March 21, 2018

Here's How Thunderstorms Are Able to Form With Temperatures Near Freezing

Loud cracks of thunder and vivid lightning are pretty far down on the list of things you expect to hear and see when the temperature is near freezing, but that's exactly what folks in North Carolina were greeted to on Tuesday night as a line of thunderstorms swept through the area. It was cold enough that some of the storms produced sleet. Thundersleet isn't a common phenomenon—especially in this part of the country—so this rare meteorological treat adds to the already-phenomenal winter North Carolina has seen this year.

Thundersnow and Thundersleet

We're used to hearing about thundersnow during the cold months. The video of Jim Cantore hollering and frolicking in a blizzard as lightning zaps around him was an instant classic. But you have to be in just the right place at the right time to see it.

Thundersnow can occur in nor'easters and lake effect snow. There's often so much lift created by the dynamics of a powerful nor'easter that this lift can have the same effect as air rising on a warm day. The strong lifting motion leads to convection which allows lightning and thunder to accompany heavy snow. Lightning is also possible during lake effect snow since the snow bands coming off the lakes form through similar processes that generate pop-up thunderstorms during the warm months.

The thundersleet we saw in parts of North Carolina and extreme southern Virginia formed through a different process.

Elevated Convection

A typical thunderstorm forms as a result of warm air rising from the surface. Air rises more quickly when there's a large temperature difference between the lower levels and the upper levels of the atmosphere. The speed at which air rises determines the strength of a thunderstorm.

A temperature inversion can stop air from rising from the surface. An inversion occurs when a warm layer of air forms on top of a cooler layer of air. The surface temperature north of Greensboro, North Carolina, on Tuesday night was 35°F when the line of thunderstorms swept through, but the air just a few thousand feet above ground level was nearly 50°F. It was too cold near the ground for surface-based convection to occur.

The atmosphere is much more complicated than the simple diagrams they teach us in school. Air doesn't always have to rise from the surface for a thunderstorm to develop. Air can actually start rising from thousands of feet above the surface if the inversion layer is warm enough.



The poorly-annotated jumble of lines above is called a SKEW-T chart, which allows us to trace the air temperature and dew point through a column of the atmosphere. Data collected by the instruments attached to weather balloons are plotted out on these SKEW-T charts, and the resulting graph is a tremendous help in all types of weather forecasting situations

The above SKEW-T chart was generated by the HRRR weather model for Greensboro, North Carolina, at 9:00 PM on Tuesday, right around the time the line of storms rolled through the area. The data shows that the air temperature at the surface was around 35°F at the time of the storms. The temperature dropped steadily for a few thousand feet above ground level before sharply rising in the inversion layer between 3,000 and 4,000 feet. The air finally started steadily cooling off again above 4,000 feet.

The inversion layer was so warm that it actually allowed air to start rising from the top of the layer all the way to the upper levels of the atmosphere, feeding the storms the instability they needed to produce heavy rain and lightning.

The SKEW-T chart above also shows a tiny little slice of the atmosphere below freezing a few hundred feet above the surface, which explains how some of the precipitation fell as sleet. Sleet forms when a snowflake falls through a layer of warm air but doesn't completely melt before reentering subfreezing air. The ice crystals left behind in the newly-formed raindrop serve as the nucleus allowing the droplet to freeze into the ice pellets we know as sleet.

Loud Thunder

If anyone wasn't aware that it was sleeting outside, they sure knew that it was thundering. The thunder was loud and it seemed like some of the cracks would never stop rumbling.

The same inversion that allowed the thunderstorms to develop also helped make the thunder so loud. The sound of the thunder was amplified through "atmospheric ducting." The sudden change in temperature acted as a sort of barrier that reflected some of the sound waves back down toward the surface and caused it to echo across long distances. The echoing effect caused by the inversion allowed one crack of thunder to reverberate for much longer than normal, leading to that ultra-satisfying rolling thunder sound.

[Radar: WSV3 | Sounding: PivotalWeather | Video: Dennis Mersereau]

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I graduated from the University of South Alabama in 2014 with a degree in political science and a minor in meteorology. I ran Gawker's The Vane for two years and I've contributed to Mental Floss, Forbes, Popular Science, and the Washington Post's Capital Weather Gang. I also teamed up with Outdoor Life to write a book called The Extreme Weather Survival Manual, which came out in October 2015.

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