{"id":6739,"date":"2026-01-06T21:37:07","date_gmt":"2026-01-06T21:37:07","guid":{"rendered":"https:\/\/marriott-stats.com\/nigels-blog\/?p=6739"},"modified":"2026-01-06T21:48:54","modified_gmt":"2026-01-06T21:48:54","slug":"uk-weather-trends-2025-calendar-year","status":"publish","type":"post","link":"https:\/\/marriott-stats.com\/nigels-blog\/uk-weather-trends-2025-calendar-year\/","title":{"rendered":"UK Weather Trends #39 – Annual Temperatures in 2025"},"content":{"rendered":"

The year 2025 was both the hottest and 2nd hottest year on record in the UK.\u00a0 Last year, I used a CUSUM Chart<\/strong><\/a> to conclude “if the calendar year average is greater than 9.48<\/strong> degrees Celsius and the meteorological year average is greater than 9.33<\/strong> degrees Celsius … (this will) signal an upwards trend change<\/em>“.\u00a0 By co-incidence, the average 24-hour temperature across the UK in 2025 was 10.09<\/strong> degrees Celsius for both the calendar (Jan to Dec) and the meteorological (Dec to Nov) years which is clearly higher than these thresholds.\u00a0 It’s only taken me four years of writing to conclude a new upward trend in UK temperatures has begun.<\/p>\n

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Learn how to decide if a trend has changed<\/strong><\/span><\/h5>\n

This is the fourth article I have written about trends in UK annual temperatures.\u00a0 My goal is to create a useful series for anyone who wants to learn how to use statistical thinking and methods to detect changes in trends for any time series.\u00a0 If this topic is new to you, I recommend you read the series from the beginning before continuing with this article.<\/p>\n

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  1. 2022<\/strong> – I explain what a Control Chart<\/strong> is<\/span><\/a> which is a technique arising out of the field of Statistical Process Control<\/a><\/strong>.\u00a0 I calculate the Control Limits<\/strong> for the control chart using the years 1997 to 2021<\/strong> and I explain why I use just those years based on an 11 year Centred Moving Average<\/strong> (CMA<\/strong>).\u00a0 When I plot 2022<\/strong> data on the control chart, I decide there is not enough evidence to conclude the trend has changed.<\/li>\n
  2. 2023<\/strong> – I update my control chart with 2023 data<\/a> but I do not change the control limits used in the chart which continue to be based on the years 1997 to 2021.\u00a0 I now conclude “a step change upwards has taken place but the pace of change is not yet clear.”<\/em><\/li>\n
  3. 2024 <\/span><\/strong>– I update my control chart with 2024 data<\/span><\/a> which continues to use control limits based on the years 1997 to 2021.\u00a0 I build another control chart using meteorological year (December to November) data instead to see if my conclusions change as a result of changing the definition of a year.\u00a0 I find that it does and I decide to withdraw my conclusion of 2023.\u00a0 I then introduce another method of detecting trend changes known as CUSUM<\/strong>.<\/li>\n
  4. 2025 (this article)<\/strong> – I update my control charts for calendar & meteorological years with 2025 data which continue to use control limits based on the years 1997 to 2021.\u00a0 I also update my CUSUM charts and draw conclusions about future trends.<\/li>\n<\/ol>\n

    Detecting changes in trends is one of the topics I cover in my training course “Identifying Trends & Making Forecasts<\/em><\/a><\/strong>“.\u00a0 I also run this course on behalf of the Royal Statistical Society<\/strong><\/span><\/a> and you can book a place on their next course here<\/a>.<\/p>\n

    In this article, I draw conclusions after reviewing three types of charts (Trends, Control, CUSUM<\/strong>) for two alternate definitions of a year (Calendar, Meteorological<\/strong>).\u00a0 I first go through the three charts for calendar years which cover January to December and then go through the three charts for meteorological years which cover December to November.\u00a0 Unlike previous years, I will no longer explain the methodology in depth behind each chart and will instead link to one of the earlier articles which provides the necessary depth.\u00a0 All temperature data shown in this article can be downloaded from the Met Office<\/a>.<\/p>\n

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    C1 – UK Annual Average 24-Hour Temperature – Calendar Year<\/strong><\/span><\/h5>\n

    In 2025, the average 24-hour temperature across the whole of the United Kingdom from 1st January to 31st December was 10.09<\/strong> degrees Celsius.\u00a0 This is the highest on record beating the previous high recorded in 2022 (10.03<\/strong> degrees).<\/p>\n

    In the chart below, I plot the UK annual average temperature for the calendar years going back to 1884 as purple dots.\u00a0 The solid black line is the 11<\/strong> year Centred Moving Average<\/strong> (CMA) which is intended to represent the underlying trend.\u00a0 The latest CMA is for 2020<\/strong> covering the years 2015 to 2025 and was 9.60<\/strong>\u00a0degrees Celsius, the highest on record.<\/p>\n

    \"\"<\/p>\n

    The 11 year CMA shows the following –<\/p>\n