Walker: We've had the perfect recipe for hoar frost recently

U OF I MASTER GARDENERDecember 19, 2013 

Frost on a Ponderosa pine seedling.

On Monday morning, the Treasure Valley woke up to frost on all our trees, shrubs and everything else. It was truly a winter wonderland!

After a foggy Monday, Tuesday’s frost was even more spectacular!

This type of frost is called hoar frost. How did we come to have such a beautiful show?

For this type of frost to form, the environment needs a lot of water vapor in the air. In our dry climate, we usually don’t have enough humidity for this type of frost to form. Since we had a nice thick fog roll in on Sunday night, we had the perfect moisture conditions for creating hoar frost.

The second ingredient needed for this type of frost is air temperature below freezing. Check!

The third requirement is an object with a surface temperature below freezing.

With the unusually cold weather we had the week before the fog, all our trees and shrubs were well chilled.

When those three conditions come into play, water vapor from the air freezes on to any cold surface it touches. Additional molecules of water vapor latch on to previously frozen molecules and soon enough, ice crystals form and grow.

The water vapor goes directly from a gas to a solid without going through the liquid state. If the water vapor went through the liquid state first (in other words, dew), frost crystals wouldn’t form. Instead, there would be a slick coating of ice on everything.

Hoar frost doesn’t necessarily require fog to form. If you’ve seen the early morning sun sparkle on top of snow, most likely what you saw was hoar frost.

During the night, moisture evaporating from below the snow’s frozen surface hits the cold air and ice crystals form. Shortly after the sun comes out, the crystals again evaporate into the atmosphere.

I’ve included a photo of a ponderosa pine seedling (about a foot tall) having a frosty hair day!

If you have particular questions about gardening you’d like to see addressed in this column, send them to highprairielandscapedesign@yahoo.com.

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