Heat wave and drought in central and northern Europe
After the unusually cold February and March, Europe experienced an extensive warm period that lasted until the end of the year. European average temperatures remained higher than the 1981-2010 average for April to December. The highest monthly anomalies during that period were in April and May (+2.4 and +2.5°C, respectively) and the lowest in June (+0.9°C). Years with a long duration of positive anomalies have not been unusual recently; it has been the case in most years of the 21st century. However, many of these years had months with only slightly-above-normal anomalies, whereas in 2018 the anomalies were much larger. In addition, the above-normal temperatures were sustained for the whole period; there were only a few days when the European mean temperature dropped below the normal. From that point of view, it was a notable year with a very warm period that started in late spring, and continued throughout summer and autumn.
The period from spring to the end of summer was exceptionally warm in central Europe, with temperatures the highest recorded since 1950 and the anomaly more than 2.5°C above the 1981-2010 average. However, the warmth was not restricted to this region or this period. Large anomalies were found for the whole of northern Europe from late spring into autumn. When considering the spatial distribution (see European temperature) of the temperature anomalies, it can be seen that much of Europe was affected. From spring to autumn, almost the whole region saw positive anomalies in the three seasonal averages, although the warm area was smaller in spring due to the cold March.
The long warm period went hand-in-hand with an extended period of low precipitation for almost the whole duration. However, only parts of Europe were affected by the precipitation anomalies, and there were differences in intensity and duration. Most affected by the relatively low rainfall were parts of central Europe, especially Germany, the Czech Republic and western Poland, and also northern Europe in large areas around the Baltic Sea (Sweden, southern Finland and the Baltic countries). These countries all had seasonal precipitation totals less than 80% of normal in all three seasons (spring, summer, autumn), spanning the main growing and harvesting period. Both the extent and intensity of the dry area increased during the course of the year and was most developed in autumn, when a large area was affected extending from France in the west to Kazakhstan in the east and from northern Sweden in the north to Greece in the south. Locally, particularly in Germany, summer and autumn totals fell below 40% of the normal. Even local thunderstorms, which brought heavy rain and hail, causing flooding and widespread damage, could not substantially reduce the drought.
The impact of this long drought was substantial, not only through agricultural losses and water restrictions, but also through low water levels in rivers causing problems for shipping. The fish population in rivers was also affected by higher water temperature, reduced fresh water and lower oxygen content. The drought in central Europe did not end until December, but even then, it continued in parts of northeastern Europe (southern Sweden, Finland, and Estonia). The ’Spotlight on...’ sections highlight the associated variations in a number of key climate variables.
The following section highlights some features of the warm period, based on examples from June and July.
A blocked weather pattern over Europe was present in June, with a remarkably high surface pressure over the British Isles and the North Sea. This was the result of a strong ridge (elongated area of relatively high pressure) from the Azores via the British Isles into Scandinavia. The blocked weather pattern over Europe also continued into July, but then high-pressure systems were also located over northeastern Europe with exceptionally high anomalies in Scandinavia and northern Russia. Strong subsidence - a downward movement of air in the high-pressure system - led to strong warming in the mid-levels and the lowest levels of the atmosphere, with record-breaking temperatures in several places. The subsidence also prevented the formation of clouds and rain, leading to a precipitation deficit, while the cloudless conditions led to increased amounts of solar radiation reaching the Earth’s surface.
Warm days and nights in June
In central and eastern Europe, the number of warm nights in June was large (above the 90th percentile). In most areas of the British Isles, southern Scandinavia and the countries neighbouring the southern Baltic Sea, the number of warm ‘day-times’ was large as well.
Monthly averages of daily minimum and maximum temperature were exceptionally high over large parts of the mid-latitudes in Europe during June.
A comparison between the 2018 and 1976 drought
The warm and dry conditions over Europe, which started in spring, gradually built up to a drought. In addition to comparing this drought to an average year, it is also interesting to compare it to a year that had a similarly long-lasting drought. For example, the drought in 1976 was also long-lasting and - like the one in 2018 - started to build up during spring.
The Standardised Precipitation Index (SPI) is often used to characterise drought. The index shows that for June 2018 and June 1976, drought levels are similar. The spatial extents differ slightly, however, with dry conditions in June 2018 not extending as far south as they did in 1976. However, northern Germany, the Netherlands and particularly southern Scandinavia were considerably drier in 2018.
Heat wave and drought in northern Europe during July
The high-pressure pattern described previously resulted in a continuation of above-normal temperatures over most of Europe in July, except for Portugal, parts of Spain and parts of southeastern Europe. The high pressure also intensified the drought in large parts of central and northern Europe. Temperature anomalies in Scandinavia reached values of more than 6°C above average. On some days, the actual temperatures in southern Scandinavia were higher than in parts of Spain.
The long period of dry days in July 2018 can be seen in the map above, which shows the largest number of 'Consecutive Dry Days'. It highlights that southern Scandinavia, the Baltics, northern Germany, Benelux, most of the UK and Ireland saw a long stretch of days without precipitation. Dry summers with high numbers of consecutive dry days are common for popular tourist destinations in southern Europe, but not in central or northern Europe. This long-lasting drought together with dry vegetation resulted in conditions of ‘high fire danger’, and a large number of wildfires were recorded in Scandinavia, the United Kingdom and Ireland. Sweden invoked emergency assistance from the other European Union countries to help fight against the flames in their wide-ranging territory. Read more in ‘Spotlight on … wildfires’.
Most of Scandinavia, the British Isles and central Europe experienced extremely high maximum temperatures during July. In Norway, it was the warmest July (together with 2014) since 1900, when weather analyses began. In most parts of Sweden, too, July 2018 became the warmest on record. Scandinavia reported several new records for maximum temperatures. The heat wave in Scandinavia lasted in many areas for more than 10 days, longer than in any other part of Europe, as illustrated by the 'Warm Spell Duration Index'.