What is the difference between average and normal?
Document Actions
The arithmetic average of a meteorological element over 30 years is defined as the climatological normal. So, the key difference between the two is that a normal is strictly for 30 years, whereas an average can be computed over any time span. Official normals are calculated by the National Climatic Data Center for temperature and precipitation elements and are updated through the end of each decade ending in zero (for example, 1971-2000). These official normals can be slightly different from a strict average because they also take into account changes in the station over the thirty-year period. These changes can include changes in the instrumentation, the location, the observing practices, the observation time, or in the surrounding environment.
Source: Midwestern Regional Climate Center
http://mcc.sws.uiuc.edu