What Do Skyscrapers, Thundersnow, and Jim Cantore Have in Common?

Thundersnow is a rare enough event to get even veteran meteorologists like The Weather Channel’s Jim Cantore excited, as one can tell by the popular clips of Cantore reacting to several thundersnow events this past winter in Massachusetts. It’s estimated that less than 1% of all U.S. lightning flashes in the winter are associated with snow. Thundersnow’s rarity, and the fact that it’s often accompanied by poor visibility, has made it tough for researchers to learn much about it. However, some important leaps of progress have occurred in the last few years. Perhaps the most surprising insight: many cloud-to-ground lightning strikes observed in thundersnow are actually “ground-to-cloud” strikes, initiated by skyscrapers, wind turbines, and other tall objects.


Figure 1. Jim Cantore reports from Plymouth, Massachusetts, early on February 15, 2015, during a lightning-studded snowstorm. Image credit: The Weather Channel.


A study published last year in the Journal of Geophysical Research: Atmospheres examined the blizzard of February 1-2, 2011, which dropped more than a foot of windblown snow from Oklahoma to Michigan and trapped hundreds of vehicles along Chicago’s Lake Shore Drive. From his office in Rapid City, South Dakota, lightning researcher Tom Warner (ZTResearch) kept tabs on weather radar and lightning data during this storm, while also keeping an eye on The Weather Channel. As Warner recalled in an email: “I started noticing isolated lightning events in the snow sector, and when I plotted the coordinates of these events in Google Earth, they were consistently falling on objects such as tall radio/TV towers, tall buildings, and wind turbines. When the activity approached Chicago, I watched Jim Cantore live as he reacted to the lightning events. They were again grouped around the tallest buildings, which he was close to.”

After the storm, Warner scrutinized the data more closely with colleagues Timothy Lang (NASA Marshall Space Flight Center) and Walt Lyons (FMA Research/WeatherVideoHD.TV). Within the zone of heavy snowfall, they found a total of 282 cloud-to-ground and intracloud lightning flashes reported by the National Lightning Detection Network. The NLDN’s array of energy-sensing instruments can track the evolution of a lightning flash from millisecond to millisecond and pin down the flash’s location to within 500 meters. The researchers then mapped the NLDN data against the locations of communication towers, tall buildings, and other towering structures. Of the 282 flashes, they found that 72% occurred within 1 km of a tall object, and most of the other flashes were within 3 km of such an object. Many of the flashes reported in the Chicago area were distinctly clustered around particular structures, such as the Willis Tower and Trump Tower. At the same time, there was very little lightning detected over the nearby waters of Lake Michigan, although radar showed heavy snow falling there.


Figure 2. Lightning flashes detected by the National Lightning Detection Network in downtown Chicago during the February 1-2, 2011, blizzard. Tight clusters of negative flashes (-CGs and -ICs) are evident around the Willis Tower (1729 feet to its tip) and Trump International Hotel and Tower (1389 feet). These flashes are related to lightning channels that initially propagate up from the towers. No lightning was reported from the nearby John Hancock Center, even though it is in the same height range (1506 feet). Image credit: Journal of Geophysical Research/American Geophysical Union.


The two ways lightning can travel upward
As its name implies, cloud-to-ground lightning strikes (CGs) involve electrical charge being lowered from the base of a thunderstorm to an object at ground level along a zigzag channel called a leader. A much brighter flash, the return stroke, occurs after the leader connects to the ground, when charge accelerates downward through the leader but in an upward-cascading fashion (somewhat like a cleared traffic jam suddenly freeing cars further back along the highway). This process can repeat itself more than a dozen times in less than a second, with lightning photos often revealing a forked structure due to new leaders that branch outward and downward from the main lightning channel.

In a typical summer storm, cloud-to-ground lightning will often strike the tallest object around--an isolated tree or house, perhaps--but the data suggest something else was going on in Chicago. When an tower is at least several hundred feet tall, the odds increase that the initial leader will actually propagate from the top of the tower up to the cloud base, rather than the other way around. Just as a downward-propagating flash can branch out over its brief lifetime, striking several points on the ground, an upward-propagating flash can branch out as well (see Figure 3 below). This may “trick” the NLDN into reporting several closely clustered ICs or CGs (as in Figure 2) instead of a single, upward-pointing flash.

Upward lightning has been recognized since the 1930s, but high-speed cameras, sophisticated electric field meters, and 3-D lightning mapping tools have greatly advanced our ability to study it. Over the last few years, scientists have analyzed upward-directed lightning at several locations, including a set of 10 radio/TV towers near Rapid City. A total of 67 upward flashes originated from these towers from 2012 through 2014, when a field project called UPLIGHTS was carried out. More than a third of those flashes (25) occurred during the intense and destructive winter storm of October 2, 2013. Prior to UPLIGHTS, 81 upward flashes were observed in the Rapid City area from 2004 to 2010. Almost every upward flash in this period occurred less than half a second after a strong CG lowered positive charge to ground less than 30 miles away. It appears the positive CGs intensify the local electric field enough to generate a compensating upward flash from a nearby tower, where charge can be readily concentrated. This is referred to as lightning-triggered upward lightning (LTUL).


Figure 3. Upward lightning from four towers in Rapid City, SD. Tall structures appear to be the source of many of the lightning flashes observed during snowstorms. (Photo © Tom A. Warner, used with permission.)


Research in South Dakota and Alabama, as well as Japan and Austria, has identified a separate phenomenon called self-initiated upward lightning (SIUL), which almost always occurs during snowstorms with low cloud bases and low freezing levels. Warner, Lang, and Lyons weren’t able to deploy the same observing tools for the February 2011 storm that were used in UPLIGHTS, but a variety of evidence strongly implies that the lightning in this snowstorm was mostly in the form of SIULs. Interestingly, SIULs seem to require winds of at least 18 mph at the top of the tall object--except when that object is a spinning wind turbine. It appears that either the wind or the motion of the turbine can dispel a protective region of coronal discharge that would otherwise inhibit the upward development of a lightning flash. With that protection gone, the enhanced electric fields at the top of the tall structures appear to be enough to generate a leader that can make it up to the regions of opposite charge in the relatively low nimbostratus clouds.

Why don’t we get more “regular” lightning in thundersnow?
One possible reason that typical cloud-to-ground strikes aren’t very common in thundersnow is the relative weakness of the convection (the upward motion within the snow-producing cloud). Cloud-to-ground lightning originates from the intense pockets of charge that develop in a thunderstorm as snowflakes, ice pellets (graupel), and unfrozen water droplets jostle each other. Radar analysis for the February 1-2 storm showed that almost every flash of thundersnow occurred where the peak radar reflectivity was less than 30 dBZ, a value normally considered too low to produce lightning. However, in most cases there were higher reflectivities up to 30 miles upstream. This suggests that charge may have been generated within small convective cells, then carried downwind by droplets and crystals. Chris Schultz (NASA) and colleagues have carried out detailed analyses of several upward lightning flashes in the vicinity of Huntsville, Alabama. One such flash, which occurred on January 10, 2011, appears to have propagated from a TV transmission tower into a cloud that featured sloped layers of mixed ice crystals, ice pellets, and supercooled water. The researchers have similar data for flashes from tall towers near Washington, D.C., and Baltimore, Maryland. “Hopefully we can get back to these cases and understand the radar measurements better, to infer what is going on microphysically within the cloud at the time of the flash,” says Schultz.

In summary, tall towers don’t seem to be an absolute requirement for thundersnow, but they do appear to facilitate it in some ways that are only now becoming more apparent. The JGR study found that even objects less than 300 feet tall, such as cellphone towers and transmission lines in rural areas, can be enough to trigger thundersnow. Patrick Market (University of Missouri) is a longtime expert on the climatology of thundersnow. In an email, he told me: “The literature supports the occurrence of thundersnow before the advent of radio, so we know that the Earth's atmosphere is capable of the phenomenon all on its own. That said, the evidence that has been presented for taller structures influencing CG activity is compelling. The outstanding question that is difficult to quantify is: Are there more flashes in winter thunderstorms because of taller man-made structures?”


Video 1. A classic “ground-to-cloud” lightning flash captured by high-speed photography (1000 frames per second) at Rapid City, South Dakota, on June 23, 2010. Bright, short-lived return strokes can be seen near the top at some distance from the main lightning channel. If embedded video does not load, you can find it at the WeatherVideoHT.TV Facebook page (posting date is April 29, 2015) or on their previewing site. Video credit: Tom A. Warner/WeatherVideoHD.TV.


Atlantic’s first named storm still possible next week
This morning’s round of model output continues to indicate the possibility of a named subtropical or tropical cyclone near the southeast U.S. coast toward the latter part of next week. See Jeff Masters’ post from earlier today for details. We’ll keep an eye on this potential system and will have an update by Monday at the latest.


This week’s WunderPoster: Fallstreak hole
The first phase of our WunderPoster series concludes today with a strange feature called a fallstreak hole, also known as a hole-punch cloud. These most often form when an airplane ascends or descends through a shallow layer of cloud made up of supercooled water droplets (droplets that remain liquid at temperatures well below freezing, due to a lack of particles on which to form ice). The local pressure drop just behind the aircraft’s propellors or wings can generate enough cooling to produce a batch of fast-growing ice crystals that consume moisture from the surrounding droplets, leaving a mid-cloud hole.

All of the 13 WunderPosters to date can be downloaded in formats suitable for posters or postcards. Watch this space in May for the release of a new set of WunderPosters inspired by photos contributed by the WU community.

Bob Henson