Dear student, For melting of snow the minimum temperature required is 5 degree celsius.When its 0 degree celsius at the ground, the temperature is lower at the mountain peaks because the atmosphere near the ground is denser and hence it accumulates heat easily.The snow at the surface of the mountain peaks will start melting as the temperature around the mountain peak reaches 0 degree celsius.But there is cooling of the snow at the surface due to the volume of snow thats present below the snow at the surface.That is why it appears that the snow at the mountain peak doesn't melt even at temperatures higher than 0 degree celsius. Regards