The following questions are considered to be those most likely to be asked. Others are welcome and may be sent to the Customer Centre.
An indication of the quality and accuracy of the grid values is given by the root-mean-square Errors (RMSE) at verification stations. For the daily grids, the RMSE values averaged over each day of a test year are as follows:
|Climate variable||Verification RMSE|
|Rainfall amount||1.23 mm|
|Mean temperature||0.94 °C|
|Maximum temperature||1.06 °C|
|Minimum temperature||1.27 °C|
For the monthly grids, the average RMSE values for 12 test months are as follows:
|Climate variable||Verification RMSE|
|Rainfall amount||16 mm|
|Mean temperature||0.36 °C|
|Maximum temperature||0.66 °C|
|Minimum temperature||0.45 °C|
RMSE values for monthly grids of other climate variables and for annual grids are given in the paper 'Generation of monthly gridded data sets for a range of climatic variables over the UK' (PDF, 590 kb).
All the monthly and annual values are given with a precision of two decimal places and the daily values are given with a precision of one decimal place. This is to facilitate onward calculation and is not a reflection of the accuracy of the values.
The station observations used to produce the grids have undergone a quality control process which corrects or removes erroneous data. In addition, some stations were excluded (e.g. due to a localised microclimate). No attempt was made to homogenise the input data. All the daily grids were checked to ensure that no erroneous station data had been used, as described in 'The generation of daily gridded data sets of temperature and rainfall for the UK' (PDF, 2.37 MB).
The number of stations used as input to the gridding varies with time, partly due to changes in the size of the observing network and partly due to the availability of digitised data. A typical starting year for daily and monthly data is 1961, although some monthly data have been digitised further back (see Q4). During the period 1961 to 2000, the average number of stations used is shown in the following table.
|Climate variable||Average no. of stations|
|Pressure, cloud, wind speed||70|
The variation in the numbers of rainfall and temperature stations used from 1960 to 2006 is shown in the following graph.
To create gridded data sets, observations from an evenly distributed network of stations are needed in digital format. For many climate variables, sufficient data only exist in the Met Office electronic databases from 1961. However, as a result of an ongoing programme to digitise published station records, the monthly sunshine series extend back to 1929 and the monthly air temperature and rainfall series extend back to 1914.
The policy for adding more gridded data, e.g. for years after 2006, is under discussion with stakeholders. In the mean time, ad hoc updating is expected. The Met Office Customer Centre is able to advise about the provision of grids for recent years and any associated costs.
The monthly grids were prepared using monthly station data as input and then, later, daily grids were produced using daily station data as input. Hence these were separate interpolation exercises. Comparisons between the monthly grids and the average/sum of the daily grids were carried out, confirming that the largest differences tend to occur over upland areas. However, no adjustments have been made to make the two types of grid equivalent.
Degree days are defined as the mean number of degrees by which the air temperature has gone above, or below, a threshold, calculated day by day and summed over a period of days. Days when the temperature has not gone above (or below) the threshold at any point in the day do not contribute to the total.
Grids of degree day values are available for three thresholds:
The formulae used for calculating degree days above a threshold are as follows (equivalent formulae are used for degree days below a threshold).
|Temperature||Day value (above threshold)|
|Tmax ≤ Tthreshold||0|
|Tmin ≥ Tthreshold||Tmean − Tthreshold|
|Tmean ≥ Tthreshold & Tmin < Tthreshold||0.5 (Tmax − Tthreshold) − 0.25 (Tthreshold − Tmin)|
|Tmean < Tthreshold & Tmax > Tthreshold||0.25 (Tmax − Tthreshold)|
The daily mean temperature Tmean is calculated from the daily maximum temperature Tmax and the daily minimum temperature Tmin as 0.5 (Tmax + Tmin)
Conventionally, maximum and minimum temperatures are recorded for 24-hour periods ending at 0900 GMT each day. Maximum temperatures tend to occur during mid-afternoon, so the relevant maximum for a given calendar day is the one recorded between 0900 on the day in question and 0900 on the following day. However, minimum temperatures generally occur around dawn, so the relevant minimum temperature for a given calendar day is the one recorded between 0900 on the previous day and 0900 on the day in question.
The majority of the stations in the Met Office network that record sunshine duration use a Campbell-Stokes recorder. However, a significant proportion of the stations that report in real time now use a Kipp & Zonen sunshine sensor (the first such sensors were installed in 2000). The characteristics of the two instruments are not identical and therefore some adjustment of the observations to a common standard is required. Currently, Kipp & Zonen monthly totals are adjusted to Campbell-Stokes equivalent values prior to generating the sunshine grids. The correction methodology is described in a paper by A. Kerr and R. Tabony: 'Comparison of sunshine recorded by Campbell-Stokes and automatic sensors' (Weather, April 2004, vol. 59,90-95), but additional overlapped data was used by M. Perry in 2007 to produce improved monthly adjustment factors (study to be published).
Hourly or three-hourly station observations of total cloud amount in oktas are converted to percentages (1 okta = one eighth of the sky = 12.5%). The average cloud amount, in per cent, is calculated for each month from these observations prior to interpolation to the 5 x 5 km grid.
The standard exposure for wind sensors is at 10 m above ground in an open, level area (such as an airfield). Hourly observations of mean wind speed are used to calculate monthly means which are then interpolated to the 5 x 5 km grid, taking into account altitude. The resulting grid therefore reflects general topography (e.g. hills, and coasts) but not the sheltering effect of urban or forested areas. The speeds are in knots, the internationally agreed meteorological unit (1 knot = 0.514 m/sec = 1.15 m.p.h.).
Where the same highest total has occurred more than once in a season or year, the date given is that of the first occurrence.
Each season is comprised of three calendar months, as follows.
For the daily precipitation data, the winter data sets are named according to the year in which January falls, e.g. Dec 1960 to Feb 1961 is 'Winter 1961'. Seasonal data sets have been produced for 46 years, from Winter 1961 through to Autumn 2006, inclusive.
The original station observations of these quantities are all integer values. However the interpolation process generates non-integer values between stations and these are presented without rounding. Note that for relatively rare events (e.g. snow), where some stations will have reported a zero value, the interpolation process will generate fractional values less than 1 at some grid points.
The 30-year period 1961 to 1990 has been designated as the international standard reference period for climate averages by the World Meteorological Organization. Averages for the period 1971 to 2000 have also been produced for the UK, but the earlier period has been chosen for the UKCIP grids as it represents a better baseline for placing recent climate change into context.
The advantages of gridded data include:
We are aware that some users are getting error messages when trying to unzip the data files. The following are some suggestions for how to get around this problem.
The 5 km grids cover a rectangular area that encompasses the whole of the British Isles and surrounding sea areas. However, climate data are only given for grid points on the land. Points in the sea, such as those in the rows and columns in the top left corner of the grid, have the value -9999. Try scrolling down and to the right and you will find some data values over the land.
When deciding which data are relevant to your chosen location, please bear in mind that the 5km values are estimates for the centre point of a 5 x 5 km grid cell, whereas the 25 km values are estimates of the average over a 25 x 25 km grid box.
The Met Office is not responsible for the content of external internet sites.