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How freezing rain forms, impacts surfaces

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Posted at 6:00 AM, Dec 03, 2020
and last updated 2020-12-03 07:01:05-05

KANSAS CITY, Mo. — The concept of liquid water, freezing on contact with surfaces, creating a glaze on all surfaces, has always been fascinating. This type of precipitation is called freezing rain.

Freezing rain can cause big problems both when there is a lot and when there is hardly any at all.

One of the worst ice storms in Kansas City history occurred January 29-31, 2002.

We had three days of freezing rain, heavy at times, with ice accumulating to one-and-a-half inches thick on trees and power lines, and they came crashing down. There were countless power outages and destroyed trees.

Trees will start to sag as ice begins to accumulate, and they reach the breaking point when the ice becomes around one half of an inch thick. So, this was an extreme weather event. Fortunately, major ice storms are rare.

Freezing drizzle, which can add up to no more than a trace to .01", can also cause huge issues. This drizzle won't accumulate enough to bring trees and power lines down but can cause roads and all surfaces to become icy in just a few minutes.

Since it is so light, it is hard to tell whether a surface is damp, wet or icy. This is sometimes called "black ice" as it takes on the color of the paved surface and can be deceiving to drivers and people walking.

This can also happen with heavier freezing rain, but sometimes when the rain is coming down hard, it can keep the ice from becoming a glaze. It can cause surfaces to become slushy slick.

Bridges, overpasses and decks can become slick quick and first. Why? If temperatures are dropping from above to below freezing or just below freezing, these structures are surrounded by the below-freezing air and cool the fastest.

Paved roads and other surfaces, like sidewalks, can retain heat from below and can freeze later. Now, if it has been below freezing for several hours or days before the freezing rain, then all surfaces can become slick quickly.

Treatment with salt and other chemicals can prevent all surfaces from becoming slick quickly. But, sometimes, the very light freezing drizzle events are not caught in time. And, sometimes, the heavier freezing rain events can wash away the salt and chemicals leaving all surfaces to become slick quick.