Flashes of lightning and booming sounds from the sky typically accompany rain, not cold white flakes. So-called thundersnow is rare. However, on Groundhog Day in February 2011, a storm in the central United States saw up to 75 centimeters of white fluff and 282 lightning flashes. The National Lightning Detection Network (NLDN) reported nearly all as negative-polarity cloud-to-ground (CG) flashes—typical of thundersnow events. Closer inspection revealed something very untypical.
During the height of the blizzard, media reporters noted lightning apparently striking Chicago’s skyscrapers. Curiously, despite strong northeast winds blowing in from Lake Michigan, no lightning was detected anywhere over the warm lake waters. In a recent paper, Warner et al. compared the reported lightning locations for each flash in a multistate region with databases for tall buildings, wind turbines, and towers. They found that up to 93% of the thundersnow flashes were likely self-initiated upward lightning (interpreted as “ordinary” negative CG lightning by NLDN).
Upward positive lightning leaders often are triggered from tall structures in thunderstorms when there is a positive-polarity CG strike nearby. However, in this case, the upward leader initiation required no prior triggering event. It appears the strong winds were blowing away the corona discharge shielding that forms on tall objects, allowing the charge accumulating on towers and skyscrapers to more readily “strike the sky.”
Were it not for the “human-built environment” and the blizzard’s strong winds, the authors note, the February 2011 storm might have produced very little thundersnow.
Similar events were uncovered in the early October 2013 blizzard in western South Dakota, in which TV and communication towers appeared to launch numerous self-initiated upward positive leaders. (Journal of Geophysical Research: Atmospheres, doi:10.1002/2014JD021691, 2014)
—Shannon Palus, Freelance Writer
Citation: Palus, S. (2015), Do cities cause thundersnow?, Eos, 96, doi:10.1029/2015EO026677. Published on 23 March 2015.