Climate normal period
Models & data Climate normals Reference period
A climate normal period is a defined span of 30 years. Its averages provide a common reference for describing weather and climate.
What does “normal for the season” mean?
One summer can be hot, wet or cool. To describe typical conditions, meteorologists combine many years. WMO uses 30-year reference periods, such as 1961–1990 and 1991–2020. Current standard normals are updated every ten years to a more recent 30-year span.
A number needs its station and period
A climate normal describes a measurement series at a particular location. A meaningful comparison needs matching reference periods, variables and station environments. An airport and a densely built neighbourhood can experience different conditions.
An average is not a timetable
The number of hot days and the amount of precipitation can vary greatly between years. A long-term average describes the reference period; it does not set a quota for an individual year.
Comparisons need the same reference
“Two degrees warmer than normal” only becomes clear when the place, period and baseline are stated. Different climate normals can produce different reported anomalies. An average also does not describe the spread of values or individual extremes.
In Hanover
Hannover as a concrete example
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| Quantity | Annual value |
|---|---|
| Mean air temperature | 10.0 °C |
| Precipitation | 626.7 mm |
| Sunshine duration | 1,597 h |
| Frost days: minimum below 0 °C | 64.6 days |
| Ice days: maximum below 0 °C | 14.5 days |
| Summer days: maximum at least 25 °C | 36.0 days |
| Hot days: maximum at least 30 °C | 7.7 days |
The figures come from the DWD station Hannover near the airport. They are not measurements covering every city district. DWD has homogenised the temperature series, checking and adjusting for influences such as station changes.