The physics of melting snow and ice

[Hypsibius]

A recent hike up in the mountains reminded me I'd like to understand better the physics behind snow and ice, in particular the combination of melting snow and ice similar to what I have encountered. 

As I walked, the impression I got was that the surface/outer layer was frozen (the surface was hard enough to support my weight), while the snow below that layer was softer: so if the icy crust cracked and collapsed under my weight, I hopelessly sank in the snow below. 

Was this observation correct in regard to the phenomenon? If so, can you explain why is it like that? 

Also, does snow begin to melt on the surface (because it's directly under the sun), or is the snow that is closer to the soil that melts first? Why? 

And why does snow around the base of trees melt faster? (there was no snow around the base of the trees, but plenty elsewhere).

Thanks in advance for any educational input you can provide. 

[Bender]

The snow on top can melt in sunshine or simply because it is no longer freezing. If it starts to freeze after that, the wet layer on top becomes ice, which is harder.

When the soil is not or only supperficially frozen, the heat of the ground can melt the ice on the bottom.

Either way, the snow in the middle won't melt for several reasons:

- snow is a good insulator, so heat can't get to the middle.

- it takes a lot of energy to melt ice/snow. Melting the top or bottom helps keeping the rest of the snow cold by absorbing heat.

[ReductiMat]

Did you know that the temperature from the top of the snowpack to the ground typically goes from air temperature to zero (C)? 

 

And that persistent cold air temperature results in snow faceting, which rots the snow near gradient closer to "warm"?  The "fresh" snow above that is ready to go and make your life hell.

 

There are a number of factors that can create the phenomenon you experienced.  Sun or wind or both.  If you are on skis when this is happening, life sucks.

 

In Canada we have AST courses that go into all of this combined with behavioral psychology as we have a tendency to seek out the worst places to play.