November 16, 2014

What's with all the Frost Flowers?

One week ago, after a long spell of milder than average weather, it suddenly got cold and clear. Winter had arrived all at once.  For six nights in a row, until last night, we hit -24 degrees C here in Lake Louise.  As expected, our lakes began to freeze, but instead of nice clean ice, which we love for skating, every lake was soon covered in "frost flowers."  They're beautiful, but last winter we managed to skate on lakes that were mostly free of these frost flowers, and we wondered, "why the difference?"

Frost flowers at Vermilion Lake
Well, it turns out that researchers Robert W. Style and Grae Worster have answered that question.  In 2009, they published "Frost flower formation on sea ice and lake ice" in a journal called "Geophysical Research Letters."  Their article showed that the flowers do not form like conventional frost, in which water vapour in the atmosphere condenses and freezes on a cold surface. (This is the normal way you end up with frost on your car windshield.)

Instead, when there is a strong temperature differential between the newly formed ice and the air temperature, moisture sublimates right out of the ice and freezes into frost crystals.  Once the flowers are in place, even if the air itself is very dry, they continue to grow as long as the air temperature remains below about -15 degrees C.

Smooth ice at one of the ponds near Field, B.C.
So the mystery is solved.... or is it?  Today, we went to Yoho Park, west of Lake Louise, and skated on the ponds near Field.  They were clear and smooth.  Where were the frost flowers?

The answer is that it needs to be calm for them to form.  Most places here in the Rockies were dead calm this week, but not Field.  It sits below Kicking Horse Pass, where the cold, high pressure air squeezed through to create a strong local wind. Wind is the enemy of frost flowers. According to the researchers, "frost flowers are typically only seen at wind speeds below about 5 metres per second: at higher wind speeds, turbulence mixes the region of local supersaturation with the drier air above."

If you like lots of math, here's the full story, but frost flowers or not, being outside on our frozen lakes has been great fun.