The beauty of a sunny winter day can be hard to beat; unfortunately these conditions can be harmful to trees.
Sunlight can significantly warm a trees bark and outer wood, causing it to expand. When the sunlight is blocked by clouds or lost at nightfall the tree quickly cools, causing its bark and outer wood to contract. These sudden fluctuations can result in vertical fissures in the trunk called frost cracks.
Sun scald is caused by the same conditions but is a different kind of damage. During the winter the cells of a tree go into dormancy, but this can be interrupted by sudden warming from direct sunlight. When the cells are awaken by the warmth they can be killed when temperatures drop back below freezing.
Young, thin-barked trees planted out in the open are especially susceptible. However, these types of damage can be prevented by using white trunk wraps to deflect the sun's rays or planting smaller plants around the tree to help shade its trunk.