On this day — weather records and history
Which year was the hottest, the coldest and the wettest on this date in your city? 85 years of climate data (1940–today) from the ERA5 reanalysis: records, what is normal for the date and the warming trend.
Frequently asked questions
Where does the historical data since 1940 come from?
From the ERA5 climate reanalysis of the European Centre for Medium-Range Weather Forecasts (ECMWF), served through the Open-Meteo archive API. A reanalysis reconstructs the atmosphere of every single day by combining all available observations (weather stations, soundings, satellites) with a modern weather model, on a grid of about 25 km. These are not official records from a specific station, but they are consistent, continuous and comparable from 1940 up to yesterday.
Why don't the records match the official station records exactly?
Because ERA5 represents the average of a grid cell of about 25 x 25 km, not the thermometer of a specific station. The point extremes of an official station (an urban observatory, a valley floor) are usually somewhat sharper than those of the reanalysis. The record years and the trends do match very well; the exact values may differ by a degree or two.
How is the normal for a date calculated?
We take that same day and month across all years since 1940 and average the maximum and minimum temperature. The rain probability is the percentage of those years in which more than 1 mm of precipitation accumulated. It is a fixed-date climatology, useful for knowing whether today is running warmer or colder than usual.
What does the per-decade trend in the chart mean?
It is the slope of a linear regression fitted to the maximum temperatures of that date across the 85 years of the series, expressed in degrees per decade. A value of +0.3 °C/decade indicates that this specific date has been warming at that average rate. It is a local, single-date indicator: the full climate change signal is assessed with annual and regional series.
Weather anniversaries: 85 years of weather history in your city
A weather anniversary answers a simple question: what happened on this day in previous years? This tool goes through every occurrence of the chosen date since 1940 — the same day and month of each year — and extracts the three milestones that define the climate memory of a date: the hottest, the coldest and the wettest year, together with the normal values and the underlying trend.
The source: ERA5 reanalysis, not station records
Let's be honest about where the data comes from: we do not query the official archives of a weather station here, but the ERA5 reanalysis of the European Centre for Medium-Range Weather Forecasts (ECMWF), distributed by Open-Meteo. A reanalysis is a physically consistent reconstruction of the atmosphere: a modern weather model assimilates all available historical observations (surface stations, weather balloons, ships, and from the 1970s onwards also satellites), producing a value for every point of a roughly 25 km grid and for every hour since 1940.
The advantage is enormous: a continuous, homogeneous, gap-free series for any point on the planet, ideal for comparing years and detecting trends. The limitation must be told too: since it represents the average of a 25 x 25 km cell, point extremes are somewhat smoothed compared with the thermometer of a specific observatory. If you are after your city's official record, the reference is your national weather service; if you want to understand how the climate of your area has changed day by day over 85 years, the reanalysis is the best tool available.
What to look for in the chart
The year-by-year chart of daily highs usually tells two stories. The first is natural variability: the same date can span years 10 degrees apart, because the weather of a specific day depends on the synoptic situation that came up that year. The second is the trend: the regression line almost always points upwards, and its slope per decade quantifies the local warming of that date. You will also notice that record-hot years increasingly cluster in the last two decades, while cold records usually sit far away, in the first half of the series.
How to use the comparison with today
If you look up today's date, we compare the current forecast with the climatology of the date: how many degrees above or below normal the daily high is expected to be and how far it is from the all-time record. It is the quickest way to answer the eternal question of whether "this heat isn't normal": sometimes it isn't, and sometimes it turns out that on this very day, back in 1950, it was even hotter.