Welcome to the first of my monthly UK weather tracker posts! Every month I will update the charts to show how exceptional or not our weather has been for the last month as well as additional charts that explore our unpredictable weather in more detail. All data used in these posts come from the UK Met Office.
It is early March and outside my window, a cherry tree has started to bud. For my wife this is a welcome sign of spring and marks the end of her second full winter of living in Britain. To go from living in sunny Texas to the unpredictable UK takes some getting used to but as we all know, 2 seasons is not enough to experience the variety of weather that is possible in the UK. I can tell her that on average the average 24-hour temperature was 5 degrees Celsius across the UK this winter but she has no context to judge whether that is a warm or cool winter.
Putting recent experience into context is a core skill of any statistician. Finding the best way to compare with today with yesterday allows anybody to easily to draw conclusions about whether there is anything exceptional about today. Before I can do this, I have to define what I mean by “yesterday”. Typically, this is a period of time which can cover anything from one day to a thousand years. For my weather tracker, I have decided to work with the concept of “living memory” which I define to be a maximum of 100 years. So I will be comparing 2017 with data drawn from 1917 to 2016. Depending on the statistic I want to use, the Met Office provides data from either 1910, 1930 or 1961 so it is not always possible to have 100 years to compare and this is indicated on my charts below.
The method of comparison I will use will be to compare each month in 2017 with 5 percentiles calculated from the “living memory” data. These are:
- 0th percentile (or MINIMUM value) observed within living memory.
- 10th percentile (or LOWER DECILE) i.e. the value corresponding the LOWEST 10% of values observed within living memory.
- 50th percentile (or MEDIAN value) observed within living memory.
- 90th percentile (or UPPER DECILE) i.e. the value corresponding with the HIGHEST 10% of values observed within living memory.
- 100th percentile (or MAXIMUM value) observed within living memory.
I have calculated these percentiles for each month separately for 6 different weather statistics as shown in the charts below. Note that the Met Office defines winter as the months December to February which is why the horizontal scale shows the year as starting in December and ending in February. This means the first black dot is for December 2016 followed by 2 black dots for January & February 2017.
The 5 percentile lines should be intuitively obvious but for completeness I will list them here.
- Solid Green (Solid Red in Rainfall charts) are Maximums
- Dashed Green (Dashed Red in Rainfall charts) are Upper Deciles
- Solid Black are Medians
- Dashed Red (Dashed Green in Rainfall charts) are Lower Deciles
- Solid Red (Solid Green in Rainfall charts) are Minimums
The 6 statistics that I will be tracking from now on are:
- Chart A: Average temperature in Celsius over 24 hours. This of course mixes up day and night time.
- Chart B: Number of hours of Sunshine per day.
- Chart C: Amount of Rainfall per day in millimetres.
- Chart D: % of days that saw frost in the morning. I understand that the met office counts snow & ice as frost.
- Chart E: % of days that saw at least 1mm of rain. I denote these days as Rainy Days.
- Chart F: Rain Intensity which is the total amount of rainfall divided by the number of rainy days.
The rain intensity allows us to distinguish between months where we get steady rain which should show up as high % of rainy days but low rain intensity and months where we get sudden deluges which should show up as low % of rain days but high rain intensity.
So what should I make of the winter of 2017 especially in terms of setting my wife’s expectations for her life in Britain? 80% of the time, I expect months to fall between the upper and lower deciles. This is exactly what has happened this winter for Sunshine, Rainfall and Frost showing that this winter was unremarkable and somewhat typical of British winters. 20% of the time (or 1 year in 5) I would expect months outside of the dashed lines which was the case for Temperature in December and February but not for January. Temperature is the odd one out of the various weather statistics as the UK climate has warmed up in the last 30 years so the norm now is to have above-median temperatures. This is something that I will explore in a separate post. As far as my wife is concerned, last winter’s temperatures might be the new normal but there is nothing to say we will not get winters in the future which are 5 degrees colder than they were this winter!