Why do some months warm faster than others?

It has been obvious to me for many years how increases in temperature in individual months in the Central England Temperatures series are far from uniform. Some months are warming much faster than others, and some by very little at all. A good example of this are the months of January and June, compare the following two charts and notice the linear trend from 1772 is rising at +0.09°C per decade in January, but is level 0.0°C in June. Why is it that mean temperatures in January has been rising at almost a degree a decade for the last 248 years, whilst the temperature in June has remained static?

To investigate this further I decided to add some code to my daily CET application to calculate a simple linear trend for each day of the year back to 1772. Twenty years ago processing 247 daily mean temperatures and calculating a linear trend for each day of the year would have taken quite a bit of time, thankfully nowadays that took just 0.038 seconds. Here’s a chart of the results that I generated.

You can clearly see the reason why December and January are the fastest two warming months, and why the days in May and particularly June have been reluctant to rise, and for some days have been cooling. It’s hard to believe that the mean temperature for June in central England has not changed at all in the last 250 years.

At the same time as adding code to calculate daily linear trends, I also added code to inspect the linear trend for any particular day of the year to see how that looked. Below are charts of two extreme examples of that, one for the 9th of January, and the other for the 15th of June. January 9th has been warming quite rapidly at 0.13°C per decade since 1772, but in direct contrast the 15th of June has been slowly cooling at 0.03°C per/decade.

The daily series of maximum and minimum CET values started in 1878 and here’s a look at what I found in an analysis of that data. You can see peaks and surges in both charts occur throughout the year, and those peaks are very large. As you can see the ones in rate of change in maximum temperature in July, October and December peak are over 0.2°C, although the values for May and June are the lowest.

I have never seen this kind of statistical analysis done before and although I say it myself I think the results that I have found are really quite fascinating. Perhaps my method is flawed in some way but I don’t think so. As to exactly what’s been happening to temperatures in central England in the last couple of centuries I’ll leave you to figure out, and if you do so, please drop me a line.

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