Storm systems and late snow event drag down irradiance in eastern and western Europe

Central parts of Europe delivered relatively strong conditions for solar generation through December 2025, as high pressure blocked Atlantic moisture and kept skies clearer than normal. In contrast, storm systems elsewhere reduced irradiance, and a late-month cold outbreak brought snow soiling impacts to the east of the continent. According to analysis using the Solcast API, Germany and neighbouring regions recorded the strongest irradiance anomalies, while Iberia, Scandinavia and Eastern Europe saw widespread underperformance.

Germany, the Low Countries, and around the eastern Mediterranean recorded the best PV generation conditions in December. Across Germany, estimated irradiance was 20-30% above the monthly average, with anomalies of 15% in the Netherlands, Belgium, and the Swiss Alps. Southern Italy, Greece and parts of the Balkans also saw irradiance levels 10-20% above normal. A negative North Atlantic Oscillation phase established a blocking pattern that diverted Atlantic moisture away from central and southeastern Europe, allowing for increased solar irradiance.

Meanwhile, much of the rest of Europe contended with persistent cloud and storm activity. Spain and Portugal experienced irradiance reductions of 15-20% below average, alongside similar deficits in Scandinavia and across parts of Eastern Europe. The British Isles fared slightly better but still recorded irradiance values around 10% below the norm.

The pattern was driven by persistent low pressure systems over the Atlantic coast, which brought successive storms. Storm Bram impacted the UK early in the month, followed by Storm Emilia, which brought flooding and strong winds to the Iberian Peninsula mid- month. Toward the end of December, Storm Joannes tracked over Scandinavia, further depressing irradiance while disrupting regional energy infrastructure.

Despite monthly mean temperatures 1–3 C above average across much of Europe, the arrival of polar air behind Storm Joannes during the final week of the month resulted in an abrupt cold outbreak across the east. This late blast of Arctic air delivered widespread snowfall to Eastern Europe, compounding the impact of low irradiance with significant snow soiling losses.

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Snowfall can significantly affect PV generation due to snow soiling, where snow accumulates on panels and blocks incoming sunlight. These impacts can be reduced through panel cleaning, so the snow soiling estimates presented in the graph represent a worst-case scenario assuming snow remained on the panels. Cities including Tallinn, Warsaw, and Zagreb had the potential for very high soiling losses during this period, with snow accumulation severely reducing irradiance at the PV panels. In contrast, Zurich experienced a relatively snow-free month, with soiling losses well below average.