Not a great deal of difference from the results of the Summer index for 2024 really, with East Anglia still having the highest index and Northern Scotland the lowest. The result for the extended summer in Northern Scotland were much improved though, from a summer index of -27 to and extended summer index of -7, thanks to a spell of good weather and record temperatures in May, and another settled warm spell in mid September.
I make the summer of 2024 [JJAJJAMeteorological Summer comprising the months June, July & August] the coldest since 2015, with a mean temperature of 15.77°C, which was 0.27°C lower than the 1991-2020 LTALTALong Term Average. This is usually defined as a 30 year period by the WMO..
Not much to say about summer [JJAJJAMeteorological Summer comprising the months June, July & August] other than it ended up a very average summer. That was because the cooler, cloudier, wetter conditions in the northwest, were almost perfectly cancelled out by the drier, sunnier and warmer weather in the east and southeast.
The summer index [JJAJJAMeteorological Summer comprising the months June, July & August] in the UKUKThe United Kingdom of Great Britain and Northern Ireland, commonly known as the United Kingdom (UK) or Britain, is a country in Europe, off the north-western coast of the continental mainland. It comprises England, Scotland, Wales and Northern Ireland. for 2024 came in at 11, which was almost the same as last summer. June had the lowest combined monthly index for temperature, precipitation and sunshine of 6 [3,1,2], whilst August bolstered the summer up with an index of 9 [5,2,2].
Regionally East Anglia, the southeast and central southern England and eastern Scotland had the highest summer index of 24. The lowest index, which I can vouch for, was -23 in the north of Scotland.
I’ve never added any animation to my Daily Central England temperature application up until now, so I thought as the weather today is still pretty dreich, I would put that right today. It’s amazing how the simple animated GIF is still going after all these years, it was declared dead many years ago at the same time as Javascript, but it’s simple and produces compact files, and there’s still not anything out there to replace it.
One of the very first articles that I 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 OfficeUKMOThe Meteorological Office is the United Kingdom's national weather service. It is an executive agency and trading fund of the Department for Business, Energy and Industrial Strategy, I have decided to revisit the summer index and update his 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 summer index, and the advantage of this data is that you can generate a summer index not only for the UKUKThe United Kingdom of Great Britain and Northern Ireland, commonly known as the United Kingdom (UK) or Britain, is a country in Europe, off the north-western coast of the continental mainland. It comprises England, Scotland, Wales and Northern Ireland., but for England, Wales, Scotland, Northern Ireland as well as any of the other twelve other regional areas. All that was required to access the latest climate data from the Met Office website was internet access. The program converted the text files I downloaded into a data structure to hold each month’s mean temperature, total rainfall and sunshine values necessary to compute each year’s summer index. The slightly tricky bit 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 [SISISummer Index]
SI = 3T + 5S – 5R – 9m
where:-
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 a 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 was unable to reach back quite to 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 summer index was first proposed by Davis in 1968, its beauty lies in its simplicity, but a good summer can be ruined by a wet last week in Autumn, so the index is far from perfect. You could dream up a summer index that looked more closely at daily values of temperature, rainfall and sunshine, but at the moment the Met Office do not make daily regional climate data available, so for now monthly data will have to suffice. How do you define what constitutes a ‘good’ summer? It is 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. 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 it’s highly likely that 1976 will be your perfect summer, younger still and it may well be the summer of 1995 or 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 have experienced in their lifetime.
The ‘best’ summers
As you can see from the table of best summers (fig 1), 1976 tops the Summer index back to 1929 for the UK which probably comes as no great to surprise to many. In fact its score is the perfect maximum of 48.
Fig 1
The extended summer index
The beauty of the algorithm is that you can also calculate an extended Summer index (May through to September), which gives an entirely different slant on what was the best summer. The table below (fig 2) shows that 1959 has the highest extended summer index of 62 (out of a possible 80), and that 1976 is only eleventh in the rankings, with an index of 28. So why was the extended summer of 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 1959 in May and September so the extended index score was reduced.
Fig 2
Because the data is also split into regional as well as national values, it’s easy to compare what kind of summer other parts of the UK experienced. As you can see in the breakdown of the extended summer of 1959 (fig 3), the northeast of England and the Midlands score a very high 72, whilst somewhat lower down the rankings came the north and west of Scotland.
Fig 3
The ‘worst’ summer
The summer of 1954 has the lowest summer index -48 of all summers in the UK since 1929 (fig 4), you just can’t get a summer index lower than -48. 1954 was the very antithesis of 1976, it was not only wet, it was cold and dull. Even if you compare 1954 using the extended summer index, it’s still has the lowest index of -64 for the UK. Just to show you how poor that summer was, here are the headlines for each month of the extended summer of 1954 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.
Fig 4
Graphical view
Finally, here is a graphical way of looking at the summer’s since 1929 as a whole by means of two scatter graphs. The first graph plots temperature against rainfall (fig 5), whilst the second graph plots temperature against sunshine (fig 6). They show at a glance just how each summer compares with each other, for instance although 1995 was very slightly drier than 1976, it was slightly less sunny and not as warm.
Fig 5Fig 6
Are summers getting any better?
The one remaining question is – are summers getting better? Well with the help of another chart (fig 8) I’ve plotted the summer index and overlaid it 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 be frowned upon (because any trend certainly wouldn’t be linear), it does help to highlight the increase in summer index that there has been since 1929. So the short answer to that question is yes, the summer index has increased over the last 87 years, whether that equates to better summers, I will leave that for you to decide.
Fig 7
References
Davis, N. E.1968. An optimum summer index. Weather23: 305-317.
Murray, R.1972.A simple summer index with an illustration for summer 1971.Weather45:161-169.
Footnote
Many of my regular readers may remember that I wrote an article about the ‘Summer Index’ in July 2015. I fully intended to get a fuller version of it published in the Weather Magazine of the Royal Met Society. I lost heart in the end, I don’t think I write in the way that they like, and I’m not good at writing in any other way. I went to a bit of trouble in putting the article together, so instead of it just languishing in a folder named ‘Weather Magazine’ in my Google drive account, I thought that I’d re-publish it in my blog just for posterity. The other thing that I like about a blog is that you can always fix typo’s or mistakes which you can’t do in a printed magazine, hopefully there are not too many of those.
Just a quick note to make a note about how late an Autumn it’s been in 2023. Its now past mid October, and many trees here in StrathpefferStrathpefferStrathpeffer (Scottish Gaelic: Srath Pheofhair) is a village and spa town in Easter Ross, Highland, Scotland, with a population of 1,469. are still have more green leaves than they do brown, and although the Service and Birch tree have lost most of their leaves, others have not. I witnessed late, even very late Autumns in Devon over the years, but not here in Scotland since we returned five years ago. Why is Autumn late? Simple enough, summer was pushed into September and early October by a rather cold mid-summer as these anomaly charts show. The numbers in the pink boxes by the way are the mean anomalies for the gridded temperatures for the whole map, the graph is for temperatures at the grid point 57.5N 5W, the closest to home. It does make you wonder if meteorological summer should now be a four month period rather than a three which also includes September, whilst Autumn should now include December. By the way the title of that graph should read Mean Temperatures and not anomalies and another thing to put right.
The summer [JJAJJAMeteorological Summer comprising the months June, July & August] consisted of the fifth warmest (mean anomaly +2.4°C) June in central England, followed by the two non-descript months of July (mean anomaly -0.7°C) and August (mean anomaly -0.2°C). You’ll notice that there were no warm spells to speak of after the 25th of June, when the summer switched to average mode. The June 10th to the 16th was a quite remarkable spell, setting 7 consecutive new maximum daily temperatures for June. The rest of the summer also set five new high daily minimum records and one for the lowest minimum on the 26th of July (8.6°C).
Summer started in a very anticyclonic mood with negative zonality (E’lies). The rot soon set in, and by the 19th of June the summer had turned when positive zonality (W’lies) was restored, and that’s basically where we’ve been since then. The meridional index has switched between spells of negative and positive, as it tends to do across our part of the world, and generally cyclonic. There were a couple of days with a gale index above the threshold of 30, but neither tie in with the named storms of Antoni (5 Aug) or Betty (18-19 Aug). The UKMOUKMOThe Meteorological Office is the United Kingdom's national weather service. It is an executive agency and trading fund of the Department for Business, Energy and Industrial Strategy, in their wisdom, decided not to name the deep low on the 15th of July that produced a GIGIGale Index of 40.5, but I reckon it was the equal of either of the two that were named.
You may have noticed that I’ve put some work into smartening up the front end of my application that visualises objective LWTLWTLamb Weather Types are often used in UK-based analyses, with individual weather patterns based on the eight primary cardinal directions (N, NE, E, SE, S, SW, W, NW) plus cyclonic (C), anticyclonic (A) and unclassified (U) types., the data for which I download from the CRU at the UEA. I’m quite proud of the app, and it works very well. I am so glad that the CRU have kept the objective series going after H.H.Lamb’s death in 1997. I do have the objective LWT algorithm, which I run on six hourly gridded reanalysis data in a seperate application as well , which gives a more detailed view of how weather types unfold.
Fine weather over central & SE England identified by Summer Index
I’ve been toying with the idea of a Real-time summer index [SISISummer Index] for many years. Of course the idea of a summer index is very subjective – everyone’s perfect summer day is different. I use a very simple algorithm of total cloud amount, present weather, relative humidity, surface visibility, mean wind speed and of course temperature to calculate it with. Results look promising. There’s no reason why the idea couldn’t be extended to use NWPNWPNumerical weather prediction uses mathematical models of the atmosphere and oceans to predict the weather based on current weather conditions. data for forecasting summer index for many days ahead. The values are using my preferences for an ideal temperature of 25°C. This is not ideal because what I would like to have used was anomalies. A fixed temperature also means the SI will oscillate through the day as things warm up, so it’s far from perfect. A perfect day to me really requires an okta of cumulus or cirrus cloud rather than complete blue skies. The perfect day also requires some wind, no more than five knots, perfect visibility and low humidities. I factor all of them into the algorithm, but thats still very much work in progress. The 25°C ideal temperature may be a little too high for me, but at that rules out SI levels much above 75 across the Mediterranean in the summer.
