Guide · 1 min
What is a climate normal?
A climate normal is the average of a weather variable at one place over a fixed thirty-year window. "Hotter than normal" always means hotter than some thirty years — and which thirty is a choice.
Same field, two yardsticks
Modelled demonstration series · not observed data
On Not Normal's 53 modelled stations, 20 sit beyond ±2.8σ of the 1951–1980 normal and 9 beyond ±2.8σ of 1991–2020. Against the thirty years from 1990 (1990 – 2019), 9. Computed from the modelled series at build time.
The standard periods
The World Meteorological Organization defines standard climate normals as thirty-year averages, recalculated every decade. The current standard period is 1991–2020. National services publish their own normals on the same window — NOAA's U.S. Climate Normals for 1991–2020 were released in 2021.
The WMO also recommends keeping an older, fixed reference period — 1961–1990 — for tracking long-term climate change, because a baseline that moves every ten years quietly absorbs the warming it is supposed to measure.
Not Normal uses 1951–1980 as its earlier yardstick, the same window NASA's GISS temperature analysis has long used for its anomaly maps.
Why the choice changes the answer
Move the baseline forward and recent warmth becomes part of "normal". A summer that was extraordinary against 1951–1980 can look ordinary against 1991–2020. Nothing about the summer changed; the yardstick did.
A normal is not a forecast
A normal describes the past. It is the centre of a distribution, not a promise about next July — and in a warming climate the most recent normal is already behind the curve by the time it is published.
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