London scoping - ukcip

Final Report
16
Table 3.1 The five most severe droughts of different duration in the Thames Region defined
using the Standard Precipitation Index (from Wade et al., 2001)
Rank
Duration
3 months 6 month 12 months 24 months 48 months
1 1938 1976 1976 1992 1891
2 1929 1921 1921 1935 1976
3 1893 1929 1922 1934 1901
4 1976 1938 1934 1997 1992
5 1989 1892 1898 1922 1890
Changes in daily precipitation occurrence and wet-day amounts are also relevant to many
environmental processes. Since the 1900’s there has been no trend in either the frequency of
winter rain days or the average length of wet-spells, or persistence of rainfall in the Thames
Region (Figure 3.5a). The observed inter-annual variability is consistent with large-scale,
atmospheric circulation changes over the North Atlantic (Wilby et al., 1997). Similarly, neither
summer rainfall frequencies nor dry-spell persistence exhibit clear trends since the 1900’s
(Figure 3.5b). However, the 1980’s witnessed above average numbers of summer rain days,
whilst dry-spell persistence has increased slightly since the 1960’s. This suggests that the
downward trend in summer rainfall totals (Figure 3.3) is partly driven by fewer rain days.