Why are south-facing slopes warmer in Northern Hemisphere mountains?

In the Northern Hemisphere, south-facing mountain slopes are warmer because they receive a greater amount of incoming solar energy per unit area compared to other slopes.21

Because the Earth is tilted on its axis, the angle at which sunlight strikes the surface varies by latitude and aspect. In temperate regions of the Northern Hemisphere, slopes facing the Equator (southward) are oriented to receive more direct solar radiation. This increased receipt of solar energy creates distinct microclimate variations, leading to higher temperatures that can influence the timing of snowmelt and the subsequent development of vegetation. Conversely, slopes that receive less direct sunlight remain cooler, which can affect factors such as the presence of permafrost.21