Welcome to the first in a climatology of thunderstorms series that will later examine other highly populated regions including North America, parts of Asia, and Oceania.
Lightning strikes. Destructive winds. Flash flooding. Damaging Hail.
These are all threats posed by thunderstorms that may cause economic loss one way or another. The insurance industry refers to them as perils.
Every year, damaged infrastructure and delayed work impart hefty costs to unfortunate businesses hit by thunderstorms. Concerningly, this has recently started to happen more often and on a bigger scale.
In 2015, Munich Re facilitated an impressively in-depth study into insurance losses 1980-2014 caused by natural disasters. They found that thunderstorms and related (convective) events have seen the largest increases in both frequency and amount of loss.
In Europe, the annual number of recorded loss events was always below 80 up to 2005. During the following 9 years, 8 brought more than 80 and 3 delivered more than 100.
To comprehend this mutating risk, it pays to know where the baseline sits. In this case, we’re taking a look at the most recent ‘standard length’ (30-year) baseline available; 1991-2020.
Winter: Sparky Southeast
When somebody says ‘winter’, most Europeans don’t think ‘thunderstorm’ – and rightly so – but down in the far southeast of the continent, it’s a different story.
Adjacent to the balmy waters of the eastern Mediterranean, winter can be quite thunderous. That warmth is the key, as it contrasts with much cooler air above it.
You see, this leads to a strong rate of temperature decrease with height – and thunderstorms love that. On top of this, the sea provides ample moisture, boosting deep (tall) cloud formation. This ripe winter environment spans the Mediterranean from the Tunisian coast to that cosy corner to the east of Cyprus.
Elsewhere, thunderstorms are rarely observed at any single location. I’m being that specific for good reason, though. Winter thunderstorms tend to be very hit-and-miss, so if you focus on just one spot, you’ll observe far fewer than if you consider an area of, say, 10 miles around you. Thunder is typically audible within about that range of a lightning bolt.
Spring: Energetics Advancing Northward
The atmosphere becomes increasingly energised during the spring, as the sun shines longer and stronger.
Being of high specific heat capacity (SHC), the seas are slow to respond; It takes a lot of energy to warm them by each degree Celsius.
Meanwhile, most of the land responds much faster to the sun’s rays, thanks to relatively low SHC.
This means that as spring goes on, we increasingly see the strongest vertical temperature gradients occur over land, not sea. The thunderstorms follow suit.
This change is most pronounced where cooling Atlantic winds struggle to reach; in the central-south and east of the continent.
Notice, though, what happens in many locations along the Mediterranean coasts. There’s a slight decrease compared to the winter season! This is due to the air warming faster than the sea, reducing the vertical temperature gradient.
For most of the northwest and Scandinavia, thunderstorms remain infrequent (but with the caveats expressed earlier). In many spots, anything more than a ‘thundery shower’ (one or two rumbles) tends not to be seen until at least mid-April.
Summer: Buzzing Mainland
In summer, atmospheric energy is sky high and in many parts of the mainland, thunderstorms need little excuse to kick off.
Sometimes, they’re severe and organised into lines spanning hundreds, even thousands of miles.
No surprise, then, that the hit rates take a major step up from those of spring.
East of a line from Paris to Lyon, most places tend to see 4 to 8 ‘thunderstorm days’ per month. A few see as many as 10, while Venice earns the ‘thunderstorm hotspot’ badge with 10 to 15 per month.
By contrast, the southern Mediterranean is even quieter than in spring. Not only is the atmosphere even warmer relative to the sea, but a large expanse of mostly stable conditions typically expands northward from northern Africa. That’s what brings those almost dependable blue sky days that characterise the summers of places such as the Balearic Isles and Malta.
Then we have Iceland, Scandinavia, the UK, northwest France, and western Spain. Activity does pick up in these areas but remains largely in the 0 to 2 days per month range. For most, this is down to the cooling influence of the Atlantic Ocean. In Scandinavia, weaker sunshine plays a part too.
Trends Over Time: Regionally Varied
Things aren’t static, though. In the eastern half of Europe, climate warming is increasing the number of days with sufficient atmospheric energy for thunderstorms (see below). Those countries are having to adapt for a more electrified future. This August 2019 study suggests that in the decades to come, the increase will accelerate in the northeast, with thunderstorms becoming 2-3 times more frequent!
In Northwest Europe, the warming atmosphere’s having the opposite effect. Again, this has much to do with reduced vertical temperature gradients as the ocean and seas warm more slowly.
Random variability may also have played a part, though, as the region endured a run of mostly cool summers from 2007 to 2012. The past few summers have been strikingly hotter and more thundery.
Autumn: Stabilising Ahead of Winter
This season, the sun’s influence wanes and for most of Europe, temperature gradients drop, meaning far fewer thunderstorms.
Not so down by the Mediterranean. Here, the waters hit their annual peak around September and only gradually cool during the remainder of autumn. That higher SHC compared to land works both ways; the waters take more energy to warm and must release more energy to cool.
So, we have a similar setup to the winter – relatively cool air over warm water. Except, the sea is much warmer, so evaporation rates are higher. As a result, autumn brings the highest thunderstorm risk of the year for most Mediterranean locations.
Thunderstorm Risk: Sensitive to Large Water Bodies
Overall, it’s clear that proximity to oceans or seas has a big impact on how thunderstorm risk behaves across the year.
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James Peacock MSc
Head Meteorologist at MetSwift