The precipitation that occurs in connection with thunderstorms in the form of ice balls or lumps of five to 50 millimetres in diameter (rarely even larger) is produced in rapidly rising, humid air streams. Depending on their size, the hailstones can perforate or completely destroy the leaves and damage the shoot tips, which in extreme cases can lead to complete defoliation. If large parts of the foliage and thus the assimilation area are affected, the growth and development of the vine is impaired, which can also affect the next harvest. A possible consequential damage caused by defoliation is, for example, grape wilt caused by sunburn.
At the time of blossoming, hail is particularly feared and leads to trickling due to poor fertilization. In extreme cases, hail can completely destroy entire harvests in a short time. During the ripening period even a light hail can damage the berries, especially red grapes are sensitive. These injuries favour the infestation by bacteria, harmful insects and fungi. Under certain weather conditions, white rot (hail disease) caused by a fungus can occur. See also other negative weather influences under drought, frost and wind, and generally on the subject under climate.