If you live in place where it's rainy and cloudy most of the time, you're probably used to seeing snails out and about during the day. In most other places, however, they hide away during bright daylight hours and come out to eat after dusk or during the early dawn in order to avoid bright sunlight and heat.
Snails are rarely seen out and about in bright sunlight. They prefer places that are dark, or at the very least shady. During the day snails are usually found under rocks or leaf litter, in hollow logs or on the underside of leaves. Some snails even burrow in the ground to get out of direct sunlight. If a snail can't find shelter on the ground, he can use a special mucus to attach himself to the side of a wall, tree or plant where there is enough shade for him during the day.
One of the reasons snails enjoy dark or shady places is because they cannot tolerate the heat. Dry heat can kill a snail, so he'll do his best to avoid it. The higher the temperature, the more important it is for a snail to remain in a dark, cool place. Snails that live in the desert will enter a state similar to hibernation during the hottest months, returning to normal activity when temperatures drop.
Snails are just as intolerant of dry conditions as they are of high temperatures. Without enough moisture, a snail's body can dry out and he will die. Since dark places are the last to dry out, snails seek out shelter that is both dark and damp. If necessary, he can also seal his shell with mucus to help retain moisture, but this is much more effective if there is also moisture in his daytime hiding place.
Even though snails typically hide away during the daytime, they will emerge during certain weather conditions. Heavy cloud cover and cool temperatures, for example, will entice a snail into the daylight. Rain often has the same effect. Snails that burrow underground to avoid light and heat usually emerge as soon as it begins to rain, burying themselves again once the sun comes out or the humidity level drops.