All precipitation begins as snow. As it falls through above-freezing air, it melts. If the above-freezing layer extends to the ground, it results in cold rain. If the entire air column is below freezing, snow forms.
In central North Carolina, the challenge arises when an above-freezing layer exists in the mid-levels of the atmosphere, causing snow to melt into rain before falling back into a below-freezing layer. The depth of the cold air near the surface determines the outcome: deeper cold air produces sleet, while a shallower layer results in freezing rain, the most hazardous type of winter precipitation.
The last major ice storm occurred in December 2002, bringing up to one inch of ice and leaving over 1.8 million people without power for up to two weeks.
So, why don't we see more big ice events? Because ice is self-limiting due to thermodynamic feedback.
When water freezes, it releases latent heat, which can warm the near-surface air and raise temperatures to or above freezing, slowing or stopping the freezing process. For example, at 32F, a slight increase to 32.1F is enough to halt ice accumulation.
For a big ice event to happen, ice needs to start building up when temperatures are in the low to mid-20s, before the air warms above freezing.
Another way is when a blast of cold Arctic air from the northeast gets funneled in by high pressure over New England. This extra cold air can counteract the latent heat release and help ice build up.
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It's been over 20 years since we've had a major ice storm here in central North Carolina, and that's because these are tough setups for us.
So, why do we see so many power outages when we do get ice? It's the weight.
When we get a glaze to a quarter inch of ice, we see some limited problems -- trees and power lines get coated, a few branches might break, and there could be sporadic power outages. But once we hit a quarter to half an inch of ice, that's when widespread issues start popping up.
Ice can add up to 500 pounds to a typical power line, which makes it sag or even break. Tree limbs often can't handle the weight and snap, sometimes landing right on a power line.
The bottom line is that we're overdue for a major ice storm here in central North Carolina. The good news, though, is that it's really hard to get the exact conditions needed for that to happen.
Fingers crossed.