By Dave Aguilera

DENVER (CBS4) – So far the winter of 2017/18 has been the winter of freezing rain and drizzle. This time around we are looking at another round of freezing drizzle along the Front Range and Eastern Plains Thursday morning.

Several of our storms this season have included either freezing rain or freezing drizzle along with snow. Making driving and even walking tough with the slick roads and sidewalks. For Denver and the Front Range freezing rain is extremely rare and freezing drizzle happens a little more often but, can also be put in the rare column.

Part of that rarity is our elevation and proximity to the mountains. Freezing rain and freezing drizzle happens quite frequently during the winter months in the southern and eastern parts of the nation. Where many storms typically, have a mixture of warm and cold air. In Colorado most of our winter storms tend to be cold at altitude and on the ground. But, occasionally, the temperature profile occurs that helps support the freezing moisture scenario.

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Freezing drizzle coats a windshield in Denver during an unusual freezing drizzle event in January 2015. (credit: Mark Neitro)

There are several different ways for both of these weather phenomenons to take place. The typical way is when you have warmer air aloft and much colder air near the ground. Ice falling from clouds melts as it falls through the warmer air layer several hundred feet above ground and then freezes on contact with the ground or other surfaces if the temperature is 32°F. The same with rain falling through the 32°F near the ground would freeze on contact.

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Trucks and cars try to get around a Denver Public Schools bus that got stuck trying to get up a hill on Yale Ave near Monaco St. pkwy. (credit: Andy Cross/The Denver Post via Getty Images)

In Denver and the Front Range there is a more common set up to freezing drizzle. Because the temperature profile is quite colder in most storms than those in the south or east. In order for liquid in clouds to freeze it has to come in contact with something in order to begin the freezing process. Typically what is called “ice nuclei”. This can even happen with dust in the atmosphere. The ideal temperature for this to happen is 14°F. Without this the water can stay a liquid even though the air temperature is below 32°F. This is called “super-cooled water.”

If the water droplets are small enough, such as the size of drizzle drops, they can stay liquid until they hit the ground at which point they freeze in contact. This is why in Colorado we have more instances of freezing drizzle than freezing rain.

Meteorologist Dave Aguilera is a Colorado native and has been forecasting weather in the Rocky Mountain region for over 25 years! Connect with Dave on Facebook and on Twitter @DaveAgCBS.


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