There are uncertainties associated with the observed climate datasets that users should be aware of. There are three significant reasons for this uncertainty:
1. Sampling and instrumental errors
These uncertainties arise from the fact that climatological instruments (mechanical and electronic) are used to sample the climate in real environments. They also arise as a result of changes in instrumentation (type, location and number/density) and from changes in the environment in which the instruments are located.
2. Interpolating point observations to a continuous grid
The observed climate datasets are presented as a uniform gridded dataset, whereby point observations of climate (the nationwide network of climatological and meteorological stations) are irregularly distributed so are used to create a 5 km grid of continuous information (a process called interpolation) for the period of record. While the interpolation process provides advantages for analysis and presentation, it also has some uncertainties when estimating the climate in locations without an observing station.
These uncertainties arise from the uneven spatial coverage (relatively few stations are located in northern and western parts of the UK) and the lack of availability of some desirable information for example limited observations in some micro-climates such as frost-hollows.
The gridded datasets are estimates of the climate for the centre point of a 5 km x 5km grid cell. The method used to generate the monthly and annual gridded datasets are described in more detail on the Met Office website .
3. Interpolating daily or monthly values
Differences have been found to occur in some places and some months between the monthly gridded values and monthly values derived from gridded daily data. These differences are largely due to the fact that the monthly values result from one (monthly) interpolation, whereas the daily values are the result of up to 31 (daily) ones.
A test of selected months showed that differences are mostly random and very small. For precipitation, larger differences were found in some upland regions, though not in lowland areas. Consideration was given to addressing these differences so that the sum of the daily values agreed with the monthly ones. However, it was decided not to do this, because the random nature of the differences in adjacent grid squares may have led to unrealistic daily patterns (especially for rainfall). Instead, users are warned on the Met Office website that these differences may occasionally occur.