The reason is that the winter sun, which shines more strongly towards the end of the season, warms up the dark tree trunk again. This causes the tree to begin pumping sap from the roots into the tissue. If the night-time temperature then cools down the tissue to temperatures below freezing, tension in the tissue results, which may tear. This damage may be latent and may therefore be unidentifiable for a long time. However, it may also occur in the form of large dead areas of bark or in thick cracks in the bark on the southern side of the tree.
You can avoid such winter damage by applying a coat of whitewash to the trunks of (fruit) trees from late autumn as a precaution. Begin painting immediately above the ground and continue up to the lower, thick branch areas. The trunk takes on a white colour, which reflects the sunlight to a great extent, meaning the otherwise dark trunk does not heat up as easily and to such a high degree. Frost cracks are not completely prevented by this measure but are significantly reduced. It is particularly advisable to paint the trunk in exposed, wintry locations. With increasing rainfall, the whitewash coat is washed off again and seeps into the soil, together with the rainwater, as liming. It is therefore recommended, with lime-sensitive quince trees and pear bushes which are grafted on quince, to forego protecting the trunk with lime every winter and instead to loosely fix reed mats around the trunks as protection against the sun during the dangerous period.
Tip: If you free the tree trunk of loose pieces of bark, green algae and attached moss with a wire brush before coating, together with the lime coating you reduce the likelihood of the tree being infested with hibernating parasites, such as greenfly or spider mite eggs.