Rime Ice in the Craggies -
Photo and Discussion by Donald F. Collins
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
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
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.
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.
photo at right shows the similar rime ice formation on Lane Pinnacle
("pointed" to by the weather vane) while the foreground mountain again
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).