In part 1 (1), motivated by an avalanche story, we saw that avalanching snow would hardly increase its temperature from all the tumbling. In fact, it would, in falling down some elevation X, warm up by an amount that would be about half of that of the increase in local air temperature at the lower elevation. So, in a sense, you might even argue that the avalanching snow has cooled relative to the surroundings. Even more surprising, you might think, is that even if the snow had been at the melting temperature before avalanching down, only a tiny fraction of it would melt, even if falling down 1000 m.

However, going back to a story about the debris-filled avalanche in the Himalaya, consider now a boulder tumbling down a mountain. It hits some snow and comes to rest. As with the tumbling snow case, the energy the boulder had above will transform to heat energy when it comes to rest. How much snow will melt?

A maximum possible amount can easily be estimated by assuming that the tumbling does not slow the boulder, the boulder itself does not heat up, and the snow is already at the melting point. The amount of melt will be in proportion to the size of the boulder, so we look at the fractional volume of snow that melts (volume of snow divided by volume of the boulder):

As you can see from the table, the boulder is more effective at melting the snow than just the tumbling snow case considered in (1). This result seems reasonable given that a boulder is denser than ice and thus has more potential energy. Even so, relatively little ice will melt.

The reason for the latter is the large amount of energy needed to melt ice. For example, to melt a chunk of ice that sits right at the melting temperature equals the amount of energy needed to warm the ice from a much cooler temperature to the melting point.

How much cooler, you might ask?

Well, about 285 degrees Fahrenheit (159 degrees C). So, if you are out in the winter and need to melt some snow at -20 C, and you've used half your stove's gas just to get it to -1 C, you might think that you are nearly there. But in fact you will need to get a lot more gas for your stove before you can even think about melting all of it.

--JN

References and notes

1. Avalanches, Part 1: Snow Hardening
https://www.storyofsnow.com/blog1.php/avalanches-part-1-snow-hardening

Why does snow seem to harden after an avalanche?

Those unfortunate few who have been buried in an avalanche often observe the snow, once stopped, has hardened. Some liken it to concrete. Whereas the powder before the avalanche seemed light and easy to move, once buried, the victim finds it impossible to move.

I have never had that experience, though I find the observation reasonable. For example, on a recent hike up a snow-covered hillside, I used snowshoes to avoid "postholing" through the snow. But when I came to a region where snow had slid down, I removed the snowshoes and easily walked on the surface.

You can see in that picture that considerable debris was present in that case, though it is unimportant for the present discussion.

An article I read last year about a similar event, yet on a vastly larger scale, reported quite the opposite: the ice melting by its tumbling fall down the mountain. Here is the relevant excerpt:

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