DENVER (CBS4) – A rare weather event hit Denver and the Front Range Thursday night turning roads, sidewalks and decks into a sheet of ice.
THE “TRADITIONAL” SETUP
When you think about temperatures in the atmosphere, especially during the winter, you assume that if it’s at or below freezing at the ground, then it’s even colder up high.
But that isn’t always the case.
In a typical freezing rain or drizzle event, there is usually a thick layer of air just above the ground that has temperatures above freezing.
As snow falls through that air, it melts, arriving at the ground as a liquid that freezes on contact.
CLOUD PHYSICS 101
A cloud, which is made of thousands of tiny liquid water droplets, has a temperature threshold for those water droplets to turn into tiny particles of ice. That temperature is right around -10°C, or 14°F.
The formation of these ice crystals serve as the building blocks for snowflakes to form.
In the case of Thursday night’s weather setup along the Front Range, there was a shallow layer of moisture in place with temperatures at or slightly warmer than the threshold for ice formation.
This allowed most of the water droplets to remain in the liquid state, even with temperatures below freezing.
In meteorology, this is called supercooled water.
Because temperatures were flirting with the -10°C threshold, some snow was mixed in with the freezing drizzle.
More times than not, in Colorado, the profile of temperature and moisture in the atmosphere sets us up for either a brief rain to snow or all snow.
But as we saw Thursday night, while prolonged freezing drizzle isn’t common, sometimes, it can happen.
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