An alpine tree line is the highest elevation that sustains trees; higher up it is too cold, or the snow cover lasts for too much of the year, to sustain trees. The climate above the tree line of mountains is called an alpine climate, and the terrain can be described as alpine tundra. Treelines on north-facing slopes in the northern hemisphere are lower than on south-facing slopes, because the increased shade on north-facing slopes means the snowpack takes longer to melt. This shortens the growing season for trees. In the southern hemisphere, the south-facing slopes have the shorter growing season.