Ask Ellen: Why does snow shrink in the winter without melting?

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GRAND RAPIDS, Mich. (WOOD) — Let’s talk about a phase change process that rarely gets the attention it deserves: sublimation.

Sublimation is what happens when a substance, like water, goes straight from its solid form to its gaseous form.

We all know when water goes from a solid to a liquid, it is called melting. Heat is used to transform water from things like snow or ice to liquid. A phase change from solid straight to gas also requires heat from the atmosphere.

Phase Changes Diagram Courtesy: https://www.studyread.com/

Sublimation can be hard to spot outside with the naked eye, but it does happen. We can see the effects of it when you see a shrinking snowpack on days when the temperatures stay well below freezing.

Keep in mind that snow naturally settles as well. The snowflakes will change grain style and quality as they age. This can also cause a shrinking snowpack. In fact, back country skiiers and snowboarders frequently monitor ice grain type and age to determine how likely avalanches are to occur.

Sublimation is the silent process that helps wick the snow up into vapor and off the ground.

SEEING SUBLIMATION AT HOME

A good way to see sublimation for yourself is to hang a wet shirt outside on a subfreezing day. Over time, the shirt will turn to ice. Eventually, the ice content will lessen as sublimation occurs.

Another fun way to see sublimation is to use a different substance other than water. Dry ice is an easy example of sublimation. You can see as the frozen solid carbon dioxide changes immediately over to vapor.

You may be more familiar with the opposite process of sublimation. This is called deposition. We see it all the time in the winter. Deposition is when water goes straight from vapor to solid form. When this happens, it creates beautiful displays as frost.

Viewer submitted photo to News 8.

In extreme circumstances, it can stack up so many ice crystals that it takes on a heavily frosted appearance. This is called hoar frost, and it is also caused by deposition as water turns from vapor straight to solid in the cold.

Hoar frost, courtesy of Stacey Anne Leeson.

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