Hailstones, once formed, can have various sizes, from a few millimetres to several centimetres in diameter (from the size of a pea to a tennis ball). The larger the size, the more severe the damage tends to be, which can consist of the destruction of leaves, damage to bunches, damage to the structure of future shoots and buds, etc. Hail can impact harvests for several years.
2.8.1.a How can hailstorms be detected?
Early detection of a hailstorm can allow time for the implementation of anti-hail protection systems. Detection is possible:
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Using pulse weather radar. This type of radar emits very short pulses followed by a much longer dead time to "detect" the return echoes from the sky (precipitation or flying objects). Only Doppler radars can detect hail signals in a thunderstorm cell with certainty. In order to have an accurate local weather forecast in real time, it is possible to install a weather radar, on an individual or group basis, to predict whether or not an approaching thunderstorm is likely to contain hail.
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Using weather warning services. There are many structures and websites offering warning services. It is possible to receive warnings by email or SMS to a mobile phone. These services are based on information coming from Doppler weather radars, which are a reliable means to detect hail but sometimes not very accurate regarding the location, duration and time of the hailstorm. For example, a hail storm forecast at a precise time may be delayed.
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By visual recognition of clouds (cumulonimbus) likely to carry hail. An assessment of the hail risk can be carried out by observing their shape (anvil-shaped at the top), by their thickness (at least 5,000 metres), by their electrical activity (presence of numerous lightning flashes), by their precipitation levels (generally heavy rainfall) and by the colour of the sky (slightly green tinge). Historical and empirical knowledge of extreme weather conditions can help predict the risk. However, this is a less reliable method than the weather radar.