I think the best way to visualise Central England Temperatures [CET] is by means (excuse the pun) of a 365 day moving average. Any shorter and it’s a little too ‘noisy’, any longer and you lose some of the more subtle nuances that are present in the underlying daily values. Here is a chart of the last 30 years, spanning the period from November 1986 to November 2016. I have annotated the graph to remind both you and me of what month or season probably caused the various upturns and downturns in the anomalies during those thirty years. At the moment we look like we are possibly at the start of a downturn on the roller coaster ride that the CET series is on, and who knows where that will lead in the coming months.
By the way, the series in the graph with a black line is a moving 365 day average of a 365 day average and that’s why the line is so smooth. It does take a bit of thinking about, but it seems to work just fine!