SE England: Was it actually cold in December and January?

The last two weeks have seen frequent snow showers in many areas in the UK, with several centimetres of snow at the higher levels. These episodes have occurred after a long period of frequent rain and mild temperatures throughout the first month of the winter season.

I have analysed and plotted the temperature anomalies observed in December 2018 and January 2019 in London Heathrow. The anomalies are computed considering the long-term averages (1981-2010), which, for London are: December (+8.3/+2.7 °C) and January (+8.07/+2.3 °C).

This analysis is simply focused on having a general overview of the observed temperatures, and on a local area; thus, it can’t be used to determine a wider analysis for the whole country. I haven’t plotted the rainfall, but data can be found in the references.

In Figure1 we can see the daily maximum and minimum temperatures anomalies observed during the last two months.

London-december-january-daily-temperature-anomaly
Figure1. Daily temperature anomaly observed in London in December 2018 (top) and January 2019 (bottom). On the left the anomaly for daily maximum temperatures, on the right the anomaly for daily minimum temperatures. Positive anomaly (red), negative anomaly (blue).

Most of December has been mild, especially in the minimum temperatures, with positive anomalies close to +10°C during the first week (when frequent rainfall has occurred). During the second week, colder and drier air has caused a decrease in temperatures (especially in the maximum values) though only for few days. Indeed, the last two weeks of December have shown mostly mild temperatures (and frequent rain) with only two days with negative anomalies (one of them it’s Christmas). No snow has been observed throughout the month.

January, instead, has shown a decrease in the positive anomaly (especially in the maximum temperatures), with colder and drier air after new year’s eve. The second week has shown settled weather, and temperatures above the average (+3/+5 °C the anomaly in the minimum values). However, from the 15th colder air (both polar maritime and arctic depending on the episode) has reached the UK, causing a strong decrease in temperatures and the first snow of the season in many areas (also in London) between the 21-23 of January and again, after few days with warmer temperatures, on the 31st and 1st of February. The last few days of January have been the coldest of the season so far, with the lowest temperatures observed before the snowfall in the morning of the 31st.

Considering the monthly analysis, thus comparing the anomaly observed in the minimum and maximum average temperatures in December 2018 and January 2019 with the historical period 1981-2019 (Figure2), we can see that December has observed a significant positive anomaly especially in the minimum values (+3.1 °C), instead January has seen values very close to the average, with a slight negative anomaly in the maximum temperatures (-0.62 °C) and positive in the minimum (+0.28 °C).

december-january-max-min-temperature-anomaly-1981-2019
Figure2. Anomaly for mean maximum (left) and mean minimum (right) temperatures for December 1981-2018 (top) and January 1981-2019 (bottom). Positive anomalies (red), negative anomalies (blue).

Overall, December 2018 is one of the hottest since 1981 (3rd for the maximum temperatures and 2nd for the minimum); instead, January has been roughly on average (27th for the maximum and 21st for the minimum temperatures).

In conclusion, even if numerous news had claimed that ‘freezing’ conditions have occurred this winter (especially in January), the actual analysis has shown that it has been a ‘warm’ winter so far, with few cold episodes mainly occurred during the last two weeks.

We will see how February will end in order to sum up the overall winter analysis.

References

https://www.meteociel.fr/cartes_obs/climato2v4.php?code=3772&mois=12&annee=2018&print=1

https://www.metoffice.gov.uk/pub/data/weather/uk/climate/stationdata/heathrowdata.txt

First two weeks of the year: Was it actually colder in S. Europe than in the north?

The first two weeks of January have seen very cold air coming from Russia towards Central-Eastern Europe, with lot of snow especially in S. Germany, Austria (more than 2 metres in the Alpine region) and the Balkans. Also in Greece and S. italy snow has fallen, at sea levels at times.

In northwestern Europe instead, and especially between France, the UK and the N. Atlantic, a strong high-pressure ridge has caused settled conditions for several consecutive days, causing also very warm air towards these areas and Scandinavia.

I have analysed and plotted the daily average temperatures in 17 countries (cities) and compared the results. This analysis is simply focused on considering the main cities, thus it is not a detailed analysis for each country.

The results (Figure1) show that in S. Europe (especially between Italy and the Balkans) the daily average temperatures have been lower than in central-northern Europe.


Figure1. Daily average temperatures observed during the first two weeks of January in Europe. Notice the cities are grouped according to the geographical region.

We can see how Rome and Istanbul observed similar or lower temperatures than London, Paris and Amsterdam (to cite a few). In addition, it is impressive to see that Reykjavik has observed a very similar mean temperature to the cities in southern Europe.

Also the Balkans have observed temperatures lower than in northern Europe, with Bucharest colder than Stockholm and observing temperatures very similar to Helsinki.

However, not all southern Europe has observed low temperatures; the Iberian pensinsula has seen warmer conditions (though with temperatures close to their climatological average), with Lisbon the warmest capital so far. This is due to the frequent flow of warm air coming from the Tropics caused by the high-pressure ridge set (since December) between W Europe and the Atlantic.

In E. Europe (Warsaw, Moscow) the lowest temperatures have been observed (which is also in accordance to their climate); these areas where directly influenced by the polar continental air coming from E/NE Russia throughout the whole period.

If we look at the daily temperature anomaly (Figure2) computed respect to the climatological average (1981-2010), we can see how Reykjavik has observed the highest values (almost +5 °C; the daily mean average temperature in January for the Icelandic city is 0 °C).

Overall, all the area from France to Scandinavia and Iceland has seen warmer temperatures, instead E Europe and S/SE Europe have experienced a colder weather.

Figure2. Daily temperature anomaly (with respect to the 1981-2010 average) in Europe during the first two weeks of this year. Negative anomalies (in blue) and positive (in red). The higher is the anomaly (in terms of absolute values) the bigger is the ‘circle’.

In conclusion, we can say that the new year has brought cold weather in the southern and eastern part of the continent (with snow in numerous and unusual areas); instead, the western and nothern areas (especially the countries along the N Atlantic coast) are still experiencing mild conditions (as occurred also in December).

N.E. Italy: A very mild autumn

Related to the analysis I made for November 2018 in Treviso (N.E. Italy), one of the warmest recorded since 1973, here I am going to analyse the temperatures of the last 3 months, that is September, October and November (‘SON’ from now onwards) in order to evaluate if Autumn 2018 has been above or below the long term average (1981-2010) in Treviso.

In Figure1 the maximum and minimum temperatures average for SON in each year are shown. Like for November 2018 analysis, we can see how the average minimum temperatures tend to increase more in the last 20 years than the maximum values and Autumn 2018 shows the highest values.

Figure1. Maximum and Minimum average temperatures for SON since 1973. The dashed lines represent the 1981-2010 average for minimum (blue) and maximum (red) temperatures in Treviso.

In fact the long-term (1981-2010) Autumn average temperatures in Treviso are (9.63/18.76 °C) and the last Autumn has recorded (13.03/21.06 °C). The magnitude of the anomaly can be seen in Figure2, where the temperatures anomalies for each year (Autumn) are shown (for both maximum and minimum average values).


Figure2. Temperature anomaly for average Maximum (left) and minimum (right) temperatures in each Autumn since 1973. Anomalies higher than 0.5 °C (red), lower than -0.5 °C (blue) and between 0.5-0.5 °C (green).

It is clear how this Autumn has observed the highest average minimum and maximum temperatures (+3.55/ +2.33 °C respectively) since 1973, with October the warmest recorded for the maximum values (anomaly +2.46 °C) and second for the minimum (anomaly +3.78 °C); also September was very mild, the 4th warmest for the maximum values (anomaly +2.78 °C) and 3rd for the minimum (+3.09 °C). 

Considering the historical analysis, though this Autumn has been the warmest since 1973, however it is not the only one observing high temperatures (especially for the minimum values). In fact, from (Figure3) we can see how the years with minimum values above the average (anomaly higher than 0.5 °C) have consequently occurred since 2008, with no Autumn recording negative anomalies in the minimum average temperatures. Instead, in the 70s and 80s, most years observed lower values. Regarding the average maximum temperatures, there is more variability, with some ‘cold’ years in the 00s and ‘hot’ years in the 80s.

Figure3. Occurrence of hot (red) and cold (blue) years (autumn). Maximum temperatures (left), minimum temperatures (right).

It would be interesting to understand why the minimum temperatures observe a higher (positive) anomaly than the maximum values and for more consecutive years; it might be due to an increase in the overcast conditions during the night which might have led to a decrease in the radiative cooling at surface.

References

https://www.meteociel.fr/climatologie/villes.php?code=16098

October 2018 analysis S.E. England

Using the data from Heathrow airport (1950-2018, Met Office), I have tried to sum up the analysis on both minimum, maximum temperatures, number of days with air frost and cumulative rainfall during October 2018 and compared with the average 1971-2000 (Figure1).


Figure1.October average max and min temperatures (top ), days with air frost (bottom left) and cumulative rainfall (bottom right) in the period 1950-2018 recorded at Heathrow Airport.

Both October 2018 minimum and maximum mean temperatures (8.50/16.50 °C) are slightly above the 30 years average (8.17/15.70 °C), especially the maximum values. This is due to the mild temperatures observed in the first two decades, which were ‘counterbalanced’ by the cold spell occurred during the last decade which has caused a day of air frost (the first one after 7 years, thus after 2010).

Regarding the rainfall, the total cumulative precipitation observed (61 mm) is slightly above the monthly mean (60 mm) and it occurred mainly in the second decade.

References:

https://www.metoffice.gov.uk/pub/data/weather/uk/climate/stationdata/heathrowdata.txt