2021-02-05 Rime Ice Close-up

Physics Photo of the Week

February 5, 2021

Needles on Needles – Rime Ice

It was a dark and spooky night a couple of weeks ago (Jan 12-13) in the Swannanoa Valley. A low-altitude dense fog had settled in. The fog did not reach the valley floor, but the base of the fog was rather low. The fog made the night very dark. We could see the direct light of houses on the valley floor from our windows, but distant glow from Swannanoa and Asheville in the sky were noticeably absent as well as houses at mid altitudes in the low surrounding mountains. Unusually dark. The temperature was also below freezing.

The freezing temperatures caused the fog droplets to freeze immediately (the droplets were supercooled – liquid below the freezing point) when they came in contact with any obstacle forming rime ice. Click on the image to see a larger view of “Needles on Needles” - needles of frost on a clump of pine needles. Every little twig in the mountain-top forest had these ice needles protruding about 5 cm (¼ inch) out from it in a delicate crystalline appearance – an amazing sight! All the frost needles point in roughly the same direction.

The dense fog did not clear until late morning revealing a glorious sunny day as the photo at left shows the last remnants of the fog lingering around the Four Brothers Knobs near Warren Wilson College.

Look at the Four Brothers carefully (click on the image to enlarge). Notice that the lower flanks have a grey, frosty tint that is absent near the tops (there are four “tops” are the “Four Brothers”!). The greyness indicates a rime ice deposit – a feature that has been featured several times in Physics Photo of the Week. (PPOW April 20, 2020) It appears from the Four Brothers' appearance that the tops were above the fog – there is no appearance of the rime ice. In fact the last remnant of the fog top is coincident with the upper level of the rime.

Because I have been very anxious to get some Physics Photos close-ups to rime ice – all the previous photos in the 17 years of PPOW have been remote photographs – I literally jumped at the possibility of getting a close-up photo! Jones Mountain – overlooking the Warren Wilson Farm – is right outside my back door, and the trail to the top only takes 35 minutes. My dog and I soon took our daily hike to the top of Jones Mountain to examine the rime that most likely would be there. Sure enough, we came across the lower edge of Jone's rime ice deposits. The top photo in this post I call “Needles on Needles” - we see needle-like frost crystals on every pine needle in this close-up photo. Notice also that the needle ice crystals all point in the same direction. That direction is directed toward the North. It is interesting that a very light wind blew from the north indicating that the ice crystals grew pointing into the wind!

Delicate (~5 mm long ice needles) arrangements of the points of the crystals pointing into the wind seems counter intuitive. Most objects (as flags on a flagpole) blow away from the wind on the downwind side of the flagpole. Why would these delicate crystals point into the wind? The wind in a fog situation is practically non-existent. On windy nights fog does not form, the wind stirs up the air too much. The very light wind is simply a slow drift. The icy fog droplets are supercooled liquid water. The below freezing water does not freeze unless there is something to initiate the freezing of the liquid into ice. The freezing of each droplet happens almost immediately when it contacts an object. Any object, even other ice crystals, will initiate freezing. By the time the fog reaches the back (leeward) side of the twigs and sticks, the droplets have been frozen out and formed on top of previous crystals preserving the geometry in the original crystal. Thus the freezing pattern consists of the delicate points all pointing into the wind.

The last photo shows the bright sunshine glinting off the myriads of rime ice crystals near the top of Jones Mountain on the morning of January 13, 2021. The dog, “Petunia”, didn't appreciate the rime, nor the cold, but she was overjoyed to go for a walk early in the morning!

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Physics Photo of the Week is published weekly during the academic year on Fridays when Warren Wilson College is in session. These photos feature interesting phenomena in the world around us.  Students, faculty, and others are invited to submit digital (or film) photographs for publication and explanation. Atmospheric phenomena are especially welcome. Please send any photos to dcollins@warren-wilson.edu.

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