While meteorologists from the Slovenian Environment Agency (Arso) measured one heat wave in most of Slovenia, which was one of the longest in the history of measurements, meteorologists across Europe defined two. The first one in the middle of the month (peaking between June 17 and 22) affected large areas of western and southern Europe, while the second one brought even more extreme temperatures and affected approximately the same area, peaking at the end of the month.

In Slovenia, we were under the influence of the same weather fronts, but around June 16th we experienced a slight cooling with storms in Istria and Primorska, so that in most Slovenian places at that time we did not really experience a heat wave, which is defined by high temperatures for at least three consecutive days. How high this temperature must be is defined according to the specific weather characteristics of each place.

Warmest start to summer in Europe

This June is the warmest June in Western Europe on record, with an average monthly temperature of 20.49 degrees Celsius, 2.81 degrees above the 1991-2020 average. It narrowly beat the previous June record from 2003 by just 0.06 degrees Celsius. June 2003 and June 2025 stand out as exceptional, as the next warmest June, in 2017, was 0.7 to 0.8 degrees Celsius colder.

Record temperatures at the end of the month

During the second heat wave, surface air temperatures exceeded 40 degrees Celsius in several countries, reaching as high as 46 degrees Celsius in Spain and Portugal. Both events were associated with persistent areas of high air pressure. So-called heat domes trap warm air and cause prolonged hot, sunny and dry weather. These weather conditions also lead to increased concentrations of ozone in the air and the risk of fires.

In Western Europe, temperatures peaked on 30 June, when the average daily temperature reached 24.9 degrees Celsius, a new June record. The same temperature was recorded on 1 July. This is one of the highest daily temperatures ever recorded in the region in summer, surpassed only by the peak heat of the season between mid-July and mid-August during the heat waves of 2003, 2018, 2019, 2022 and 2023.

Impact on well-being

In addition to high air temperatures, much of Europe was also hit by above-average felt temperatures and an above-average number of days with high heat stress in June. This is a combination of temperature and humidity, which makes it harder for the body to cool down.

Felt temperatures above 38 degrees Celsius, corresponding to very high heat stress, were measured across much of southern and western Europe, particularly in Portugal, Spain, France, Italy and much of the Balkans. Northeast of Lisbon, the highest felt temperatures reached 48 degrees Celsius.

According to a preliminary study by scientists at King's College London, 2,300 people died from heat-related illnesses in the 12 major cities they analyzed during the heatwave between June 23 and July 2. In Milan alone, the heatwave is estimated to have caused around 500 additional deaths. Climate change is estimated to be responsible for two-thirds of these deaths.

Precise data on the impact of high temperatures on human health is not yet available. Death estimates are statistical, as the vast majority of cases involve older people with chronic health problems. Researchers are comparing mortality data with previous years.

Tropical nights

Many parts of southern Europe also saw an above-average number of tropical nights – nights when temperatures do not fall below 20 degrees Celsius. Parts of Spain saw up to 24 tropical nights, 18 more than the average for June, and around the Mediterranean Sea between 10 and 15, while these areas typically have none in June. Tropical nights make it harder for the body to recover from the heat of the day, increasing the risk of heat-related health problems.

Sea temperature

The biggest deviation from long-term averages is seen in sea surface temperatures. These were exceptionally high in the western Mediterranean last month, further contributing to the intensity of the heatwaves. In the Ligurian Sea, the surface temperature exceeded 28 degrees Celsius, more than five degrees above average.

This year's sea temperature in the western Mediterranean  Photo: C3S/ECMW

The average sea temperature in the western Mediterranean began to increase sharply in early June, reaching a peak of -27.0 degrees Celsius on 30 June, the highest June temperature on record. It was 3.7 degrees above average, the highest daily anomaly.