- Climate change is going to lead to fewer, but larger hailstones.
- Stones are now the size of tennis balls.
- Bordeaux is one of the regions that will be most affected.
On June 20th as the city of Bordeaux hosted a conference that was largely focused on climate change. Some 7,000ha of vines, other agriculture, forests, towns and villages were hit by hail.
Hail is hardly a novelty on the list of challenges faced by winegrowers, but, as the local newspaper, la Dépêche reported the ferocity of the storm took the region by surprise.
There were hail stones the size of quail’s eggs, ping pong balls and, most dramatically, tennis balls - large enough to break car windscreens and solar panels.
This storm, which affected 4,000 ha of vineyards in Bordeaux followed others that hit 7,000ha in a wide range of regions at the beginning of the month, and preceded ones in Burgundy, the south and south west on the 23rd.
Not enough protection
As Vitisphere reported on June 22nd, while regions like Bordeaux are equipped with anti-hail generators that seed the clouds with silver iodide, the eficacy of these is limited, even when forecasting allows them to be deployed three hours before the storms are due. Bordeaux currently has 138 generators and, local experts say, could do with 40 more. None are currently situated to protect forests or the city of Bordeaux, but these gaps in the protective umbrella enable the storms to regain strength. Severe winds can also exacerbate the damage caused by the stones.
In any case, even the best protective measures are limited in their effect. Research in France and Spain suggests that, at best, “the hailfall energy of the most severe hail days “ can only be decreased by “about 50%”.
Other research into the impact of climate change on hail, suggests that larger, more damaging stones are becoming more common, globally. Some 70,000 homes in Calgary, Canada were hit by a storm in July 2020 that included similar tennis-ball size stones to the ones seen in Bordeaux. The cost of the damage was estimated at just under a US$1bn. Hail damage in the US costs arount ten times that sum every year.
More complex than many imagine
It is often imagined that hailstones are frozen rain drops, but they are far more complex than that. In fact they are created as frozen droplets that start to fall from a cloud but are forced back by an updraft. As they rise they meet and chill liquid droplets and combine with them to make larger stones. This process of falling, rising and growing continues until the updrafts are too weak to lift stones which are, in turn, too weighty to be held within the cloud. The more moisture in the air, the faster the stones will grow, and the stronger the updraft, and the higher the stones are carried, the larger they will grow. According to US National Oceanic and Atmospheric Administration, updrafts of 103km/h (64 mph) can support a 42.7mm diameter golf ball-sized stone but one that is 27% faster will carry a one of 76mm - larger than a tennis ball.
A 2015 paper by Dessens, Berthet and Sanchez, worryingly suggests that, between 2000 and 2040, while the frequency of hailstorms will remain unchanged, conditions in Bordeaux will lead to a decrease of 12% in the number of stones and an increase of 40% in their kinetic energy - and potential size. The implications for grape growers of fewer, larger stones are not yet clear, but, given the current limited efficiency of anti-hail generators, the trend has to be worrying for them - and incentivising for anyone looking to come up with a better solution.