As fierce winter storms pummeled much of North America in February, lightning danced over the Gulf Coast. “Thundersnow”—thunderstorm activity during a winter snowstorm—is rare, and researchers are now poring over data from the Houston Lightning Mapping Array network to better understand this elusive phenomenon.
Most thunderstorms tend to occur in spring and summer, and atmospheric science provides an explanation: Warmer conditions are conducive to lifting parcels of air, which transport water vapor upward. This convection is critical to the formation of thunderclouds, said Tim Logan, an atmospheric scientist at Texas A&M University in College Station. “Storms need energy to develop.”
A Boost from the Cold
Because temperatures are lower in winter, there’s less convection. That makes for far fewer wintertime thunderstorms. But they’re possible if something physically forces air upward, said Logan. Advancing cold fronts can provide that boost because they tend to shove air out of the way—and upward—via displacement, he said. “Winter season thunderstorms need dynamical lifting.”
When a winter storm spawns a thunderstorm, the result is known as “thundersnow” or “thundersleet,” depending on the type of precipitation it accompanies. Wintertime thunderstorms are elusive, said Christopher Schultz, an atmospheric scientist at Marshall Space Flight Center in Huntsville, Ala., not involved in the new research. A “very conservative” guess is that they’re about a thousand times less common than their warm-weather counterparts, he said. “It’s a rare phenomenon.”
But earlier this year, Logan and his colleagues had the opportunity to study thundersnow occurring nearly in their own backyards.
Thundersnow in the Lone Star State
Starting just before Valentine’s Day, winter storms swept over a wide swath of North America. They dumped record-setting amounts of snow and ice, sent temperatures plummeting to unprecedented lows, and left hundreds of thousands of people without power. The Houston area was hit on 14 and 15 February. Logan, who was working from home in College Station, monitored reports of thundersnow in the area. “There was lightning observed within 5 miles of my house,” he said.
Logan and his colleagues are keen to understand how wintertime thunderstorms differ from the storms more commonly observed in the spring and summer. To do so, they’ve been analyzing data from the Houston Lightning Mapping Array.
The network, directed by Logan, consists of 12 solar-powered sensors spread around Houston. Antennas detect radio frequency emissions from lightning, and the measurements are then fed into software that pinpoints the altitude, latitude, and longitude of the lightning. “It gives you a three-dimensional view of where the lightning initiates and how it moves through the atmosphere,” said Logan.
No Lower than Usual
Logan and his collaborators focused on 835 flashes of lightning detected during the February storms by the Houston Lightning Mapping Array. The researchers found that the flashes originated at an altitude of roughly 9 kilometers. That’s surprisingly high, said Logan. Ice, a critical ingredient of thunderstorms, would have been forming at lower than normal altitudes during February’s storm, so it’d make sense if lightning were also occurring at lower altitudes. “It was actually at what’s considered a normal height,” said Logan.
The team also investigated the thunderstorms’ electrical nature using data from both the Houston Lightning Mapping Array network and the National Lightning Detection Network. Lightning can be classified as negative or positive: Negative lightning, by far the most common, transfers a net negative charge. Positive lightning does the opposite.
More Positive in the Winter
Logan and his colleagues found that roughly 30% of the lightning they analyzed was positive. That’s significantly higher than the normal fraction of about 10%. However, that result isn’t wholly surprising, Logan and his collaborators suggested. Wintertime thunderclouds often contain more ice crystals than usual, and those particles tend to take on a positive charge.
But there are downsides to positive lightning. It’s more likely to be associated with severe weather like hail and tornadoes, and it also often delivers a stronger punch, said Schultz. “Positive flashes are generally more powerful.”
The Houston Lightning Mapping Array—and other lightning detection networks—will continue to stand sentry for thundersnow. It’s a fascinating phenomenon, said Logan, but it’s unlikely to be spotted again over the Houston area this century. “To see something like this here over the Gulf Coast is a treat.”
—Katherine Kornei (@KatherineKornei), Science Writer