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| Earth on Feb 13, 2014

The science of freezing rain

The phenomenon of freezing rain: What causes the dangerous winter weather element that can paralyze cities.

Shannon typically wakes up at six a.m. to get ready for work at eight a.m. It typically takes her 30 minutes to drive to work, but she usually leaves 30 minutes early in case the traffic gets too bad. She can hear the rain falling outside her house, and realizes she should leave just a little earlier since the roads will be wet. She grabs her keys, and runs to her car. Immediately, she notices how cold it is outside.

“Man, if it is going to be this cold, it should just snow!”

Shannon cranks up her car, and proceeds to drive out of her yard and onto the interstate to get to her job. As she pulls out of her driveway, she notices she is having a difficult time moving up the hill. In fact, her car practically stalls and begins to slip downward. Shannon immediately freaks out as she realizes she has no control over her vehicle. She glances at the temperature reading in her car, and the thermometer reads “30°F”. She finally gains control of her car, stops the engine, and gets out of her vehicle. As she tries to look around, she notices everything is covered in ice. Not a spot of snow anywhere on the ground.

“Why is it raining when it is below freezing?”

Shannon is asking a great question. Why is it raining when temperatures are below freezing? What is going on? This phenomenon is simply called freezing rain. In this post, I will explain what causes it and how it can be the most problematic weather feature in the wintertime, especially in urban areas.

Freezing rain in Kentucky. Image Credit: NWS/NOAA

Freezing rain is simply rain that falls through a shallow layer of cold temperatures at or below 0 degrees Celsius (32 degrees Fahrenheit) near the surface. When this rain becomes supercooled, it can freeze on contact with roads, bridges, trees, power lines, and vehicles. When freezing rain accumulates, it can add a lot of weight on trees which can result in numerous power outages and damage to homes. Freezing rain is typically the weather threat that creates the most car accidents, injuries, and deaths in winter storms. Many people can drive in the rain and snow, but when the roads become icy, it is almost impossible to drive. Just a quarter of an inch of ice can add 500 pounds of weight on trees which can easily bring them down. Some of the most severe ice storms can shut down large cities, result in thousands of power outages, and the most violent ones can also become billion dollar disasters (rare).

Here is a diagram that shows the importance of how precipitation falls as rain, sleet, snow, or freezing rain. It is important to know the difference between snow, sleet, and freezing rain.

1) Snow forms when the entire layer of air is sub-freezing. Snow consists of ice crystals and is white and fluffy.

2) Sleet forms when the layer of sub-freezing air is fairly deep, 3,000 to 4,000 feet. This allows time for the water droplet to freeze into a tiny piece of ice and become sleet as it falls to the surface. Precipitation in the wintertime that falls as tiny ice pellets is sleet. Hail is only associated with strong thunderstorms and are larger in size and can cause damage.

3) Freezing rain forms when the sub-freezing layer is very shallow. 2,000 feet from the surface, temperatures are above freezing,so any precipitation that falls is liquid. Once rain hits that shallow, cold air near the surface, it freezes on contact with any object.

Ice storms can cause major problems on the roads. Image Credit: NWS

Shallow, cold air at the surface can sometimes occur thanks to cold air damming. Cold air damming, also abbreviated as CAD, is where a low level cold air mass is trapped topographically. These events can be very common near or around mountain regions, and is known to occur across the eastern United States thanks to the Appalachian Mountains. Some of the worst ice storms to form were thanks to this CAD effect that is also known as the “wedge”. The term is constantly used because shallow cold air is wedged down the Appalachian Mountains thanks to a ridge of high pressure typically located across New England, eastern Canada, or the Mid-Atlantic.

When it comes to freezing rain, it is the weight of the ice on the trees that cause major problems. They can fall over and crush cars, houses, and power lines. According to Steve Nix, brittle tree species typically take the brunt of heavy icing. Trees such as poplars, silver maples, birches, willows and hack-berries are more likely to break and fall over due to the weight of the ice. One of the big reasons these trees break and fall over first is because they are fast growers. They also develop weak, V-shaped crotches that can easily split apart under the added weight of ice.

Image Credit: NWS Hastings by Marla Doxey

Bottom line: Freezing rain is simply rain that falls into a shallow layer of cold temperatures that is below freezing. When this supercooled droplet hits an object, it then freezes and becomes ice. Freezing rain is the most dangerous winter weather element as it can paralyze cities and cause a lot of damage. Freezing rain is the culprit for ice storms, and it can create a large nightmare for travelers on the roads.