What is a climate normal?

Saying "the July normal is 19.4 °C" means nothing without saying which years the average was taken over. A normal is always a pair: a value and a reference period. Two normals computed over different periods are not comparable, and that is the first reading error — comparing a figure from 1961-1990 with one from 1991-2020 measures warming without meaning to.

The World Meteorological Organization recommends thirty-year periods. The refresh rule has itself changed: until 2020 standard normals succeeded one another every thirty years — 1931-1960, then 1961-1990 — and since 2021 they are refreshed every ten years, the current one being 1991-2020. 1961-1990 also remains in use as a reference period for measuring climate change, because it predates most of the warming. Thirty years is a compromise, not a physical truth: long enough for exceptional years to cancel out, short enough that the period still describes the climate people actually live in.

Weather varies enormously from year to year. One July can run three degrees above the next without any climate having changed — that is meteorological noise, not signal. Averaging over a long period removes the noise and leaves what is stable.

The mathematics behind this is simple: uncertainty on an average falls as the square root of the number of years. Going from five years to thirty does not divide the error by six but by a little over two — which is still the difference between a figure you can quote and a figure that mostly describes the five years you happened to get.

This is also why a "five-year normal" is not a normal. It describes recent weather, which is legitimate information — it is simply not the same information.

Climate Memory does not serve a single normal but several sliding windows — 5, 10, 20 and 50 years — plus a "max" window anchored to 1940, which is where the ERA5 archive begins.

That is not the WMO standard period, and the departure is deliberate. A 1991-2020 normal answers "what is the weather usually like here". It cannot answer "how much has this place warmed", because it discards precisely the decades you would need to see it. Both questions come from the same visitors, so the site serves both.

The window shown by default is the longest one, 1940 to the most recent complete calendar year. It is where warming can be told across nearly a century, so it is what search engines should see. The shorter windows remain available; they are simply not what the page leads with.

Every normal on this site is computed over complete calendar years. The current year never enters the calculation until it has finished.

The reason is seasonal: folding in the first eight months of a year pours in a winter and a spring without the autumn that balances them. The annual mean would be skewed by an amount that depends on which month you happen to look. A normal that changes with the day you consult it is not a normal.

A normal is an average, so it flattens the distribution. Two cities with the same annual mean can have very different climates: one steady all year, the other split between scorching summers and hard winters. That is exactly why this site also shows records, frost days, hot days and a month-by-month breakdown — the mean alone is a summary, not a description.

A normal predicts nothing either. It describes what was observed over a past period. In a changing climate it is structurally behind: a 1991-2020 normal describes a world slightly cooler than the one of 2026.