Why freezing rain can be so much more dangerous than snow

Why freezing rain can be so much more dangerous than snow

By Andrea Thompson edited by Claire Cameron

This past weekend’s winter storm blanketed huge swaths of the country in snow, with up to two feet falling in some areas. But the widespread power outages that came with the storm—an estimated one million people from Texas to Kentucky had lost power by Sunday afternoon—had less to do with the snow and much more to do with the 0.5 to one inch of ice that built up as a result of freezing rain.

Here’s why freezing rain can end up being so much more damaging to infrastructure than snow.

First, it helps to know what freezing rain is. In a winter storm, the temperature at different layers of the atmosphere determines what kind of precipitation falls. When the atmosphere is below freezing from the surface upward, snow falls. But if there is a layer of warm air between higher levels of the atmosphere and the surface (what is called a temperature inversion), that snow melts into rain. And if there is a deep enough layer of freezing air below the inversion, the falling rain refreezes into hard pellets of ice called sleet. If it’s not deep enough, however, the rain stays liquid but freezes on contact with cold surfaces, especially exposed ones, such as bridges, tree branches—and power lines.

On supporting science journalism

If you’re enjoying this article, consider supporting our award-winning journalism by subscribing. By purchasing a subscription you are helping to ensure the future of impactful stories about the discoveries and ideas shaping our world today.

For a large part of the southern U.S., the latter scenario is exactly what came to pass with this storm, as warm, moist air from the Gulf of Mexico was pulled into the weather system, melting the snow and turning it into freezing rain that hardened into ice on the ground.

The reason that ice can cause power outages is, essentially, that “the freezing rain sticks and the snow doesn’t,” says Seth Guikema, a civil and environmental engineer at the University of Michigan. Though a small layer of snow can collect on top of some surfaces, wind will often blow it off. Meanwhile ice can accrete on both the top and underside of branches and power lines. That ice exerts enormous weight—ice can add 500 pounds of weight to power lines, according to the Air Force Safety Center.

Power outages occur when ice breaks power lines or other infrastructure, but they primarily happen when ice breaks tree limbs that then take out the power lines. Where power lines are aboveground rather than buried underground, there is often more damage, and that can have socioeconomic implications. Research shows that underground lines are more prevalent in areas where there are “bigger, higher-value and newer homes,” Guikema says. People in poorer areas who are subjected to more outages are also less likely to have their own electric generators as a backup, he adds. “There’s a real disparity in how people can deal with this,” Guikema says.

The regions affected by this past weekend’s storm also underscored its effect. “Keep in mind that most of the areas impacted by this ice are rural in nature so this is a pretty incredible number of outages, and as you can see many counties and parishes in the impacted area have more than half of their populace without power,” wrote Alan Gerard, a meteorologist and CEO of Balanced Weather, in a blog post on Sunday.

How long the outages last will depend on several factors, including how widespread they are, what the degree of damage is and how fast crews can access sites in need of repair. Outages can last longer in winter than they do after summer thunderstorms because crews have to contend with icy roads and snow, and the damage to the power system can be worse, even requiring rebuilding, Guikema says. “There’s just so much of it that it’s going to take time,” he says.

It’s Time to Stand Up for Science

If you enjoyed this article, I’d like to ask for your support. Scientific American has served as an advocate for science and industry for 180 years, and right now may be the most critical moment in that two-century history.

I’ve been a Scientific American subscriber since I was 12 years old, and it helped shape the way I look at the world. SciAm always educates and delights me, and inspires a sense of awe for our vast, beautiful universe. I hope it does that for you, too.

If you subscribe to Scientific American, you help ensure that our coverage is centered on meaningful research and discovery; that we have the resources to report on the decisions that threaten labs across the U.S.; and that we support both budding and working scientists at a time when the value of science itself too often goes unrecognized.

In return, you get essential news, captivating podcasts, brilliant infographics, can’t-miss newsletters, must-watch videos, challenging games, and the science world’s best writing and reporting. You can even gift someone a subscription.

There has never been a more important time for us to stand up and show why science matters. I hope you’ll support us in that mission.