PRECIPITATION OVER SCOTLAND, 1757-1992 - SOME ASPECTS OF TEMPORAL VARIABILITY

A 236-year time series (1757-1992) of monthly areal average precipitation for Scotland is produced using the 'official' record complied by the Meterological Office (UKMO) from 1869 onwards, corrected for known inhomogeneities, and extended back to 1757 using previously unpublished data from the UKMO. The complete series is evaluated and examined for fluctuations and extreme values with particular reference to observed precipitation increases over the last two decades. Recent increases in annual precipitation, and associated seasonal variations, are judged to be unusual despite the recognition of a longer term trend to higher annual and winter values. Correlations of precipitation since 1869 with Lamb weather types show differences within the record and from other work, which suggest that this index does not fully capture the synoptic conditions relevant to precipitation over Scotland as a whole. The series since 1869 was also used to test the predictive value of climatic normals for 3, 5, 10, 15, 20, and 25 years alongside the standard 30-year averages of precipitation. It is suggested that the best estimate of seasonal and annual precipitation totals for the following year can be achieved by the application of 3- and 5-year normals, rather than by using precipitation averages of longer duration.