Meteoologists define autumn in the UK to be the period from September to November so autumn is now over and we are officially in winter. The 2017 autumn was completely unremarkable but this hides the fact that our autumnal climate has changed quite notably over the last 25 years.
I analyse the long term trends in the UK weather using a statistical tool known as STANDARDISATION. This means that the 3 key variables of Temperature, Sunshine and Rainfall are recalculated so that they all have the same units, which is number of standard deviations from the mean. Such variables are known as Z-SCORES. For more information on how I have done this, please read my post on trends in the UK summer.
The Z-Scores for Temperature, Sunshine and Rainfall are shown in the 3 charts below. Each chart also contains an 11-year centred moving average which gives an idea of the underlying trend.
Standardised variables aid interpretation of data in many ways. If the standardised value is positive, it means that the value is above your average or expected value. If it is negative, then the value is below your expected value. If the original variable is approximately normal in its distribution then the vertical scale gives us an idea of how typical or atypical each year is. Z-Scores in the range -1 to +1 are considered typical values and completely unremarkable. Z-scores in the ranges -2 to -1 and +1 to +2 are considered to be uncommon values but still entirely plausible and such values should not cause us concern. When Z-Scores get into the ranges -3 to -2 and +2 to +3, we should start paying closer attention and asking ourselves if something has changed especially if we get a sequence of successive points in these ranges. Finally, if the Z-scores are less than -3 or greater than +3, that is normally regarded as a clear call to action. There are in fact many ways of interpreting Z-Scores and what I have said so far merely a gives an overview of the most basic interpretations. A whole field of study known as Statistical Process Control (SPC) is dedicated to building and interpreting such charts (known as a CONTROL CHART).
For the autumn of 2017, the z-scores for temperature, sunshine and rainfall were respectively +0.6, -0.7 and 0. Officially this means autumn was warmer and darker than normal with normal rainfall but the magnitude of the z-scores clearly point to an entirely unremarkable autumn. If you have been following my monthly weather tracker then this should be no surprise as all 3 months stayed pretty close to normal.
Since the 3 moving averages in the above 3 charts all use the same units, they can be plotted onto the same chart as below.
This clearly shows a shift in our autumnal climate over the last 100 years of almost 1 standard deviation. Recall that the baseline for the z-score calculation is based on the idea of “living memory” which I have defined to be the last 50 years of 1967 to 2016. We can characterise our autumns broadly as follows:
- 1915-1925 – we had cold dry autumns.
- 1935-1975 – we had stable autumns which were on the cool, dull and dry side.
- 1975-1995 – similar to the previous period of 1935-75 but with a brief spell in the early 80’s when we had wet autumns.
- 1995-today – a clear shift in our climate occurred and we now have warm, bright and wet autumns.
So when I said that 2017 was unremarkable, in fact it was unremarkable only in the context of living memory. Compared to the 1940s, 2017 was actually warm, bright and wet.
A further point may be apparent from the above chart. The z-scores for temperature, sunshine and rainfall all appear to be correlated. In fact this is somewhat illusory as the above chart uses moving averages. If we look at the actual z-scores, we can see in the 3 scatter plots below that there is only a limited correlation between the 3 variables.
The brown square in each chart is 2017. In all 3 cases, the charts reinforce the ordinariness of 2017. However, scatter plots can be useful to identify unusual years that do not follow the normal relationships. They can also be used to identify the true dimensionality of our weather, a concept I will explore in my next post on UK winter weather trends.