Are Winters Getting Less Frosty in Herefordshire

Photo 1: Frosty Morn in Hereford (2nd December 2023)

This graphic (Figure 1) appeared on the BBC website today (1st March 2024) in an article discussing how wet and warm it has been in the UK this Winter. Meteorological winter in the northern hemisphere is defined as the months of December, January and February (DJF). There lots of ups and downs in Figure 1 but also a general downward trend in the number of frosty winter days. Over the 60 year period covered by Figure 1, the number of winter frosts has decreased, on average, from about 57 to 43 days.

Figure 1: Time Series of UK Ground Frost Days in Winter (DJF) 

You might think fewer frosts would be a good thing and there are certainly some very obvious benefits such as lower heating bills and less road gritting. However, there are also some downsides, especially in the way it affects nature. The BBC article mentions one of the disadvantages: apple trees that need a period of dormancy, provided by subzero temperatures, to encourage fruit bud formation and a healthy crop. Apples are very important to Herefordshire which, alledgedly, produces over half the cider drunk in the UK. For gardeners, frosts are also good for killing off overwintering pests, making the soil more friable, and as a good excuse to procrastinate (though most gardeners don't need an excuse).

Considering the importance of the apple industry to Herefordshire, I thought it worth looking at whether the UK trend in fewer frosts was also evident in our County (tl:dr not yet). There is also a personal interest in that we have eight apple trees in our garden; they are small and not all perfectly formed but, since it is more than five, does constitute an orchard.

Figure 2 reports the number of frost days recorded by my Davis Weather Station during the winter months, with the green line representing the Winter (DJF) data.

Figure 2: Number of Frost Days in Hereford during the Winter Months

The sawtooth pattern observed locally (Figure 2) for the winter season (DJF) mimics the trend observed nationally (Figure 1). December and February, but not January, have a similar sawtooth pattern to that seen for the winter season. [Note: it is the exceptionally high number of frosts in January 2022 that hides a potential sawtooth effect for the January time series]

Interestingly, January 2022 was not a cold month in Hereford despite the high number of frosts. It was, however, the third sunniest January on record (since 1880) for the UK and above average in Herefordshire (Figure 3).

Figure 3: January 2022 Sunshine Hours

Perhaps even more interesting is the fact that the data from my Weather Station (Figure 4 and Figure 5) did not show an unusually sunny January in 2022 (relative to the 2020 -2024 period). The yellow bars show the January 2022 data. Figures 4 & 5 indicate that January 2022 in Hereford City was (a) warm, (b) had lots of relatively mild frosts (minimum temperature), (c) very low rainfall with plenty of dry days, (d) average sunshine, and (e) was relatively calm (maximum wind speed).  

Figure 4: January Temperature & Frost Data for Hereford City (2020 - 2024)

Figure 5: January Rain/Wind/Sunshine Data for Hereford City (2020 - 2024)

Apart from only average sunshine hours and general warmth, all the other factors are expected to be favourable to frosts (e.g. frosts less likely in windy conditions).

As a reality check, I looked up the January sunshine hours reported for the Met Office's Ross-on-Wye weather station, about 15 miles south of Hereford. In January 2022, Ross-on-Wye was located in that slightly sunnier part of Herefordshire (Figure 3). As can be seen from Figure 6, the sunshine profiles for Hereford (Figure 5) and Ross-on-Wye are different but not widely different. And January 2022 was sunnier in Ross-on-Wye than in Hereford (as indicated in Figure 3). [Note: I estimate Hereford sunshine hours using a correlation between my solar radiation readings and the Ross-on-Wye sunshine hours]

Figure 6: January Sunshine Hours for Ross-on-Wye

As a further check, I compared the number of January frost days in both locations (Figure 7). Did Ross-on-Wye have a couple of extra frost days in January 2022 because it had more sunshine than Hereford?

Figure 7: Number of January Frost Days in Hereford and Ross-on-Wye

If you can remember that far back, the original question was whether Hereford, and indeed Herefordshire, was witnessing a decrease in winter frosts. My weather data for Hereford is inconclusive because the time series is far too short. So let's look at the historical data for the Met Office's Ross-on-Wye weather station. Figure 8 shows the equivalent plot to the one that appeared on the BBC website (Figure 1). The similarities are evident despite one being based on a single location and the other on multiple locations over a vastly larger area.

Figure 8: Ross-on-Wye Winter (DJF) Frost Days from 1961/2 to 2022/3

However, as we have more data for the Ross-on-Wye station, we should use it. Figure 9 plots the number of winter frost days recorded by the Ross-on-Wye weather station over the past 94 winters. Winter, as always, is defined as the months of December, January and February (DJF).

Figure 9: Ross-on-Wye Winter (DJF) Frost Days from 1930/1 to 2023/4

While the statistics indicates a small negative trend in the number of winter frost days with time, the correlation (as denoted by the correlation coefficient, R²) is not significant; i.e. there is no evidence to suggest, based on available data, that winter frost days are decreasing in Herefordshire. For now, the apple crop is probably OK.


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