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I frequently see references to the "standard reference period" for weather data and it seems generally to be quoted (eg by Philip Eden) as 1971-2000. On looking for a source, I found however that, "The 30-year period 1961 to 1990 has been designated as the international standard reference period for climate averages by the World Meteorological Organization. Averages for the period 1971 to 2000 have also been produced for the UK, but the earlier period has been chosen for the UKCIP grids as it represents a better baseline for placing recent climate change into context." http://www.metoffice.gov.uk/climatechange/science/monitoring/ukcp09/faq. html#faq The WMO site states "Under the Technical Regulations (WMO No. 49), climatological standard normals are averages of climatological data computed for the following consecutive periods of 30 years: 1 January 1901 to 31 December 1930, 1 January 1931 to 31 December 1960, and so forth. The most recent period for climatological standard normals is from 1961 to 1990, and the next period for the calculation of climatological standard normals will be 1991 to 2020." http://www.wmo.int/pages/prog/gcos/documents/gruanmanuals/WCP_CCl/guide_ third_edition_draft_may2007.pdf What statistical fiddle is being perpetrated by having the UK reference period roll forward a decade? Allan *********************************************************************************** This email and any attachments are intended for the named recipient only. Its unauthorised use, distribution, disclosure, storage or copying is not permitted. If you have received it in error, please destroy all copies and notify the sender. In messages of a non-business nature, the views and opinions expressed are the author's own and do not necessarily reflect those of the organisation from which it is sent. All emails may be subject to monitoring. ***********************************************************************************