Atmospheric Scientist here at Weather Underground, with serious nerd love for tropical cyclones and climate change. Twitter: @WunderAngela
By: Angela Fritz , 7:37 PM GMT on January 26, 2013
Of all the types of precipitation, sleet and freezing rain are typically the most dangerous. Freezing rain coats the roads, sidewalks, and runways with ice making travel not only difficult, but often deadly. Even a thin coating of ice can result in a travel nightmare, while heavier amounts will severely damage trees and power lines. Strong winds can add extra force to already weighed down tree branches and power lines, increasing the likelihood of significant damage.
Ice from a freezing rain storm coats the branches in Duncanville, Texas.
Sleet is frozen precipitation that falls as ice pellets that you may see bouncing off your windshield, roof or the ground. Depending on the intensity and duration, sleet can accumulate in a similar way to snow. Freezing rain on the other hand, falls just like normal rain only it freezes on contact with roads, trees, power lines and other structures since temperatures are 32 degrees or below at the surface. Even light accumulations may cause dangerous travel, while heavier amounts can be very damaging.
What causes these two types of wintry precipitation?
Snow melts in a warm layer above ground in both cases. The depth of cold the layer near the surface determines whether rain drops form into sleet (left) or freeze on contact to surfaces (right). In the sleet scenario, snow falling high above ground goes through a warm layer where it melts into rain. After exiting this warmer layer it enters another layer of very cold sub-freezing air, and the raindrops then refreeze into pellets of ice near and just above the surface of the earth. For freezing rain, the setup essentially remains the same except for one key difference. Snow falling high above ground goes through a warm layer where the flakes melt into rain. The difference is the warm layer is much deeper and extends closer to the ground making the layer of sub-freezing air near the earth's surface much thinner. As a result, the drops themselves do not freeze before they hit the ground. This is a phenomenon called "supercooling", where liquid is below freezing but hasn't solidified into its frozen, solid state. When supercooled drops hit the ground, road, car, or any surface that has a temperature below 0 °C (32 °F), the drops instantly freeze, forming a layer of ice, which can continue to accumulate.
Freezing Rain Facts
• Ice can increase the weight of branches by 30 times
• A 1/2" accumulation on power lines can add 500 pounds of extra weight
• An ice storm in 2009 centered from northern Arkansas to the Ohio Valley knocked out power to 1.3 million
• In 1998, an ice storm in northern New York and northern New England damaged millions of trees and caused $1.4 billion in damage. Accumulations were as much as three inches thick!
Freezing rain coats a post in Nesbit, Mississippi.
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Stay safe out there!
Chris Dolce contributed to this blog.
The views of the author are his/her own and do not necessarily represent the position of The Weather Company or its parent, IBM.