A temperature anomaly is the departure, positive or negative, of a temperature from a base temperature that is normally chosen as an average of temperatures over a certain reference period, often called a base period. Commonly, the average temperature is calculated over a period of at least 30 years over a homogeneous geographic region, or globally over the entire planet. Temperatures are obtained from surface and offshore weather stations or inferred from meteorological satellite data. Anomalies can be calculated based on datasets of surface and upper-air atmospheric temperatures or sea surface temperatures.
