Updated: Jan 12
It might not be something you typically think of, but during cold spells when everything is freezing these conditions can be lethal to snails who are mostly made up of water. In order to prevent this from happening Snails make a dried lid of slime over the entrance to their shells, sealing it for several months. To keep as warm as possible, snails will typically hibernate in tight gaps away from wintry gusts and where frost can get in. Snails often hibernate in groups as can be seen in the photo below.
In a similar process of retaining moisture, during very hot times snails may also estivate. When snails are doing this they can usually be recognised by a rather brightly coloured shell, reflecting the sunlight to minimise evaporation.
You may also find this article 'How do snails cope with the winter freeze' on the BBC Earth website of interest.