Physics Photo of the Week - Feb 18, 2022

Physics Photo of the Week

Feb. 18, 2022

 Frosty Silk



On the morning of Jan 4, 2022 we woke up to a very rare occurrence of rime ice on the trees in our yard.  I believe the temperature was not terribly cold but a few degrees below freezing.  This resembles ordinary frost that forms on surfaces, but believe it is rime ice judging from the long needle-like crystals forming on all the spruce needles as well as along a couple of old spider silks that stretched between small spruce branches.  

This phenomenon is rime ice that was formed from supercooled fog that had descended on our yard during the night.  The fog droplets were below freezing temperature but still liquid when they were suspended as fog.  As soon as the droplets contacted an object (spruce needle, spider web, leaf point, or airplane wing) they freeze instantly.  Rime ice is different from "ordinary frost" or "hoar  frost" in that hoar frost forms by direct sublimation of water vapor directly into solid ice without going through the liquid phase.  This is is called sublimation.  Hoar frost commonly appears on grass, leaves, car windows.  The frost particles on hoar frost are generally smaller and not departing much from the surface that they were formed upon.  The long needles of ice in the photo above are indicators of rime formation called "soft rime".  The needles in soft rime are very delicate and easily broken or shaken off the tree branches.

Hard rime - slightly different from the soft rime - is much more solid.  Winter photos of the weather station equipment atop Mt. Washington in New Hampshire  .  Those of us who grew up before auto-defrost refrigerators became common remember the major chore of defrosting the thick hard rime-like frost that built-up on the chilling units of refrigerators and on the freezer compartments of combination refrigerator-freezers.  Hard rime, I believe, is very hazardous for airplanes flying through clouds that are below freezing temperatures.  The air ram effect essentially forces the needles on soft rime to collapse and fuse into a thick coating of ice that destroys the lifting surfaces on the aircraft wings.

Rime ice was featured about one year ago and is republished at this link: Feb 5, 2021.
 

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