In climate data, what does the term anomaly mean?

• A. A value that deviates significantly from the baseline for that period.
• B. A value that is within one standard deviation of the mean.
• C. A value that represents the maximum value in the series.
• D. A value that is the average of the series.

Answer: (A)

An anomaly in climate data is the difference between the observed value and a baseline value for that period. This baseline, or climatology, is a reference average for the same period (like a specific month or season) over a long time, often about 30 years. For example, if January’s observed temperature is 2°C warmer than the January baseline, the anomaly is +2°C. Positive anomalies mean conditions are warmer (or wetter, depending on the variable) than normal, while negative anomalies mean cooler or drier than normal. This framing helps compare conditions across years and places by removing the regular seasonal cycle, rather than implying anything about being within a statistical range like one standard deviation, or about the maximum or the average value.