eli5 How does ice and snow melt when its below freezing.

Question

like not melting to water but an ice cube left out side will slowly get smaller even when its super cold out.

In: 5

Answer 1

There is a misconception that people have that below the freezing point ALL the water will be a solid. Above this and below the boiling point ALL water will be liquid. Above the boiling point ALL water will be gaseous. This is incorrect (I’m assuming just water here, but it works for all the other substances too).

If you look at an ice cube its just a bunch of H2O molecules that on average have such low energy that they pack themselves for the MOST part into a crystal structure. Some of the H2O molecules on the surface may get hit by molecules in the liquid or air surrounding the ice. These molecules may gain enough energy from these collisions to escape their neighbors and become part of a liquid or gas surrounding the ice cube. Some H2O molecules may also hit the ice cube and stick to it. Pretty much all ice (even well below freezing) will have a very thin layer of liquid water around it. And if the ice cube is in a gas, there will be some H2O in gaseous form around it.

What you are describing with the ice cube getting smaller even when it is super cold is not really melting, but it is sublimation. Sublimation is a phase transition going directly from a solid to a gaseous form (dry ice is famous for this). Sublimation like evaporation or freezing is a macroscopic description of what is actually going on. If you zoomed in enough you’d more likely see the ice becoming part of the thing liquid film surrounding the ice cube and then becoming gaseous.

A great description of what is going on can be found in [Chapter 1 of Feynman Lectures on Physics](https://www.feynmanlectures.caltech.edu/I_01.html#Ch1-S1).

EDIT:

Although it doesn’t match the text in your description, you can also melt ice below its freezing point by putting it under pressure. This is what happens when you are skiing. Your weight pushing down on the snow causes a little bit of it to melt and creates a layer of water between your skis and the snow. This reduces the friction allowing you to easily ski down the slope.