Christmas 2018 in Italy: very different weather conditions

Christmas day in Italy has shown both a wintry and mild aspect. Looking at the minimum and maximum temperatures (Figure1), we can see how values below 0°C were observed in the morning in the Po valley and Alpine region, with frost especially in the countryside. Values close to 0 °C were recorded also in central-southern Italy on the Appennines, instead mild values (10-15 °C) were observed mainly in Sicily and Sardinia.

The night and morning has been sunny mostly everywhere; only in Calabria and Sicily some showers have occurred and foggy conditions were observed especially on the plains in N.E. Italy.

The afternoon has been settled mostly everywhere, with sunny conditions along the peninsula. However, in N.E. Italy, especially close to the Adriatic coast (Figure2), foggy conditions have lasted during the whole day, causing cold temperatures in the plains.

Indeed, the maximum temperatures (Figure1) were just above 0 °C in the area between Venice, Verona and Ferrara; instead, in N.W. Italy, were bright conditions have occurred most of the day, maximum temperatures reached 10-11 °C.

Also in central-southern Italy this Christmas has not been cold (though with temperatures close to the average), with maximum values between 10-15 °C mostly everywhere (lower values along the Adriatic coast due to the colder air moving towards the Balkans). Finally, the warmest temperatures were observed in Sardinia and Sicily, with peaks of 16-18 °C in some areas.

Figure1. Minimum (left) and maximum (right) temperatures in Italy on Christmas day. (source: Meteociel).
Figure2. Satellite imagery of Christmas afternoon (13 UTC). (source: sat24).

The very different temperatures observed were due to both local orography/climate and larger scale weather patterns.

The foggy (and cold) conditions on the plains in N.E. Italy were due to the absence of winds at surface in the area and to anticyclonic conditions in the western Mediterranean region. Moreover, being the area surrounded by mountains (Alps and Appennines), the cold air is ‘trapped’ at surface along with humidity and pollutants (which contribute to the formation of the fog).

Instead, in N.W. Italy, the winds were ‘stronger’ and coming from the Alps, leading to a decrease in the humidity and to an increase in temperatures during the day.

Along the peninsula, the colder air on the Balkans has caused a decrease in temperatures on the Adriatic coast with strong northerly winds; instead, the Tyrrhenian side, ‘protected’ by the Appennines, has seen a little decrease in temperatures.

The minimum and maximum temperatures observed in the main italian cities on Christmas day, compared with the 1971-2000 average, are shown in Table1.

Table1. Minimum and maximum temperatures obseved this Christmas in Italy and compared with the long term (1971-2000) average. Values below the average (blue), values above the average (red).

We can see how Venice has observed low temperatures (-3 °C of anomaly in both minimum and maximum values), instead the cities in N.W. Italy (especially Torino) have shown maximum values well above the average (+7 °C of anomaly in Torino). Also the cities on the Adriatic coast (Ancona, Bari) have shown temperatures below the average, though only for the maximum values, instead Roma and Palermo (Sicily) were warmer than the average.

These weather conditions will last until Saturday, with the high-pressure still positioned on W. Europe bringing mild and settled weather on the peninsula (though foggy conditions are still possible in the Po valley with temperature inversion at surface). Then, from Sunday, a colder air mass coming from Scandinavia should cause a decrease in temperatures especially in northern-central Italy. However, it is too early to talk about it in details, especially regarding the likelihood of precipitation (e.g. snow).

Italy weather analysis Autumn 2018

In this article I am going to analyse the temperatures observed in Autumn 2018. I have extracted temperature data (minimum and maximum) for each main city in every region in Italy, and plotted the results. To notice that this is only a general overview, which doesn’t consider differences between mountainous regions and the plains or the local climate.

The anomaly is calculated with respect to the long term average 1971-2000 for each city.

It is clear how all the three months have observed temperatures anomalies well above the average (for both max and min temperatures) mostly everywhere. Only in southern Italy negative anomalies (-0.5/-1.0 °C) have been observed (especially in September); instead in the north (in particular in the N.W. Alps), the anomalies have reached +5 °C especially in September and November. This is strictly related with what found in the Autumn analysis in N.E. Italy.

In summary, the last Autumn has been very mild especially in the north, with continuos significant positive anomaly in both maximum and minimum temperatures due to the presence of the high-pressure positioned in the western Mediterranean (in September) and to the southerly winds (in October and November) which have brought lot of rain but very mild temperatures.


N. Italy: 10 °C of difference in 100 km?

Today the maximum temperatures in N. Italy have observed very different values between the north and south of the Po valley. In fact the temperatures observed in the afternoon (Figure1) were close to 10 °C between Veneto and Friuli Venezia Giulia, instead in Emilia Romagna and S. Lombardy the maximum values were close to 0°C, with some areas (e.g. Parma, Piacenza) observing negative values throughout the whole day.

Figure1. Maximum temperatures today 18/12/2018 (source: Meteociel).

The high-pressure system positioned east of Italy has contributed to this significant difference, causing a temperature inversion (colder air at surface) especially in the areas with snow at the ground (Emilia Romagna). The snow has increased the albedo effect which has caused a decrease in the observed temperatures (especially in the minimum values). The second element is the stationary fog (Figure2) in the south of the Po valley which has caused a decrease in the solar radiation (thus in the warming effect) during the day, increasing the temperature difference between the north of the plains and the south. The stationarity was caused by the (weak) easterly winds coming from Slovenia and blowing in N.E. Italy which have decreased the humidity in this area, and have ‘pushed’ the fog towards the northern Appennines. In addition, in the south of the Po valley, the (weaker) winds were mostly south-westerly, and the area where the fog ‘ends’ (limit between foggy conditions and clear sky) was at the convergence of the two different winds.

Figure2. Satellite image of today 18/12/2018 (source: Sat24).

Italy: first snow on the northern plains

The first coldwave of the winter has brought snow at low levels and in some areas (especially on the south of the Po valley) on the plains. Overall, temperatures have decreased towards value below the average, especially in central-northern Italy. The maximum temperatures observed on Sunday (Figure1, when most of the snow occurred) where below 5 °C in all N. Italy (except the Ligurian coast), Tuscany and Umbria. On the Alps and N. Appennines maximum temperatures were below 0°C. In central-southern Italy, the temperatures reached values between 10-13 °C and only in Sicily and Sardinia mild conditions were still observed (14-17 °C).

Figure1. Maximum temperatures on Sunday (Meteociel).

Snowfall has occurred mainly between Sunday afternoon and Monday morning due to a low-pressure system moving westward from France and colder air coming from the Balkans. However, only in few areas (mainly in Emilia Romagna) few centimeters of snow have been observed, instead elsewhere the precipitation was too weak to lead to some accumulation.

This is the first coldwave of the season, after the first 10 days of December when temperatures above the average have been observed. The next few days (until Friday, when milder air coming from the Atlantic should cause an increase in temperatures especially in central-southern Italy) will see still cold air coming from the east (though not as cold as the previous one) with minimum temperatures below 0°C especially in the areas with snow at the ground and maximum values slightly below or close to the average mostly everywhere. This will contribute to reduce the significant positive anomaly observed until last week.

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.


Italy: warm week before possible cold spell

Today and the next ones are going to be mild, with both minimum and maximum temperatures above the average, after the short cold period occurred during the last week of November. As we can see from the satellite imagery of this afternoon (Figure1), showers have occurred in central-southern Italy, especially along the Appennines, instead in the north clear skies were mostly dominant, except for some fog along the Po valley during the night/early morning. On the west (towards Spain) a high pressure system is moving eastward, bringing very mild (Tropical) air in the Mediterranean region.

Figure1. Satellite imagery of this afternoon (Sat24).

The temperatures are already well above the average (especially in central-northern Italy and in the Alps), with maximum values above 10 °C mostly everywhere and close to 20 °C in Sardinia (Figure2). Temperatures (both minimum and maximum) will increase also tomorrow and this condition should last until the weekend (when colder air is expected from Central-Europe).  The anomalies are between 2/5 °C above the average. Only in Emilia Romagna and extreme S. Italy the maximum temperatures are closer to the normal values for December.

Figure2. Temperatures in Italy this afternoon (Meteociel).

The next days the weather conditions won’t change, with still mild air coming from the mid-Atlantic and N. Africa causing only few showers (especially on the Alps and Appennines) and high temperatures (for this month) with the 0 °C level above 3000 m on the western Alps both tomorrow and Friday, and no frost at low levels is expected. Moreover, in some areas (especially in Sardinia and Sicily) the maximum temperatures might reach 20 °C, especially tomorrow.

From the weekend, colder air coming from central Europe should reach the Alps, causing unsettled weather in the north (snow above 800-1000 m), and leading to a decrease in temperatures towards the average values. 

For next week a colder air mass might reach the Mediterranean region, decreasing the temperatures towards values below the average, but it is still too early to talk about it in a detailed way.

November 2018 in N.E. Italy. Close to the record for high temperatures.

November (in Italy) has just finished with colder air coming from eastern Europe, which has caused a decrease in temperatures especially in the north.

However, these few days of temperatures below the long term averages are not enough to reduce the (significant) positive anomaly recorded in N.E. Italy during the last month. 

Related to the previous historical analysis made for Treviso, in this article I am going to analyse the temperatures observed during November 2018 and compared with the long term average (1981-2010).

To notice that the average (for both minimum and maximum temperatures) is computed on a monthly basis, thus the 10-day average is not considered (the first week of the month has a higher long term average temperature than the last one), which would have led to a more robust analysis to determine the significance of the daily temperature anomaly .

This analysis is focused on comparing November 2018 with the previous ones, considering the overall monthly average temperatures (thus between maximum and minimum values), and on computing the daily temperature anomaly in the past month in both minimum and maximum values, being conscius that its ‘significance’ cannot be quantified in a precise way because of the above mentioned limitation.

Regarding the daily analysis, in Figure1 the daily temperature anomalies (for both maximum and minimum values) are shown, considering a long term average of (4.6/12.7 °C).

Figure1. Max (top) and min (bottom) daily temperature anomaly referred to the long term maximum and minimum averages in Treviso. Positive anomaly (red), negative anomaly (blue).

It is clear how only in few days temperatures below the average occurred and the positive anomaly is observed especially in the minimum values. In fact, though we can’t exactly quantify the significance of this anomaly, the overall analysis shows how the positive difference from the monthly mean is not counterbalanced by negative anomalies. For the minimum temperatures, 13 days have recorded values 5 °C above the average, instead the same negative anomaly was never observed.

Regarding the maximum temperatures, the magnitude of the positive anomaly is lower. This might be due to the overcast conditions observed in the first week of the month, when the southerly winds caused prolonged rainfall events limiting the solar radiation and thus, an increase in the maximum temperatures.

However, November 2018 (in Treviso) has been the second warmest since 1973 (Figure2). 

Figure2. Comparison of the deviation in the average November temperatures from the long term average (1981-2010). Deviation equal or above 0.5 °C (red), between -0.5/+0.5 °C (green), below -0.5 °C (blue).

The plot shows the temperature deviation for each November and it is clear how November 2018 has the second highest anomaly (+2.79 °C). The warmest November occurred in 2014 (+3.84 °C) and the third one in 2002 (+2.74 °C). Interesting, as mentioned in a first analysis , how since the 00s there are no months observing values below the average (deviation bigger than -0.5 °C), with most years observing a significant (positive) anomaly.

November 2018 has shown as average minimum and maximum values : +8.5/ 14.4 °C, thus, respectively, (+3.9/+1.69 °C) above the long term average. This confirms what shown previously with the daily analysis, where the most significant anomalies are observed in the minimum temperatures.

We will see how December will be, though for the next few days no cold spells are expected and temperatures will increase (again) towards values above the average, even if without reaching the positive anomalies observed in November.