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'Clipping' is the name given to the process used in UKCP09 to exclude tails (or outliers) from the sampled data. In practice this means that the projections that are virtually certain not to occur are not included in the data.

In detail

The method developed by the Met Office Hadley Centre to derive Probabilistic climate projection is designed to span the full range of uncertainty. However, there are issues in the tails of the distribution because a number of statistical processing steps are combined and the statistical approximations can have a large influence on the tails of the distribution.

All variables in the sampled data were clipped at the 1st and 99th percentile. A more significant level of clipping was applied to the following variables:

VariableLow percentileHigh percentileLong name
precip variance595Variance of daily precipitation rate
temp variance595Variance of mean air temperature at 1.5 metres
precip skewness595Skewness of daily precipitation rate

Please note that these three variables that have been clipped at the 5th and 95th percentiles are not available through the UI. They are only used as inputs to generate change factors when running the Weather Generator.