On Monday, April 28, 2025, around 12:30 PM local time, a massive power outage swept across nearly the entire Iberian Peninsula, cutting electricity for up to 50 million people in Spain and Portugal. In some areas, power was out for as long as 18 hours.
Right after the event, there was a wave of speculation. One early explanation suggested that extreme temperature swings in central Spain may have triggered a rare effect called induced atmospheric vibration on high-voltage lines — possibly leading to loss of synchronization in the grid. This theory was briefly linked to Portugal’s grid operator REN, but the company later clarified it had not issued such a statement.
More concrete findings came from ENTSO-E, the European association of grid operators. Their early technical review indicated that the outage began around 12:33 PM with a sudden drop of roughly 2,200 MW in power generation across the regions of Granada, Badajoz, and Sevilla in southern Spain. This loss was accompanied by a dip in grid frequency and an unexpected voltage increase — both of which put serious pressure on grid stability.
What made this situation particularly fragile was Spain’s energy mix at the time: around 52% of the electricity generated during those hours came from solar power, and renewables accounted for close to 77% of total production. While these sources are clean and essential for a sustainable future, they also make the system more sensitive to quick changes. Real-time balance is crucial. When things shift — a plant trips offline, a voltage level drifts — there’s little room for error if the grid lacks immediate storage or fast-responding backup.
In fact, at least two solar farms in southwestern Spain reportedly disconnected shortly before the blackout. Preliminary reports point to voltage irregularities as a possible cause — not weather-related damage, but likely instability within the grid itself.
So, what role did the weather play?
By most accounts, April 28 was an unremarkable day, meteorologically speaking. No severe storms, no organized weather systems, no standout warnings. From the surface, it all seemed… fine.
But that’s where it gets interesting.
What we often call “fair weather” can hide complexities — especially for solar power. Conditions may look calm, but certain types of clouds, like small cumulus, can form quickly and behave in ways that make them hard to model. They don’t bring rain or lightning. Yet they can cast sudden shadows over solar panels, dropping output sharply for a few minutes at a time — enough to disrupt balance if it happens across many sites simultaneously.
As part of my post-event review, I analyzed satellite images from the Meteosat Third Generation FCI instrument (EUMETSAT). Between 09:30 AM and 12:50 PM, eastern and northeastern Spain — where many PV installations are concentrated — saw a shift from clear skies to a mosaic of shallow cumulus clouds appearing occasionally. These formations, while visually appealing, could have triggered scattered, rapid fluctuations in solar output at precisely the wrong time.
Interestingly, what stood out to me was the spatial coherence of these cumulus clouds. Their simultaneous development across a broad region could represent a valuable case study in smart grid stability and protection. A localized cloud event is one thing — but when similar shortwave disturbances hit multiple nodes of solar generation at once, the impact on grid resilience becomes much more complex and worthy of deeper analysis.
Danish Meteorological Institute researcher Irene Livia Kruse independently confirmed the presence of the same cumulus cloud formations in her own satellite review. Her observations closely match mine, and support the idea that even small-scale, transient cloud development can be a serious operational challenge for solar-heavy grids.
On the meteorological side, I also checked instability parameters such as CAPE (Convective Available Potential Energy), lapse rates, and low-level moisture profiles. The values were not consistent with conditions favorable for deep convection or severe thunderstorms. In other words, the atmosphere didn’t look threatening — at least not in any traditional sense.
Did these clouds cause the outage? That’s unclear. But they might have been one piece of a much larger, more fragile puzzle.
This event is a powerful reminder that even “nice” weather isn’t always safe when it comes to renewable-heavy power systems. It’s not just storms and heatwave extremes that matter — sometimes, the ordinary days are the most deceptive.
If we’re serious about building a climate resilient, green energy future, we need to get better at forecasting all types of weather. And we need closer collaboration between weather services and grid operators — not just when something breaks, but in day-to-day operations and long-term planning.
Kutay Mihliardic
Co-Founder | WeatherX