A simple summer index: 1929 – 2018

One of the first articles that I ever read in the Weather Magazine as a young outstation assistant was entitled “A simple summer index with an illustration for summer 1971” by R. Murray which was published in April 1972. Now over forty years later as a retired programmer with the Met Office, I’ve decided to revisit the summer index [SI] and update his original record.

I have a number of advantages that Murray could only dream of, and they are a powerful personal computer, up to date freely accessible climate data, and of course the Internet to access that data from. The Met Office provide the data in the form of monthly regional and national gridded climate data back to 1910. This provides you with all the temperature, rainfall and sunshine that you require to calculate a SI, and the beauty of this data is that you can generate a SI not only for the UK, but for England, Wales, Scotland, Northern Ireland as well as any of the other twelve regional areas.

The program parses the text files that I had downloaded into a data structure to hold each months mean temperature, total rainfall and sunshine values that are necessary to compute each year’s SI. The slightly tricky part was to calculate the quintiles of each month’s mean maximum temperature, and the terciles of the total sunshine and rainfall that the formula requires.

The summer index [SI]

This is what the SI looks like:

SI = 3T + 5S – 5R – 9m


  • m = number of months
  • T = sum over m months of quintiles of monthly mean temperature
  • S = sum over m months of terciles of monthly sunshine
  • R = sum over m months of terciles of monthly rainfall

Quintiles and terciles are statistical terms used with any series of data arranged in order of magnitude. Rainfall is conventionally divided into three equal classes; the driest third being tercile 1, and the wettest tercile 3. With temperature the data is divided into five equal classes; quintile 1 refers to the coldest and quintile 5 to the warmest. There is one slight drawback in using the climate data series from the Met Office, although the temperature and rainfall series extend back to 1910, the sunshine series is only available from 1929, so I can’t reach back quite as far as 1881 as Murray did originally. Using Murray’s formula the absolute best ‘meteorological’ summer can score a maximum SI of 48 and the absolute worst a SI of -48.

A simple summer index

The beauty of the optimum SI first put forward by Davis in 1968 is that it’s simple. A good summer can be ruined by a wet last week in Autumn, so the index is far from definitive. You could devise a SI that looked more closely at daily values of temperature, rainfall and sunshine, but at the moment the Met Office don’t make daily regional climate data available, so for now monthly data will have to suffice.

The ‘best’ summers

How do you define what constitutes a ‘good’ summer? It’s usually very subjective, and as we grow older it may have less to do with weather and more to do with other things that are going on in our lives at the time. Keeping it strictly meteorological, and if you’re older than 70, you probably look back at the summer of 1959 as being the best, older than 50 and its highly likely that 1976 will be your perfect summer, younger still and it may well be the summer of 2003. The worst summer in contrast is not so easy to quantify, and many people if asked will struggle to name the worst summer that they’ve experienced in their lifetime. As you can see 1976 tops the SI back to 1929 for the UK which probably comes as no great to surprise to many. In fact the SI for 1976 is 48 the perfect maximum.

Best summers in the UK 1929-2018 (fig 1)

The ‘worst’ summers

The worst summers in the UK 1929-2018 (fig 2)

The summer of 1954 scored the lowest SI of -42 of any summer in the UK since 1929 (fig 2), remember the lowest possible SI is -48. 1954 was the very antithesis of 1976, it was not only wet, it was cold and it was very dull. Even if you compare 1954 using the extended SI, it’s still has the lowest index of -58 for the UK. Just to show you how poor the summer of 1954 was, here are the headlines for each month of the extended summer that I’ve copied from the Monthly Weather Report.

  • May 1954 mainly dull and wet, with frequent thunderstorms; large variations of temperature.
  • June 1954 mainly dull and cool; periods of rain, heavy at times.
  • July 1954 notably cool and dull; wet in some areas.
  • August 1954 cool and dull, mainly wet in England, Wales and southern Scotland.
  • September 1954 cool and unsettled; wet in most areas; sunny on the whole.

The extended summer index

The beauty of the algorithm is that you can also calculate an extended SI (May through to September) which gives an entirely different slant on what was the best summer compared to the one that uses the three summer months of June, July and August. The table below (fig 3) shows that 1949 has the highest SI of 61 (out of a possible 80) and that 1976 trails in the fourteenth position in the rankings with an index of 28. Why is 1976 so much worse? If you compare the various quintiles and terciles for 1976 and 1959, you will see that 1976 was in fact duller and wetter than 1949 during both May and September, so the extended SI was reduced.

The best extended summers in the UK 1929-2018 (fig 3)

Because the data is also split into regional as well as national values, it’s also easy to compare what kind of a summer other regional areas of the UK experienced. As you can see in the breakdown of the extended summer of 1959 (fig 4), the south of England scored a very high SI of 72, whilst somewhat lower down the rankings came the north and west of Scotland.

Regional extended SI for 1959 (fig 4)

What about Summer 2018?

To a lot of people, especially those in the south and east the summer of 2018 was very good, and the eagle eyed amongst you will have noticed that although it only managed sixth highest in the table of UK meteorological summers (fig 1), it ranks third highest in the table of extended summers (fig 3) since 1929. But how did the extended summer of 2018 look regionally? According to the extended summer index northeast England fared best in 2016 with a summer index of 64 (fig 5), closely followed by southern England, East Anglia and northern England.

Regional extended SI for 2018 (fig 5)

Graphical view

Finally, here is a graphical way of looking at the data by means of two scatter graphs (figs 6 & 7). They plot a correlation between sunshine-temperature and rainfall-temperature. They show at a glance just how each of the summer compares. For instance although 2006 was warmer than 1959, it was neither as dry or sunny, whilst 1989 although a little sunnier than 1959, was also cooler and a little wetter.

Sunshine – temperature correlation (fig 6)
Rainfall – temperature correlation (fig 7)

Are summers getting any better?

So much for last summer and the best and the worst summers in the UK, the one remaining question is – are summers getting better? Well with the help of another chart (fig 8) I’ve plotted the SI as a bar chart and overlaid with a five year centred moving average (dashed line with a yellow outline). I’ve also added a simple linear trend (dashed black line), although climatologically this may not be a good idea (because any trend certainly wouldn’t be linear), it does help to highlight the increase in SI that there’s been since 1929. So the short answer to that question is yes, the SI has increased over the last 89 years, whether that equates to better summers, I’ll leave that for you to decide.

UK Summer Index 1929-2018 (fig 8)


  • Davis, N. E.1968. An optimum summer index. Weather 23: 305-317.
  • Murray, R.1972.A simple summer index with an illustration for summer 1971.Weather 45:161-169.

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