Analysis of rainfall records for all of the UK from 1990 to 2010 shows an upward trend in the rain rate exceeded for 0.01% time, a parameter used for fixed-link assignments. The rate of change tends to increase from north to south.
When Ofcom's current rain map was brought into use in 2001 it tended to over-estimate the 0.01% rate compared to the 20-year trends found in this study. Since then the degree of over-estimation has decreased due to rising 0.01% rates. By 2015 the Ofcom map is expected still to over-estimate the 0.01% rate over most of the UK, although by then there will be smaller but increasing areas where it under-estimates.
A new UK rain map has been prepared of the rain rate expected to be exceeded for 0.01% time in the year 2015, based on the trends found in the analysis.
The 0.01% rate is increasing sufficiently rapidly, approaching 0.8 mm/h per year in some places, that it would be advisable to update the analysis of rainfall data on a regular basis. This would be facilitated by standardising the format in which the required data can be downloaded from the agencies involved in making the measurements.