ERA5 reanalysis explained: where our weather history comes from
Every past-weather number on WeatherArchive comes from ERA5, produced by the European Centre for Medium-Range Weather Forecasts (ECMWF) for the Copernicus Climate Change Service, and served by Open-Meteo. Here is what that means, and when to trust it.
What a reanalysis is
A reanalysis re-runs a modern weather-forecast model over the past, hour by hour, and at every step nudges it towards whatever was actually observed: weather stations, radiosonde balloons, ships, buoys, aircraft and, since the 1970s, satellites. The result is a complete, physically consistent picture of the atmosphere on a regular grid, with no gaps where there were no stations.
What it is good at
Temperature and humidity over flat or gently rolling land, pressure, wind at larger scales, and long consistent series for comparing years or places. That makes it ideal for climate averages, frost dates, degree days and “how did this week compare with normal” questions.
Where it falls short
Small-scale rain. A thunderstorm a few kilometres wide is smeared across a grid cell, so daily totals can be too low at the storm and too high nearby. Mountains and coasts. A grid cell averages valley and peak, land and sea; expect differences of a few degrees. Extremes. Record station highs and lows are usually more extreme than the grid-cell value. Early decades. Before satellites (roughly pre-1979), the model had far fewer observations, especially in the southern hemisphere and over oceans.
How WeatherArchive uses it
We query the Open-Meteo Historical Weather API, which combines ERA5, ERA5-Land and, for the most recent days, ECMWF’s operational model, and delivers it in the place’s local time. Requests go straight from your browser to Open-Meteo when you press a button; we don’t store your queries. Recent results are cached in your browser so repeat views are instant. Sun times are computed locally with NOAA’s solar equations, and nothing about them depends on weather data.
We never fill gaps: if the source has no value for a variable, the page says “no data”.
Credit and licence
Weather data by Open-Meteo.com, licensed under CC BY 4.0. Contains modified Copernicus Climate Change Service information; neither the European Commission nor ECMWF is responsible for any use of it. Hersbach, H. et al. (2020), “The ERA5 global reanalysis”, Quarterly Journal of the Royal Meteorological Society 146: 1999–2049. Geocoding by Open-Meteo using GeoNames data (CC BY 4.0).