Physics Photo of the Week

December 2, 2005

Rime Ice in the Craggies - Photo and Discussion by Donald F. Collins

Two days before Thanksgiving, Nov. 22, 2005, a winter storm deposited snow and rime ice on the tops of the Great Craggy Mountains near Warren Wilson College.  Rime ice on the mountain tops is a fairly common winter occurance.  It forms during winter storms when clouds enshroud the mountains and the clouds' temperature is below the freezing point of water.  The microscopic water droplets in the clouds are liquid, however.  The existence of liquid water below the freezing point is in a state called "supercooled liquid".  For the water droplets to freeze, they must have something to serve as a "nucleation" site on which to freeze.  The twigs of the trees form the nucleation sites and the cloud droplets freeze immediately upon contact with the tree branches.  The storm of Nov. 22 is rather unusual in that it both snow and rime ice were deposited.  The rime ice coating the trees whitens all the top of Craggy Gardens (center) and Craggy Dome (background).  At lower elevations just below the Blue Ridge Parkway, the mountain trees are bare, but snow has accumulated on the ground.  The southeast facing slope of the mountain (the right side of the mountain) was facing into the wind and that side received both the rime ice and snow giving it a solid white appearance.  The whole effect accentuates the topography of the mountains and the direction of the wind when the rime formed.

The foreground mountains at an elevation of about 3000 ft recieved snow and no rime.  The Warren Wilson College Farm at an elevation of 2200 ft (not visible in the photo) received no snow accumulation. 

The photo at right shows the similar rime ice formation on Lane Pinnacle ("pointed" to by the weather vane) while the foreground mountain again shows only snow with bare trees.

Rime ice formation is very dangerous to aircraft when flying through supercooled clouds.  The rapid buildup of ice on the wings and propellers soon weights down the aircraft and adds prohibitive drag.  Commercial aircraft are equipped with de-icers to minimize the hazard of ice buildup.  Inflight de-icers usually consist of inflatable leading edges (the surface expands and contracts to break-away ice build-up) or by heating.  Aircraft are often de-iced on the ground before takeoff using freezing point depression fluids (antifreeze-like compounds).

The clouds that form rime ice are often very acidic - presumably due to sulfate content (acid rain).  Active programs are now studying the ion content and acidity of rime at Mount Washington Observatory in New Hampshire.  Study of the acidity and ion content of rime ice should be an interesting undergraduate research project if the logistics of sample collection can be overcome (the Blue Ridge Parkway is closed at high elevations in winter).