What causes the bark on the sunny side of my young tree is cracked, sunken, and peeling off?

Winter sun, not summer heat, despite the name. On a clear cold day the sun warms a dark trunk enough that the tissue beneath comes out of dormancy; when the sun drops behind a building or the horizon, that tissue refreezes within minutes and the cells rupture. The southwest face takes the most intense low-angle afternoon sun and the steepest drop, which is why the injury lands there, and snow reflecting light back onto the trunk makes it worse. Young trees are hit because their bark is thin and unfurrowed. A tree stripped of the low branches that shaded its own trunk, or moved from a crowded nursery row into open lawn, is at highest risk. The exposure window in central Indiana runs roughly November through March.