2.8.1.b What preventive measures can be taken against hail?

Cannons

One preventive measure is to use anti-hail cannons to generate shock waves. These consist of an explosion chamber and a conical diffuser. They prevent the enlargement of hailstone embryos by crystallizing the outer layer of supercooled water, thereby preventing them from agglomerating. The cannons use a mixture of explosive gas (acetylene or butane) and air. This type of protection is not, or only marginally, effective on hailstones that have already formed, so it must be triggered in advance, 5 to 20 minutes before the first hailstones fall. However, the success of such an operation tends to be subject to too many variables to be considered reliable. The effectiveness of anti-hail cannons has not been scientifically demonstrated.

Cloud seeding

A well-known method to prevent hail formation is cloud seeding using certain compounds.

• Silver iodide: seeding clouds with silver iodide can reduce hailstone size by multiplying the freezing nuclei in the clouds. The result is a larger number of smaller hailstones, thereby reducing the intensity of the impact and damage to the vineyard. It consists of a generator that has to run for several hours, representing an approximate cost of 8€/ha/year and requires the use of radar to predict storms. The silver iodide used to be delivered using hail rockets which have now been banned, so other seeding techniques are used such as dispersal by plane. As with the previous technique, in addition to the associated dangers and ecological issues that should not be overlooked, its effectiveness appears to be somewhat random.

• Hygroscopic salts: another seeding technique, these provide an alternative to silver iodide. Helium balloons carrying a pyrotechnic torch containing hygroscopic salts are projected into the storm cell. This method reduces the number but not the size of the hailstones. The hygroscopic salts produce condensation nuclei which accelerate rain formation and reduce the formation of hailstones. The use of balloons also requires weather radar to warn of the presence of a thunderstorm cell within a few dozen kilometres.

Hail nets

This is one of the most reliable systems on the market and the only means of protection recognized by insurance companies in France. The installation costs are significant. These nets are frequently used in vineyards with table grapes, for example in Moissac (South-West France) or Argentine vineyards (Mendoza region), to protect the vines against hail that destroys 20% of the harvest every year. Two types of nets exist:

• Horizontal nets for vineyards where the row spacing is greater than three metres. These nets are not authorized in France.

• Only vertical single-row nets or row-by-row nets are authorized in France. The mesh size of a few millimeters prevents hailstones from passing through and damaging the grapes and foliage. These nets are made of polypropylene or polyethylene and can be reinforced with braided wire. Mechanisms exist that enable the nets to be rolled up for canopy management work and harvesting. These systems are installed at the beginning of the growth season until the grapes are ripe and there is no longer a risk of hail. However, depending on their colour and the material, these meshes can impact photosynthesis and therefore the production of sugars by the plant. The vines' health must be monitored more closely, as the nets can also restrict the circulation of air. The application of spray treatments may also be affected. Winegrowers can be put off by the major investment required, which can range from €10,000 to €25,000 €/ha, and then be tempted to consider other solutions (hygroscopic salt balloons for example).