AVERAGING PERIODS IN CLIMATE
The appropriate averaging period to use for climate data has long been an area of discussion, as the dual needs of having sufficient data for a stable average while being reasonably current in a changing climate are taken into account. The issue is complicated by the fact that climate averages effectively serve two purposes - one as an implicit prediction of the most likely conditions to occur in a location in the near future, the other as a benchmark against which current (or past) conditions are referenced.
The World Meteorological Organization currently defines the ‘standard normal’ period as covering the 30 years from 1961 to 1990. While the warming trend globally means that 1961-90 averages for temperature are no longer representative of the most likely values to be experienced in 2007, they still provide a suitable reference benchmark, and are used as the base for the Australian temperature data set (table 1.5).
Averages for shorter, or more recent, periods are useful in monitoring current conditions, or in allowing averages to be calculated at stations which do not have data available for the full 1961-90 period. Table 1.7 uses 1971-2000 averages for this reason.