This winter, North America has endured countless cold waves, blizzards, “bomb cyclones,” and nor’easters. Two of the most notable storms were January’s “Storm Grayson” and March’s “Storm Riley,” which dropped nearly 2.5 feet of snow across New York, with one location receiving as much as 40 inches. While this heavy snow did bring snow days and peaceful views, it was not always quiet. Up until recently, a phenomenon known as “thundersnow” was relatively rare, but lightning mapping has revealed that numerous cracks of thunder were heard during these past storms.
Thunderstorms occur when warm air near the ground rises, cools, and condenses to form a cloud. This process continues as temperatures above the clouds drop below freezing, turning the water-vapor into ice. Small clumps of ice bump into each other, leading to a buildup of electric charge. Eventually positive charges from the ground and negative charges from the cloud base cause a spark — lightning. Thunder is a product of this electricity passing through the air and creating vibrations which are heard as sound. This scenario is quite common during the spring and summer because hot air rises. However, frigid winter storms develop in a different way.
“In the case of a thundersnow event, air closer to the ground is just warm enough to rise and create atmospheric mixing, but it’s still cold enough that falling snow can stay frozen,” said Texas State Climatologist and member of the American Meteorological Society Dr. Nielsen-Gammon. “The air is cold enough down low that as the frozen precipitation falls out of it, it never encounters a warm enough layer of air to melt, so instead of rain and lightning, there’s snow and lightning.”
People are now wondering why thundersnow has become more common. Observation of recent New England storms reveals that towers like the 906 foot Prudential Tower in Boston, Massachusetts and the 104 story Freedom Tower almost exclusively harbored these strikes. This occurred as a result of rare conditions occurring simultaneously: low-cloud bases, high winds and heavy snow. It is known that an interaction between positive and negative charges of clouds and outside objects causes lightning. However, this does not mean that charges just leap throughout the air.
“Space charges” are created around objects that experience a charge buildup which shield it from being struck. However, high speed winds are enough to blow this shielding space charge away, leaving objects exposed, thus increasing the odds of it being struck. This, coupled with low cloud bases, allowed the shieldless-points of large towers to trigger lightening by poking into the underside of clouds. The points focused charges in the clouds and boosted it tenfold, creating a perfect recipe for the sky-wide flash and ear-splitting boom that is thundersnow.
This new research and data collected may soon enable meteorologists to predict which towers and man-made structures are at risk of a strike based on cloud-base altitudes and wind speed levels.