Snails and slugs come out when it rains because they need a high degree of humidity to carry out some important physiological activities, such as feeding and reproduction. They also make small holes in the ground to lay their eggs, a task that is made more difficult by hard, dry soils.
Although rain is very important for the above-mentioned tasks, snails and slugs can survive in more adverse weather conditions. When they live in dry or very dry climates, they take advantage of the humidity of a cloudy day or at night, when there is dew. They can carry out these activities always close to the place where they are hiding.
However, although rain is directly related to the appearance of snails, they do not usually come out while it rains, but afterwards, because during the rain both water droplets and hail could kill them on impact.
Answered by Alberto Martínez Ortí, professor of parasitology at the University of Valencia.
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