Årsnormalen

Årsnormalen is a climatological concept used in meteorology to describe the long-term, typical weather for a location. It represents the average value of a weather variable over a reference period of 30 consecutive years. By averaging daily or monthly observations across this 30-year window, a normal value is established for each day or month, such as the normal temperature for January 15 or the normal precipitation for March.

Purpose and use are to provide a stable baseline against which current weather can be compared. The difference between observed values and the normal value is called an anomaly, which helps communicate how unusual current conditions are relative to the long-term climate.

Reference periods are periodically updated to reflect climate change and changing conditions. The World Meteorological Organization recommends 30-year normals, and many national services adopt a recent window such as 1991–2020 as the current normal period (replacing earlier periods like 1961–1990). Normals can be established for various variables, including temperature, precipitation, sunshine duration, and humidity, and they are applicable at national, regional, or local levels.

Calculation typically involves the arithmetic mean of the 30-year data for each specific day, month, or variable. Some reports also present variability measures, such as standard deviations, to indicate typical spread around the normal.

Limitations include that normals are not predictions; they describe past conditions and may gradually shift as the climate changes. Therefore, anomalies relative to the current normal are often more informative for assessing recent weather than the raw values alone.

Årsnormalen is used in weather forecasting, climate reporting, agriculture, infrastructure planning, and media communication to describe what constitutes typical conditions.