As we have seen over recent weeks and months, observations for the UK are essential to put recent weather into context and to detect variations and possible long-term trends in UK climate. So, when the Met Office quotes “the wettest on record” what does that mean?
All our time-series of rainfall come from observations made by rain gauges and their length is determined by how long the recording stations have been open.
Stations with long records are a very important part of the UK’s weather station network. These time series provide an accurate picture of rainfall for that particular location, provided there are no significant changes in instrument type or station exposure. One of the longest in the UK is the weather station at Oxford Radcliffe Observatory, which holds nearly 250 years of rainfall observations from 1767 to the present day and is maintained by Oxford University School of Geography and the Environment.
This will give an accurate picture of rainfall for the city and will be broadly representative of the year-to-year variations of rainfall across Oxfordshire or even central England, but it won’t be useful when looking more widely across England and Wales or the UK.
So, to accurately make comparisons across regions and countries we need to create a different time-series.
One way of doing this is to calculate the average of different weather stations across an area – for example England and Wales. The England and Wales Precipitation series (EWP) is such a series of monthly rainfall totals from 1766 to the present day. For the most recent decades, the EWP series is based on over 100 stations, although this number decreases as you go back through the 20th, 19th and 18th Centuries.
The EWP series is much more reliable than a single station at representing rainfall for England and Wales overall and, because it extends back to 1766, it is very important climate series.
However, it does not cover the whole of the UK or take full advantage of the complete network of several thousand stations currently recording rainfall across the country.
Met Office national records are created using gridded datasets which interpolate observations from the full network of stations onto a 5km by 5km grid covering the UK. The gridding method is a more sophisticated approach for analysing rainfall than simply taking an average of station data. However, because it is a digital series it is shorter than the EWP – the number of rain gauges with data in our electronic archive decreases rapidly by the early 20th Century.
So, the UK’s national climate series – the records you will see quoted when the Met Office releases statistics – is a comprehensive gridded rainfall analyses back to 1910. This series provides the best estimate of overall rainfall and its distribution across the UK.
The gridded rainfall analysis also enables us to produce maps showing UK rainfall patterns, for example January 2014:
How do the series compare?
Here are the headlines for January 2014:
- For the England & Wales areal series, January 2014 (173.5mm) was the wettest January since 1948 (176.8mm) and the second wettest January in the series from 1910
- For the EWP series, January 2014 (185.0mm) was the wettest January in the series from 1766, marginally wetter than 1948 (176.8mm)
- For Oxford, January 2014 (146.9mm) was the wettest January in the series from 1767, wetter than 1852 (138.7mm)
You can see from the graph below that the EWP and the national England & Wales series both represent the same area and are very similar. Rainfall totals for the Oxford Radcliffe Observatory series are generally lower.
For any individual year there can be significant differences between series. For example we would probably conclude that January 1988 was climatologically more extreme for England and Wales than for Oxford, similarly 1997 is the driest January for England and Wales by a reasonable margin, but there are a number of similarly dry or drier Januarys than 1997 for Oxford.
So which is the best series to use?
Well, the answer is that we need to use them all. The Met Office routinely quotes rainfall statistics based on the gridded data, because these are considered the most reliable estimates, are based on the full network of observations, and can provide the regional pattern of rainfall.
The EWP series is an invaluable climate series because it provides a much longer near 250-year perspective but has less regional detail.
However, our climate analyses would not be possible without the long running high quality individual station series such as Oxford. These are the foundations of historical climate analysis.