jul 2006 19 Jul 2022 anomaly: mean +11.0 °C, max +18.1 °C; 61% of UK area above its 1991-2020 July record === Step 6: export to Cloud Optimised GeoTIFF === era5_uk_tmax_anomaly_2022-07.tif: GTiff, 3 bands float32, 44 x 46, EPSG:4326, res (0.25, 0.25), layout COG, 14 KB bounds (-8.875, 49.625, 2.125, 61.125); band 2 at Heathrow: 14.1 °C band descriptions: ('July 2022 mean Tmax anomaly', '19 July 2022 Tmax anomaly', '19 July 2022 Tmax minus 1991-2020 July record') === Step 7: compare with Met Office historic stations === 37 stations, 3244 July means; 31 stations reported July 2022 6 of 37 stations are nearer a sea or coastal cell than the centre of a land cell July 2022 mean Tmax, ERA5 vs station nearest cell: n=31, bias -2.38 °C, MAE 2.38 °C, r=0.93 + 6.5 °C/km lapse-rate correction: n=31, bias -2.23 °C, MAE 2.23 °C, r=0.94 nearest land cell + lapse rate: n=31, bias -1.58 °C, MAE 1.70 °C, r=0.94 1991-2020 July mean Tmax, nearest land cell + lapse rate: n=31, bias -1.40 °C, MAE 1.46 °C, r=0.93 alt era_z lsm obs_2022 era5_2022 era5_2022_land diff_2022 station Hurn 10.0 23.0 0.6 24.7 20.9 21.0 -3.7 Whitby Coastguard 60.0 53.0 0.3 22.8 17.1 19.5 -3.3 Leuchars 10.0 56.0 0.7 22.0 18.4 18.7 -3.3 Aberporth 133.0 53.0 0.3 19.8 18.6 19.9 0.1 Lerwick 82.0 16.0 0.4 14.2 12.3 14.7 0.5 Camborne 87.0 33.0 0.4 20.8 20.8 22.0 1.2 Heathrow July means 1991-2020: r=0.99 between station and ERA5 year to year Heathrow 19 Jul 2022: ERA5 cell 35.2 °C (the station recorded 40.2 °C that day) === Figures === figures written: 01-era5-tmax-19-july-2022.png, 02-clip-mask-and-weights.png, 03-anomaly-maps.png, 04-uk-daily-tmax-july-2022.png, 05-station-comparison.png