Guide · 1 min
What a sigma anomaly means
Sigma (σ) measures how far a reading is from normal in units of that place's own year-to-year variability. It turns "4 °C above average" into "how surprising is that, here?"
The arithmetic
σ = (reading − baseline mean) ÷ baseline standard deviation
A place whose summers barely vary has a small standard deviation, so a modest departure is a large σ. A place with wildly variable summers needs a much bigger departure to register.
What the numbers mean
| Anomaly | Years inside it | Beyond it, one side |
|---|---|---|
| ±1σ | about 68% | 1 year in 6 |
| ±2σ | about 95% | 1 year in 44 |
| ±3σ | about 99.7% | 1 year in 741 |
| ±4σ | about 99.994% | 1 year in 32,000 |
These assume values follow a normal (bell-curve) distribution around the baseline. Rainfall and river flow are skewed, which is why they are usually discussed as percentiles instead.
Why not just use degrees?
Because the same number of degrees means different things in different places. +3 °C in a tropical city with steady temperatures can be a once-in-centuries event; +3 °C in a continental interior can be an ordinary week. σ puts every place on one scale — which is what lets Not Normal draw one surface across the whole planet.
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