From CBS Denver (Chris Spears):
My experience with freezing rain comes from the traditional setup which is common in the eastern and southern United States.
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.
THE “COLORADO” SETUP
In Colorado, it’s a much more compilecated scenario because many times the temperature of the air during the winter season, both at and above the ground, is below freezing.
That begs the question of then how do you get freezing drizzle and not snow.
There answer lies in the very complicated subject of cloud physics.
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.