Why is it possible to sunbathe in the sun in the mountains at the same time (sometimes t = + 25оС) and ski?

For the transition from solid to liquid, the entire mass of a solid must be heated to the melting point.
Ice and snow begin to melt (melt) when their entire mass heats up to 0 ° C. In the mountains, where the mass of ice and snow is very large, it does not heat up to a temperature of 0оС during the day, despite the rather high air temperatures (sometimes t = + 25оС). Therefore, on such days, you can sunbathe in the sun and go skiing or sledding. But if part of the snow or ice “breaks away” from the bulk of the glacier, for example, finds itself on a boulder, it will quickly heat up in the sun and melt.
Answer: The mass of ice and snow does not heat up to the melting point.