Satellite Images Reveal ‘Hidden’ Tornado Tracks

When a strong tornado roars through a city, it often leaves behind demolished buildings, broken tree limbs, and trails of debris. But an equally powerful storm making landfall on barren, bare land is much harder to spot in the rearview mirror.

Now, satellite images have revealed a 60-kilometre-long wet dirt track in Arkansas that was invisible to human eyes. The feature was presumably dug out by a tornado when it swept away the top layer of soilresearchers report in the March 28 Geophysical Investigation Letters. This method of looking for “hidden” tornado tracks is particularly valuable for better understanding storms that strike in winter, when there is less vegetation, the researchers suggest. And recent research has shown that winter storms are likely to increase in intensity as weather warms (Serial number: 12/16/21).

More than 1000 tornadoes strike the United States every year, according to the National Weather Service. But not all are equally likely to be studied, says Darrel Kingfield, a meteorologist with the National Oceanic and Atmospheric Administration in Boulder, Colorado, who was not involved in the research.

For starters, storms that pass over populated areas are more apt to be analyzed. “Historically there has been quite a large population bias,” says Kingfield. Storms that occur over vegetated regions also tend to be well studied, simply because they leave obvious scars on the landscape. Uprooted grasses or downed trees function as beacons to indicate the path of a storm, says Kingfield, who has studied forests damaged by tornadoes.

Spring and summer are peak storm seasons in the United States: More than 70 percent of tornadoes occur from March to September, according to NOAA. But on December 10, 2021, a group of storms began to run across the central and southern United States. Those tornadoes, which claimed more than 80 lives, leveled cities as well as farmland, much of which had already been harvested for the season.

Jingyu Wang, a physical geographer at Nanyang Technological University in Singapore, and his colleagues set out to detect the signatures of these deadly storms in barren, unpopulated landscapes.

Swirling winds, even relatively weak ones, can suck up several inches of soil. And since the deeper layers of the ground tend to be wetter, a tornado should leave behind a telltale signature: a long strip of wetter-than-usual ground. Two properties related to the moisture level of the soil, its texture and temperature, in turn affect the amount of near-infrared light that the soil reflects.

Wang and his colleagues analyzed near-infrared data collected by NASA’s Terra and Aqua satellites and looked for changes in ground moisture consistent with the passage of a tornado.

When the team analyzed data obtained shortly after the 2021 storm’s outbreak, they noticed a signal in northeast Arkansas. The feature was consistent with a wet soil runway approximately 60 kilometers long. Tornadoes had previously been reported in that area, outside of the city of Osceola, so it is likely that this feature was created by a powerful storm, the team concluded.

That makes sense, Kingfield says, and observations like these can reveal signs of tornadoes that might otherwise go undetected. However, it’s important to recognize that this new technique works best in places where soils are capable of holding water, she says. “You need to have clay-rich soils.”

Still, these results hold promise for looking at other tornadoes, Kingfield says. It’s always helpful to have a new tool to estimate a storm’s strength, track and structure, but many storms go relatively unnoticed simply because of where and when they occur, she says. “Now we have this new basic truth.”