Why aren’t days of the year warming at the same rate in central England?

It seems like the days of the year aren’t warming at the same rate in the daily central England temperature series that started in 1878. Of course you wouldn’t expect each day of the year to be warming at exactly the same rate but if you graph the linear trend for each day of the year and plot the results hopefully you’ll get the same kind of graph as I produced above. As you can see the scatter graph results vary from just below zero to well over +0.25°C. I’ve overlayed a seven year centred moving average to see if there is any trend in the daily rate of change though the year. Lower warming, even cooling, seems to occur from late January and into late February, the same seems to occur in a spell from late May and into June but not quite as markedly. There are strong warming peaks in the second week of January, and for much of March, but the highest warming trends seem to occur in October with with another short and sharp peak in the week before Christmas.
The highest rate of change I found is on the 23rd of December which as a day is warming at the rate of 0.29°C per decade, that’s almost 3°C every 100 years! The lowest rate of change is on the 4th of June which is cooling at a rate of -0.04°C per decade.
Don’t ask me why central England is warming (or cooling) at such an uniform rate though the year. All I know is that these daily rates of change do affect monthly means, which I have known for a long time have meant that months such as June has changed little temperature wise in central England since 1659.

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