What is the difference between averages and normals?

A normal of temperature, precipitation or any other weather element is defined as the arithmetic average of the observed values of that element. A normal is strictly for 30 years, whereas an average can be computed over any time span. There is an international agreement that normals should be from a 30-year period from 1961 to 1990.

To calculate the normal daytime maximum temperature for Pretoria on say April 15, we would add together all the daily maximum temperatures on this day over the 30-year period from 1961 to 1990, divide the total by 30, to get the normal. Normal values are supposed to be representative, not necessarily what should occur.


    What are period averages?

Period averages are averages of climatological data computed for any period of at least ten years starting on 1 January of a year ending with digit 1.


    What are climatological standard normals?

Climatological standard normals are averages of climatological data computed for the following consecutive periods of 30 years:
1 January 1931 to 31 December 1960
1 January 1961 to 31 December 1990