Over the past 30 years, a proliferation of new technologies (especially cell phones) has increased the number of antenna towers in the United States more than threefold. Advancements in broadcasting technologies also assisted in the development of the National Lightning Detection Network (NLDN), a web of 100 sensors that can detect the electromagnetic signals emitted when lightning strikes the ground. Within seconds, these towers transmit data on the location, time, and polarity (positive or negative electrical charge) of the lightning strike to a global database via satellite.
The NLDN database is the crux of numerous climate studies, as it catalogs lightning strokes and flashes across a vast area. Following an upgrade in 1995, this U.S. network has consistently detected cloud-to-ground lightning strikes—the classic bolt of lightning—95% of the time.
However, it’s an imperfect system. Studies dating back to the 1960s show that antenna towers attract lightning strikes to a greater extent than mountain peaks at similar elevations. However, many studies summarize lightning over wide areas (10–20 kilometers), potentially masking smaller-scale lightning anomalies. Thus, lightning driven by other human-made structures might be, in reality, behaving differently than what is reflected in broad use of the data.
To test the accuracy of current lightning measurements, Kingfield et al. (a research group from The University of Oklahoma in Norman, Okla., the heart of storm country) mapped 20 years of NLDN cloud-to-ground lightning data in a grid spaced into 500-meter cells. The researchers found that nearly all (99.8%) of the grid cells with more than 100 cloud-to-ground lightning strikes recorded were within a kilometer of an antenna tower registered with the Federal Communications Commission. They also found that the taller the tower was, the greater the likelihood of a cloud-to-ground lightning strike occurring was.
For instance, 619 cloud-to-ground lightning strikes, the most measured in a single grid cell, were recorded near a 331-meter-tall tower located in the Boston Mountains 30.6 kilometers southeast of Fayetteville, Ark., whereas 163 cloud-to-ground strikes were measured near the Willis Tower (520 meters tall) in Chicago, Ill., over the 20-year period. Furthermore, there was a 631% increase in lightning near a 512-meter tower in northern Wisconsin when compared to an area roughly 2–5 kilometers away.
Most past studies have examined limited geographies and seasons, both of which have a significant effect on lightning frequency. The researchers, however, decided to cover a wide range of locations and dates. For example, they found that from September to February, throughout the northern Great Plains, the frequency of all cloud-to-ground lightning strikes near a tower was about 138% higher than in a region about 2–5 kilometers away. From March to August, the frequency at the same locations was about 117% higher. An exceptionally surprising find was the identification and tracking of so-called hot spots where cloud-to-ground lightning increased immediately after a tower’s construction.
As a whole, the study quantifies the increased likelihood of lightning strikes occurring near human-made towers, especially the tallest of these towers. Its illustration of the variability, yet predictability, of this common atmospheric phenomenon will inform many meteorological and climatological studies to come. (Geophysical Research Letters, https://doi.org/10.1002/2017GL073449, 2017)
—Sarah Witman, Freelance Writer