The rainfall data was obtained from the Cornwall Association of Rainfall Recorders (CARR); the secretary is Mrs Ros Smith of Duchy College, at Rosewarne. The CARR are a mix of amateurs and professionals who have been recording rainfall in Cornwall since 1940 and submitting their results to be recorded centrally. The rainfall measurements are made accurately and the rain gauges used conform to British meteorological standards.
The data analysed was monthly rainfalls, though daily data does exist. As time has passed, some gauges have closed and new ones have opened. The number of gauges read each month and each year also varies. It was therefore necessary to use a scientifically sound method for analysing the data and calculating county-wide averages. This was done as follows:
1. For each individual station, the map coordinates were found. For some of the older stations this required a fair bit of detective work.
2. For each month, from 1940 to present, the measured rainfall was tabulated against location. For example, the data for October 1972 can be seen by clicking here.
3. For each month, a topographic rainfall surface was generated by mapping the data points onto a regular 1 km resolution grid. Figure 1 shows an example from October 1972, 30 years ago, about halfway through the record.
4. This surface was then superimposed onto a map of Cornwall to provide a representative rainfall volume, Figure 2.
5. The total rainfall volume for the month was then divided by the land area of Cornwall to provide a representative rainfall depth for the month.
This procedure was considered to be the most accurate available when dealing with areally distributed and variable location data, and was applied consistently. It is therefore unlikely that the trends resulting from the analysis are due to differences in the locations of, or numbers of, rain gauging stations throughout the record.
Fig 1 Rainfall isohyets for the month of October 1972. Contours are in 25 mm intervals.
Fig 2 Prism for calculating the volumetric rainfall in Cornwall during October 1972.
The time series data was then plotted in terms of annual rainfall against year, Figure 3. The best fit straight line trend was calculated by regression analysis and added to the graph. Annual rainfall has increased in Cornwall by some 22% since 1940. The data were then separated into winter (October-March) and summer (April-September) 6 monthly periods and plotted as time series with linear trends, shown in blue and red respectively in Figure 3. Summer rainfalls have increased by 8% in 60 years (which is probably statistically negligible) but winter rainfall has increased by some 35%. These statistics remain even when the winter of 2000/2001 is removed from the data, indicating that the trend is robust and real.
Fig 3 Division of annual rainfall for Cornwall (black) into winter (blue) and summer (red) components.
Though the winter of 2000/01 was extreme, it did not include the wettest month (in Cornwall). This accolade goes to December 1999, the previous winter. Winter 2000/01 does however, contain the wettest 2, 3, 4, 5 and 6 month periods.
Click here to see the extreme (wettest and driest) months for both Cornwall and England, for comparison.
Comparing the data for Cornwall with average monthly rainfall for England, the trends are somewhat different, Figure 4, and also no discernible difference in trend is noticed between winter and summer, Figure 5, for England generally. This may not be surprising, as it is generally perceived to be becoming wetter in the west and drier in the east. A similar analysis of the east, say Kent for example, may show the opposite trend to Cornwall. In this case the average may not change, though the extremes may.
Fig 4 Comparison between annual rainfall data for Cornwall (black) and for England generally (green).
Fig 5 Division of annual rainfall for England (green) into winter (blue) and summer (red) components.
The data for England was obtained from the Meteorological Office's Hadley Centre and may be accessed by clicking here.
The monthly rainfall data for Cornwall may be accessed by clicking here.
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