Summer 2023 – the balance

Temperatures at the absolute top

The meteorological summer of 2023 came to an end on Thursday, and it was one of the warmest since records began. True, it did not come close to 2003 (1st place) and 2022 (2nd place). But for a long time it was ahead of 2015 in 3rd place, and after the cool end of the month it now ranks just behind summer 2019 in 5th place. Across the country, the summer was 1.7 degrees too warm compared to the new climate norm for 1991 to 2020.

The foundation was already laid in June. This month was practically consistently warm, but brought only a few hot days with more than 30 degrees. In the end, it was 2.6 degrees too warm nationwide – the second warmest June since records began. The first heat wave followed in July, with highs of between 33 and 36 degrees across the country between July 9 and 11, and even hotter temperatures of 36 to 37 degrees in Geneva and Chur. In Chur, 37.6 degrees were reached on July 11, the second highest value ever measured at this station. Moreover, this remained the Swiss maximum for the current year well into August. Until mid-month, the nationwide temperature surplus was still over 2 degrees, but then the temperature level declined.

With a short interruption (August 2 was very warm to hot), the first decade of August also remained distinctly cool. On August 6 and 7, highs were mostly well below 20 degrees, and the snowline temporarily dropped to 2000 meters. In the high Alps, plenty of fresh snow fell. Then, however, the weather situation changed again and it quickly became warmer and warmer. In the west, a pronounced heat wave set in from August 12, while in the east it began 1 to 2 days later. It was exceptional in many respects. It was late, with a large part of it occurring in the second half of August. It was very long, in Geneva there were 15 heat days in a row. This is, together with July 2022, the second longest uninterrupted series – only in July 1983 there were 16 heat days in a row. And it brought very high temperatures for the advanced time of year. On August 24, Geneva virtually pulverized the highest temperature in Switzerland up to that point (37.6 degrees in Chur). Supported by southwesterly winds, 39.3 degrees were reached – August record and the second highest value both at this station and generally on the northern side of the Alps. There were also new monthly and all-time records at many other stations.

Another superlative occurred in the night of August 20-21, the radiosonde launched in Payerne registered a zero degree limit of 5298 meters. Also a new record! At the end of the month, the temperature level was then partly far below average.

 

From great drought to torrential rain

June was too dry practically throughout the country, but the dryness was particularly pronounced on the northern side of the Alps. In July, however, the pattern began to shift, rain showers and partly heavy thunderstorms brought spurts of precipitation, which, however, were rather inhomogeneously distributed due to the convective character. In the far west, on the other hand, it remained clearly too dry. In August, there was also rain here from time to time, but the bottom line remains a clear deficit even after the third month of summer. The picture is different between Upper Valais, Ticino and Lake Constance. Already in the first decade of August, abundant rain fell in the east, and in the second half of the last decade, a lot of rain fell again. High water rivers, floods and landslides were the result. All details can be found here.

Across the country, the summer of 2023 was actually a bit too dry – this was mainly due to June. However, there is a strong contrast between the west and the east and south. Locally, precipitation totals were well above average. However, this averaging somewhat blurs the real problem, in fact, this summer went from one extreme to the other!

Strong thunderstorms!

Extreme weather changes and high temperatures are also the breeding ground for thunderstorms. In June they were rare, but in July and August there were staggered true thunderstorms with hail, gale-force winds and hurricane-force gusts(217 km/h in La Chaux-de-Fonds) as well as extremely large amounts of lightning. In the last three months, there were over 360,000 lightning discharges in Switzerland, which is over 100,000 more than in 2022! In the summer of 2021, however, there were 440,000 lightning strikes.

Most lightning discharges occurred in the large cantons of Graubünden and Bern, but the highest lightning density was in the north and east!

Below are a few more facts about lightning:

• The strongest lightning was registered with 322 kA on July 4 at 23:47 over the municipality of Ilanz
• The canton with the highest lightning density is Appenzell Innerrhoden (14.6/km²)
• The day with the most lightning in Switzerland was July 12 (59,845 lightning bolts).

Note: The lightning data are taken from the world's leading and highly precise lightning measuring network LINET by nowcast.

Sunshine duration almost within the norm

June brought new records for sunshine duration in many places, in the other two months this surplus was reduced bit by bit. In the Alps and in the south, sunshine was within the range of the norm or only slightly above it, in the north it shone 10% to 18% more often compared to the long-term average.