A climate normal is the average of a meteorological element (temperature, precipitation, etc.) computed over a 30-year period defined by the WMO. The current reference period is 1991–2020, replacing 1981–2010 in January 2021.
Practical use
When we say "Madrid's temperature is 5 °C above normal", we are comparing with the 1991–2020 average for that day. Normals help identify anomalies, assess whether an event is extraordinary, and communicate a location's climate. However, with climate change, normals shift: the current period already includes some warming, so a "+2 °C anomaly" relative to 1991–2020 would be larger against earlier baselines.