Why do mountaineers often begin climbing snow slopes early in the day?

Mountaineers often begin climbing snow slopes early in the day because it is desirable to traverse the terrain while the snow is in a hard, stable condition.1

As the day progresses, the sun’s heat causes the snow to melt, which can significantly alter its consistency and increase various hazards. By starting early, climbers can complete the most arduous parts of their ascent before the warming effects of the sun make the snow soft, less stable, and more prone to avalanche danger. Careful consideration of these changing conditions is a fundamental aspect of sound judgment in mountaineering.1