In Earth Science you learned that climate is the pattern of weather over 30 years or more. Meteorologists make that precise with climate normals, and they describe any day, month or year by how far it is from normal, using signed numbers.
Key idea
A climate normal is a 30-year average of a weather element, for a date, month, season or year. NOAA's current U.S. normals cover 1991–2020 and are updated every ten years. An anomaly is a value minus its normal: $$\text{anomaly} = \text{value} - \text{normal}$$ Positive means above normal; negative, below.
Why it works
One day's weather swings far more than climate does, so you need a long average to see what is typical: 30 years is long enough to smooth out odd years, short enough to describe the climate of today. Anomalies let you compare unlike places and seasons fairly. A 25 °C day is cool for Phoenix in July and hot for Seattle in March; their anomalies show it at once. Global temperature records are kept as anomalies for a deeper reason: the anomaly at a station far from the next one still tells the same regional story, even when their actual temperatures differ because of height or coastline.
Worked example: A hot July day
The normal high for July 15 in a city is 88 °F. This July 15 the high was 95 °F. What was the anomaly?
- Anomaly = value − normal = $95 - 88 = +7$ °F.
Answer: +7 °F
Worked example: A cold month
A town's January normal is −3.4 °C, and this January averaged −6.1 °C. What was the anomaly?
- Anomaly = $-6.1 - (-3.4) = -6.1 + 3.4 = -2.7$ °C.
- January was 2.7 °C colder than normal.
Answer: −2.7 °C
Common mistake
Reading an anomaly map as a map of actual temperatures. A blue (negative) area in Arizona in July can still be over 40 °C, just a little cooler than its own normal; a red area in Antarctica is still far below freezing. Check the legend: an anomaly map is labeled in degrees above or below normal.
Changing the baseline period moves the zero line, not the trend. NASA reports global temperatures relative to 1951–1980; NOAA uses the 20th century; the IPCC uses 1850–1900. The same year gets different anomaly values, but the warming from decade to decade is the same in all three.