WEATHER · FIELD GUIDE
What Is a Climate Normal? Average Weather, Explained (1991–2020)
Everyone talks about the 'average' temperature for a place — but average of what, over how long? What a climate normal really is, why it's exactly 30 years, and why it quietly changes every decade.
Ask what the temperature "usually" is in a city in July and you'll get a confident number. But average of what, exactly — and over how many years? The answer is more precise than most people expect, and it's the same standard used by weather services worldwide. It's called a climate normal, and this guide is the plain-language version: what it measures, why it's exactly 30 years, why it quietly shifts every decade, and how our city climate pages turn it into a simple month-by-month table.
A normal is a 30-year average
A climate normal is the average of a weather figure — mean temperature, total rainfall, and so on — over a fixed 30-year reference period. The current international standard, set by the World Meteorological Organization (WMO), is 1991–2020. So when a page here says London averages about 17.5°C in July, that's the mean of every July across those thirty years, not a reading from one warm afternoon.
Thirty years is a deliberate choice. Weather is noisy: one scorching summer or one washout August tells you almost nothing about a place's climate. Averaging three decades smooths that noise into a stable picture — yet it's still short enough to describe the climate now, rather than a century ago.
Why it changes every decade
Here's the part that surprises people: the normal isn't fixed forever. The reference period rolls forward every ten years. It used to be 1961–1990, then 1981–2010, and it's currently 1991–2020; around 2031 it will become 2001–2030.
That's why a place can feel like it keeps "beating its average." As the climate has warmed, each new 30-year window drops some cooler early years and adds warmer recent ones, nudging the temperature normals upward. The bar quietly moves.
A normal is also, importantly, not a forecast. It's a rear-view mirror. Knowing Singapore sits near 27°C every month tells you what to pack, not what Tuesday will be. Any single day, month or year can and will depart from the normal — the normal is just the baseline those departures are measured from. (The live "yesterday" line on each city page makes that concrete: it shows how the most recent day actually compared with the month's norm.)
Where these numbers come from: ERA5
Our normals aren't read off a single thermometer. They come from ERA5, a reanalysis produced by the Copernicus Climate Change Service and ECMWF. A reanalysis takes decades of real observations — surface stations, ships, weather balloons, aircraft and satellites — and blends them with a physics-based weather model to reconstruct past conditions on a complete, even grid, with no gaps. It's one of the most widely used climate datasets in the world.
The trade-off is worth stating plainly, and every city page says it: each value is the average for a grid cell (about 25 km across), not a specific weather station. For most inland cities that's barely noticeable. For a strongly coastal city like Cape Town or a desert one like Cairo, the nearest cell can read a touch cooler, warmer, wetter or drier than the city centre. What a reanalysis captures best is the shape of the year — when it's hot, when it's cold, when the rain comes.
Normal vs record vs forecast
Three words get muddled, so it's worth separating them:
- A normal is the long-run average (what a place is usually like).
- A record is the single most extreme value ever measured (the hottest day, the wettest month).
- A forecast is a prediction for a specific upcoming day.
Our climate pages are firmly in the first category. They show the average monthly temperature and rainfall — because that's what a climate normal is, and it's what answers the questions people actually ask: when's the warm season, when's the wet season, how big is the swing between summer and winter. Record highs and lows live in the day-by-day record, a separate and much larger dataset.
See it for yourself
Every one of our city climate pages is the same idea made concrete: a 12-month table of average temperature and rainfall, the warmest, coldest and wettest months at a glance, and yesterday's reading set against the month's norm. Compare a monsoon city, a Mediterranean one and an equatorial one and the different shapes of climate jump straight out.
For what's happening right now rather than on average, the live weather map shows conditions across the planet in real time — the moment-to-moment weather that, averaged over thirty years, becomes the normals here.
Frequently asked questions
What is a climate normal?
A climate normal is the average of a weather figure — temperature, rainfall, and so on — over a fixed 30-year reference period. It describes what a place is typically like at a given time of year, rather than forecasting any particular day. The current international standard period is 1991–2020, set by the World Meteorological Organization.
Why is the period exactly 30 years?
Thirty years is long enough to average out the noise of individual hot summers, cold winters and wet or dry years, but short enough to still describe the present-day climate rather than the distant past. The World Meteorological Organization adopted 30 years as the standard early in the 20th century, and it has stuck because it strikes that balance well.
Why does the normal change every decade?
The reference period rolls forward every ten years so the 'normal' keeps describing the recent climate. It was 1961–1990, then 1981–2010, and it is now 1991–2020; around 2031 it will become 2001–2030. Because the climate has warmed, each new period tends to shift the temperature normals upward, which is one reason a place can feel like it keeps beating its 'average.'
Is a climate normal a forecast?
No. A normal is a backward-looking average, not a prediction. It tells you July in a city has averaged, say, 25°C over 30 years — not that next July will be 25°C. Any single day, month or year routinely departs from the normal; the normal is simply the long-run baseline those departures are measured against.
What is a reanalysis, and why not just use a weather station?
A reanalysis takes decades of real observations — from stations, ships, balloons, aircraft and satellites — and blends them with a physics-based weather model to produce a complete, even grid of past conditions, with no gaps. ERA5, the reanalysis behind these pages, is one of the most widely used. The trade-off is that each value is the average for a grid cell rather than a single station, so a coastal or mountain city can read slightly different from its exact centre.
Why do these pages show average temperature and rainfall, but not record highs and lows?
They are built from ERA5 monthly means, which carry the average conditions for each month. Record daily highs and lows would need the full day-by-day record, which is a separate and much larger dataset. Average temperature and rainfall are the core of a climate normal and answer the questions most people are actually asking — when it's hot, when it's cold, when it rains.
SEE IT LIVE
Everything in this guide is on the live weather map — open it and watch the real thing.