Sunday, January 23, 2022

The Ice Hoar Cometh

Do you know the difference between Hoar Frost and Rime Ice? Most people don’t.

Rime ice is a deposit of ice formed during a freezing fog, often accompanied by high wind and temperatures well below freezing. The supercooled water droplets freeze rapidly when coming in contact with objects such as branches, fence posts, etc..


Rime ice definition from the American Meteorological Society's Glossary of Meteorology:

“A white or milky and opaque granular deposit of ice formed by the rapid freezing of supercooled water drops as they impinge upon an exposed object. It is denser and harder than hoarfrost, but lighter, softer, and less transparent than glaze. Rime is composed essentially of discrete ice granules and has densities as low as 0.2–0.3 g cm-3.

Boy, leave it to a meteorologist to take the romance out of everything. Here’s a cool snap of rime ice formed in high wind.

rime ice formed when weather conditions produce both freezing fog and strong winds at the same time.

So how does this differ from hoar frost? Again, let’s have the AMS take the fun out of it:

(Hoar frost) is a deposit of interlocking ice crystals (hoar crystals) formed by direct deposition on objects, usually those of small diameter freely exposed to the air, such as tree branches, plant stems and leaf edges, wires, poles, etc.

And a slightly less technical explanation from Cal Tech physics professor Dr. Ken Libbrecht.

Frost is to dew as snowflakes are to raindrops.  When water vapor condenses into liquid water, you get raindrops and dew.  When water vapor condenses directly into ice, then you get snowflakes and frost. Snowflakes are not frozen raindrops, and likewise, frost is not frozen dew.


Here’s a close up of the crystalline structure.

lacy hoarfrost (2)

My non-technical (and probably non-correct) distinction: rime ice forms spikes during freezing fog, hoar frost forms crystals during non-foggy but moist cold snaps. Most people can’t tell the difference and don’t care. They just enjoy the scene. This is a picture I took through the windshield on the way to Home Depot the other day. Given there was dense fog at first light that day I’m going with rime ice.

hoar frost on old ranch road3

It was fuzzy ice and it made everything it touched look enchanted. What more do you need to know?

As with all appealing things though, there is a downside to rime ice. It’s the same phenomena that causes you an additional half hour on the tarmac waiting for the deicing treatment before take off. without the deicing rime ice would attach to the plane’s wings while flying through super-cooled moist air and decrease the wings’ lift. Not good. And in the mountains a thick layer of rime ice makes it hard for fresh snow to form a good bond thus making slippage of the top layer more likely -avalanche!

Apologies to those of you who thought this post was going to be about Kamala Harris just because she once compared ICE to the KKK. 

kamala harris whorevia Peoples Cube