Spectacular icy tree show likely again this weekend; "Severe" to "Extreme" winter so far
Was it hoarfrost or rime ice? And what's the difference?
It’s a visually spectacular benefit of a Minnesota winter.
Parts of Minnesota awoke to a thick crystalline ice display on trees and other objects Friday morning. Hoarfrost and rime ice both produce these amazing shows.
But which was it at your house and what’s the difference?
Hoarfrost
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Hoarfrost usually occurs on clear cold calm nights. Water vapor in the air crystallizes directly onto surfaces like trees, fences, or anything that will allow attachment. This process skips the liquid phase, with water vapor (gas) turning into ice crystals (solid) on contact with surfaces.
Hoarfrost often has a more feathery appearance than rime ice.
Rime Ice
Rime ice occurs when super-cooled water droplets in fog freeze on contact with surfaces like trees. Rime ice goes from the liquid phase (water droplets) to the ice phase.
Rime ice produces heavy ice deposits that can weigh down trees and power lines. It also is very dangerous to aircraft. Here’s a great video from the Canadian Broadcast Corporation that explains hoarfrost and rime ice.
Severe to extreme winter so far
We’ve been blasted with extreme cold and copious snowfall so far this meteorological winter. (Dec-Feb)
So just how extreme has our winter been so far in Minnesota?
According to the Midwest Regional Climate Center’s Accumulated Winter Season Severity Index (AWSSI), this ranks in the top 20% of severe winters in Minnesota to date. The AWSSI calculates winter severity based on cold and snow over the season.
The Twin Cities AWSSI through January 5 is 526, which falls near the top 20% of winter severity so far.
The Red River Valley and Duluth are showing AWSSI reading in the extreme range. The AWSSI in Duluth is 762, into the extreme range.
We’re getting a break in the storm track with little snowfall for a bit, and temperatures look milder over the next couple of weeks.
But this winter is off to a vigorous start.