Wednesday, December 28, 2011


Today if you are in the high country, you might getting a first hand look at rime, which forms as supercooled cloud droplets freeze when they contact solid objects.  

I got a pretty good coat of it on my googles in upper Collins gulch while skiing through the altostratus deck that was draped over the Wasatch Mountains early this afternoon.  

Source: Alta Ski Area
In fact, I even resorted to a maneuver I call the Snoqualmie swipe, which involves rubbing my thumb across my goggles to remove the rime between turns.  I don't use the Snoqualmie swipe much in Utah, but I was a real pro at it when I lived in Seattle and skied frequently at Alpental in Snoqualmie Pass, where rime is practically an every day occurrence.

Unless heated, no object is safe from rime.  One of the Snowbird cameras at the top of the tram was coated with it.

Source: Snowbird Ski Area
Rime is produced by clouds that contain large supercooled cloud droplets or drizzle.  Supercooled means that the droplets or drizzle are below 0ºC but are comprised of liquid water rather than ice.  Water does not necessarily freeze when it is below 0ºC.  To freeze, it needs a particle, known as an ice nuclei, to help it transition to ice.  Without such a particle, it can become supercooled.  

At temperatures just below 0ºC, there aren't many particles that can serve as ice nuclei.  So, shallow clouds that are just below 0ºC are prone to riming.  Today as we have a shallow layer of altostratus hanging over the Wasatch Range.  The temperature at Alta-Collins, which sits right at cloud base, is about -1 to -2 ºC.  

Riming is a concern for aviators.  The buildup of rime ice on aircraft not only adds weight to an aircraft, but also changes the wing aerodynamics.  The National Weather Service Aviation Weather Center produces analyses of icing severity, on which they overlay pilot reports of icing.  Light icing is presently being reported by pilots over northern Utah.  They won't linger long at altitudes where this is a concern.

Source: NWS/AWC

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