Tried the search function couln't find earlier thread where this has been discussed though obviously it has probably been done to death ;)
It's all down to the WMO apparently, according to this article in Norwegian at http://www.yr.no/nyheter/1.7378341
Here is a translation, of course it still rasies many questions
When meteorologists talk about normal weather, it is to the average weather in the period 1961-1990. This seems odd to yr.no reader Terje Kristiansen. He asks:
- Why is the average temperature based on what I would call historical data (1961 - 1990) and not the actual average past 30 years?
A normal is an expression of the average value over a specific 30-year period. It is internationally determined that the normal period for which we use today is 1961-1990. This average alone is used as references to look at changes in temperature and precipitation. To say something about the changes from year to year it requires that the reference value will be the same.
It is the World Meteorological Organization which determines the normal periods. The previous normal period was 1931-1960. If we continue to follow this system in 2021, we will adopt a new normal, namely, the average values for the period 1991-2020.
Since the normal period is determined internationally as agreed by most meteorological institutes around the world they use the same normal period. It makes it easier to compare precipitation and temperature changes across borders.
Although the period 1961-1990 has been selected as the default period, there is nothing to prevent calculation of the average values for other periods. For example, the seNorge [ data services division of Norway Meteorological Institute ] also provide normal values for the period 1971-2000.