-Consider the period of overlap between the records at the two sites (or a period of overlap with a nearby third site if there is little or no overlap between the two sites).
-Calculate the 5, 10, 15, ...., 95 percentile temperatures at each of the two sites for each month of the year during the period of overlap.
-Use these percentile points as a set of 'matching points' for equating temperatures at the old site to temperatures at the new site. Linear interpolation between the data points is used for temperatures falling between the 5 and 95 percentiles; for temperatures falling outside this range, it is assumed that the temperature difference between the sites is equal to the difference between the 5 or 95 percentile (as appropriate) points at the two sites.
The method is described more fully in a forthcoming article in the International Journal of Climatology (B.C.Trewin and A.C.F.Trevitt, The Development Of Composite Temperature Records).
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