The fog comes on little cat feet. -Carl Sandburg
Another batch of freezing fog crept into southern Minnesota Monday morning. The super-cooled liquid droplets froze on contact with trees and other sub-freezing objects. The magical result, a beautiful display of rime ice on trees.
— MPR Weather (@MPRweather) December 10, 2018
Rime ice, or hoar frost?
Today’s icy display looks like rime ice to me. Rime forms when supercooled water droplets freeze on contact the accumulate on the windward side of sub-freezing objects like a tree branch or an airplane wing. This is how trees near the Weather Lab look today on the southwest (windward) side.
Here’s the definition for rime in the American Meteorological Society’s Glossary of Meteorology.
A white or milky and opaque granular deposit of ice formed by the rapid freezing of supercooled water drops as they impinge upon an exposed object.
The Twin Cities NWS office agrees with the rime ice assessment.
The trees behind our office have a coating of rime ice this morning, making the branches appear white. The supercooled water droplets from the fog froze on impact with the trees, forming the rime ice. #MNwx pic.twitter.com/Q8hcFu1SqU
— NWS Twin Cities (@NWSTwinCities) December 10, 2018
So, how is rime different from hoar frost? Hoar frost tends to develop on clear calm nights. You might think of hoar frost as the frozen equivalent of dew. Feathery needles of ice form in humid, sub-freezing air.
Here’s the AMS definition for hoar frost.
Frost is to dew as snowflakes are to raindrops. When water vapor condenses into liquid water, you get raindrops and dew. When water vapor condenses directly into ice, then you get snowflakes and frost (see the Snowflake Primer). Snowflakes are not frozen raindrops, and likewise, frost is not frozen dew.
When frost forms as minute ice crystals covering the ground, we just call it all frost. But sometimes the frost grains grow larger and are called hoarfrost crystals. Good hoarfrost is not that uncommon if you watch for it. Hoarfrost grows whenever it’s cold outside and there is an ample source of water vapor nearby.
Our current weather pattern across Minnesota is favorable for more late night freezing fog. Daylight may reveal more surprising and beautiful scenes in the coming week.