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By DAN ROBINSON
Storm Highway Editor/Cameraman |
Tall television broadcast towers and mega-skyscrapers are lightning magnets - no other objects on earth are as frequently and predictably struck. Many of these structures experience over one hundred direct hits each year. This article will examine the unique type of lightning that is common with tall structures as well as how this phenomenon correlates with some common myths about lightning.
In This Article:
- Upward-moving or 'Ground-to-Cloud' lightning
- The stratiform precip region
- Lightning attraction myths
- 'Degree of influence' from metal objects
- Cloud-to-ground lightning to tall structures
- Lightning strikes twice
- Lightning leader phenomena
- Photographs
- Video Clips
- Links
Upward-moving or 'Ground-to-Cloud' lightning
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While most lightning strikes to earth are the cloud-to-ground variety, the vast majority of lightning discharges to tall structures are of the distinctly different 'ground-to-cloud' or 'upward-moving' type. Unlike a cloud-to-ground stroke's downward-moving (and downward-branched) stepped leader, a ground-to-cloud lightning discharge initiates as an upward-propagating, upward-branching leader from the tip of the structure skyward into the cloud. To illustrate the difference between cloud-to-ground and ground-to-cloud strokes, we'll examine frame-by-frame part of two video clips as well as simulated animations of both types of lightning strikes.
>> Comparison: Cloud-to-ground lightning
This first sequence of images are successive still frames from a typical cloud-to-ground lightning stroke, showing the downward-branching progression of the stepped leader:
     
Fig. 1: From video of a distant cloud-to-ground strike near Gothenburg, Nebraska: Frames 1 through 4 show the stepped leader descending, Frame 5 shows the intense first return stroke, Frame 6 shows the decaying first return stroke.
The following animation depicts the descent of the stepped leader and subsequent return strokes with a cloud-to-ground lightning stroke:
[ restart animation ]
Fig. 2: Animation depicting stepped leader, upward leaders, first return stroke, and secondary return strokes.
>> Ground-to-cloud lightning
The next set of images show the upward-propagating nature of a ground-to-cloud lightning discharge to a television tower:
Fig. 3: Frames from video of an upward-moving discharge from a television tower near St. Albans, West Virginia.
>> Ground-to-cloud lightning: two variations
Ground-to-cloud discharges have been observed in two distinct forms. The first variation, 'Type A', is less common but much more visually spectacular, and consists of a tree-like branch network literally 'sprouting' skyward off of the tip of the structure. As a 'Type A' discharge continues, the number of branches diminishes until only one or two main channels remain to carry secondary return strokes. The second 'Type B' is more common and consists of a single, branchless leader rocketing upward from the structure tip. The video images above have captured a Type B stroke. Although 'Type B' ground-to-cloud strokes show no low-level branching, they usually exhibit upward branching at some point near or above the cloud base. The following animation depicts the two types of upward-propagating 'ground-to-cloud' discharges, Type A and Type B, to a television tower.
[ restart animation ]
 
Fig. 4: Animation depicting two forms of upward-moving or 'ground-to-cloud' discharges to a television tower. 'Type A' (left) and Type B (right).
>> Ground-to-cloud lightning: natural and artificial triggers
A purely natural upward-moving ground-to-cloud lightning stroke is actually a rarity. It favors a unique terrain feature characterized by an isolated, small peak at very high elevations relative to its immediate surroundings. One location in the United States where ground-to-cloud lightning is known to occur naturally is at Pilot Peak, near Yellowstone National Park in Wyoming. Pilot Peak is a tall, sharp-pointed, pyramid-shaped peak rising high above its surroundings, and at least one photograph has been obtained of a ground-to-cloud strike to its summit (see links section below).
While ground-to-cloud lightning is indeed a natural phenomenon, it is man-made structures that have clearly brought this type of discharge to the 'mainstream'. Today, thanks to urban development, ground-to-cloud lightning strikes are very common occurances and can be observed anywhere a thunderstorm encounters a tall structure. Broadcast towers and buildings that rise to heights above 1,000 feet above ground (AGL) are especially prolific hotspots for upward-moving lightning, with multiple direct strikes common even during a single storm.
Some well-known skyscrapers that are frequently targets of direct ground-to-cloud lightning strikes include Chicago's Sears Tower and John Hancock Center (see photos below), Toronto's CN Tower, New York's Empire State Building and the former World Trade Center towers (see links section below). Numerous photographs exist of these buildings taking direct hits from the distinctive upward-moving lightning. Ground-to-cloud lightning can usually be observed in any major metropolitan area, where very tall television broadcast towers are commonplace.
 
Fig. 5: Tall skyscrapers are frequent targets of upward-moving lightning. In these photos, Chicago's Sears Tower (left) and John Hancock Center (right) take direct strikes. In the image below, both buildings are hit simultaneously during a single discharge event.
In general, the taller the structure, the more lightning strikes it will experience. A 2,000-foot TV tower will receive more strikes than a 1,000-foot structure.
Frequency of strikes to skyscrapers and towers
If you were to perform an internet search to find the number of times lightning strikes a specific structure in an average year (the Sears Tower or Empire State Building, for example), you'll find figures ranging from the hundreds to the thousands. Which numbers are right? Based on our observations and collected images of actual tower/skyscraper strikes, a structure in the midwestern and northeastern USA with a height over 1,500 feet AGL (above ground level) receives an average of between 2 and 6 strikes during each thunderstorm that passes directly over the structure (a few thunderstorms produce no strikes to the structures, while in rare instances some can produce a dozen or more).
Given the fact that places like downtown Chicago, downtown New York City, and the WVAH tower site at West Virginia receive roughly 10-15 thunderstorm events that pass directly over the structures annually, we can conclude that these structures receive an estimated 40 to 90 strikes per year, with anomalous years likely producing no more than 125.
Tall structures in locations such as central Florida (regions that see many more thunderstorm days per year than the midwest or northeast) are more likely to have storms pass directly overhead. Therefore it is plausible that any tall broadcast tower (over 1,200 feet AGL) in the Florida 'lightning alley' would see well over 100 strikes per year.
The stratiform precip region
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Strikes to towers and skyscrapers are most prolific in the electrified stratiform precipitation region of a thunderstorm complex (MCS) or convective squall line (see Fig. 5a below). The stratiform precip region usually extends from 20 to 200 miles or more behind a line of thunderstorms, and is characterized by light to moderate rain with intermittent lightning activity. While tower strikes can and do occur in the primary heavy cores of a thunderstorm, it is within the stratiform regions where they are most numerous and dramatic. Due to the lighter precipitation rates, the cloud bases and attendant visibilities are much higher in a stratiform region, allowing for a much greater length of lightning channels to be visible.
Fig. 5a: Radar image showing a convective squall line and attendant trailing stratiform precipitation region.
Upward lightning strokes from towers and skyscrapers in the stratiform precip regions are usually triggered by an intracloud discharge above the structure, and typically begin several minutes after the main core of heavy rain and lightning in the squall line passes. Many cases there will be a lull in lightning activity after the passage of the main squall line, leading an observer to prematurely conclude that the storm is over. After this lull, which can be up to 10 to 15 minutes long, upward discharges will suddenly begin to occur and continue every 2 to 5 minutes until the electrified portion of the stratiform region has passed. In most cases, two to five upward discharges are typical with the common storm complex, but larger complexes with extensive stratiform regions can produce more than a dozen strokes, since the tower remains under the electrified region for a longer period of time.
Very large thunderstorm complexes (usually associated with severe weather outbreaks) with extensive and highly electrified stratiform regions may produce upward lightning off of towers and skyscrapers even after the precipitation stops falling. As a general rule, if an electrified cloud mass (where lightning is still present) attached to a storm complex is over a tall structure, the potential exists for upward-moving lightning strokes. This can continue for over an hour after the passage of a squall line and result in numerous upward discharges.
Lightning attraction myths
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Storm chasers and scientists can tell you, from their years of experience and observation, that lightning routinely defies its most prevalent myths. In reality, lightning doesn't always strike the tallest object, doesn't always strike the most conductive (metal) object and it is not attracted, influenced or drawn to small objects on the ground, metallic or non-metallic. Photos like this one give compelling evidence that lightning strikes wherever it pleases:
(Click for full-size photo)
Fig. 6: Lightning strikes the ground very close to a metal light pole near Pittsburgh, Pennsylvania.
A person standing in the grassy field where the above bolt hit the ground would see quickly that the tall metal light pole close by didn't help draw the lightning away. The bare dirt and grass certainly wasn't the tallest and most conductive path in the area for the lightning to follow. The 'degree of influence' of the light pole was not enough to affect this strike to ground, even though it was less than fifty feet away.
'Degree of influence' from metal objects
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It has been found that the 'degree of influence' of metal objects on lightning is proportional to the size of the object. Photographic and laboratory evidence suggests that a conductive object will only attract a lightning channel at a distance at or less than the object's longest vertically-oriented dimension. That is, a three-foot high umbrella will not attract or influence a lightning channel that strikes more than three feet away (see Figure 6 illustration below). A metal earring will only attract a lightning bolt that is less than one-half of an inch away! A house or building may attract a lightning bolt that comes down at or less than a distance equal to its height. In other words, for most objects on the ground, a lightning strike must already be occuring at extremely close range for any attraction effects to come into play. This makes any relevance to safety a moot point, as lightning striking within a few feet of a person standing outside is usually just as lethal as a direct hit.
Fig. 7: Small metal objects will not attract a lightning channel that is further away than a distance equal to the object's length. Lightning would have to strike within three feet of this umbrella before it could be 'attracted' to the umbrella.
A tall television broadcast tower or a mega-skyscraper introduces a huge leap in size, and the resultant 'degree of influence', from an umbrella, earring or house. Not only is their immense size incomparable to small metal objects on the ground, these structures significantly reduce the insulating air gap bewteen a thunderstorm cloud and ground - something a house, golf club or umbrella fails to do. Using the degree of influence concept, we can conclude that a broadcast tower that is 1,500 feet high is likely to draw a lightning strike that is occuring within a 1,500-foot radius of its antenna tip. Photographic evidence of lightning strokes to these structures have reinforced this concept.
Cloud-to-ground lightning: attraction to tall structures
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While upward-moving ground-to-cloud strikes account for most discharges to tall structures, these structures do on occasion experience a direct strike from a cloud-to-ground lightning flash. In these cases, a stepped leader for a forming cloud-to-ground discharge must already be descending in the general vicinity of the tower before it can be drawn to the tip of the structure. Using the degree of influence concept, if a stepped leader happens to come down near a tower closer than a distance equal to the structure's height, it may make a last-second horizontal jump over to the tower. The photo below illustrates a clear instance of this occuring to a tower in Oklahoma City, Oklahoma. The lightning's downward branching is the main feature identifying it as a cloud-to-ground strike as opposed to a ground-to-cloud discharge.
Fig. 8: A cloud-to-ground strike is drawn to a tall tower in Oklahoma City, Oklahoma.
Tall structures: Lightning strikes twice
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The old saying that lightning never strikes the same place twice is another myth that any veteran storm observer or researcher has seen nature defy. Lightning can strike any location more than once. In fact, given enough time, it is actually inevitable. It may take as little as less than ten minutes within a single thunderstorm, or longer than a million years - but lightning will eventually strike the same spot again and again. A strike to any location does nothing to change the electrical activity in the storm above, which will produce another strike as soon as it 'recharges'. The previously hit location is then just as fair game for the next discharge as any other spot.
   
   
 
Fig. 8: Lightning strikes twice? How about eleven times - in 20 minutes! These images show ten out of a total of eleven strikes in a 20 minute time frame to the WKYT / WTVQ towers in Lexington, Kentucky during a storm on February 5, 2008. (video clip)
Tall television towers and large skyscrapers blow the 'lightning never strikes twice' myth out of the water. A television tower's antenna often experiences a direct strike as frequently as every thirty seconds during more intense thunderstorms, with a total of three to over a dozen strikes per every half-hour interval that a storm is overhead. A observer wishing to witness a predictable close lightning strike has to go no further than his local television tower during a storm. Towers or skyscrapers that reach or exceed the 1,000 foot mark are virtually guaranteed to take at least one direct hit during every thunderstorm that passes overhead.
Lightning leader phenomena
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Fig. 11: Incomplete upward lightning leaders eminating from the tops of five broadcast towers in Oklahoma City, Oklahoma in response to a simultaneous large 'anvil crawler' discharge directly overhead.
CASE STUDY LINK: Oklahoma City lightning leader event with photo and video documentation.
Most upward lightning events to tall structures occur in conjunction with, and possibly as a result of, an in-cloud lightning discharge above the structure tip. Video evidence suggests that intracloud lightning discharges are often the precursors of, and most likely the triggers for, leader (or 'streamer') initiation from the tops of towers and skyscrapers. Video has also shown that in-cloud lightning flashes initiate leaders off of tall structures that either do or do not go on to connect to the in-cloud discharge event (see photo and video clip linked above). The leaders that do not connect to full discharge will extend only a short distance before terminating in mid-air. Some of these leaders can produce audible thunder that will end abruptly due to the channel being very short in length (watch video clip in linked case study above). The following images are video captures of short lightning leaders that did not go on to complete a full discharge to the cloud. Note that the leader length can vary from less than a few feet to over 200 feet before the electrical breakdown ceases and the channel ends:
Fig. 10: Upward-propagating leaders ('streamers') off of a television tower that did not connect to full discharge.
In the second image above, the leader channel produced audible thunder that lasted for less than 1/5 of a second, indicating the channel was very short in length. The first image shows a very small leader which did not produce any thunder audible to the observer.
Most leader initiation events, however, do result in a full ground-to-cloud lightning discharge, as the leader propagates all the way into the cloud and connects to the intracloud flash already in progress. Video cameras can sometimes capture one or two frames of leader propagation prior to the return stroke of an upward lightning flash:
Fig. 11: Upward-propagating leaders ('streamers') off of a television tower that ended up connecting to a full lightning discharge.
Lightning and tall structures: Photographs
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The following thumbnail images can be clicked to access larger versions of each photo.
>> Lightning strikes five times - two and three towers at once
This cluster of three towers near Clayton, North Carolina carry the antennas for several Raleigh-Durham area television and radio stations, including those for WRAL, WRAZ, WRDC, WQDR, WNCN and WLFL. All three towers rise to a height of just under 2,000 feet, making them prime targets for lightning strikes during any storm that happens to pass overhead. On April 10, 2004 we filmed a relatively small thunderstorm passing over the site, which produced five direct hits to the towers during a ten-minute timespan. Three of the discharges produced strikes to either two or all three towers simultaneously. The center tower, home to WRAL's antenna, took a direct hit during each of the five discharge events. We employed two cameras for the shoot, with one zoomed in close on the antennas and another at wide-angle.
    
>> Lightning's favorite West Virgina target
The ridgetop at Coal Mountain, just west of St. Albans, West Virginia near the Kanawha/Putnam county line, is home to the transmitter sites of several Charleston and Huntington area television and radio stations. For many years, Coal Mountain was home to two of the state's tallest structures, the towers housing antennas for WCHS, WVAH and WFYV. After a February 2003 ice storm brought down the massive WVAH structure, a new tower was built to replace both it and the smaller WCHS structure nearby. After the WCHS tower was removed, the new 1,500-foot structure remains the only tall tower on Coal Mountain. The old WVAH and WCHS towers were guaranteed lightning targets during every thunderstorm that passed overhead, and the new tower continues the tradition. The new tower is routinely hit between 3 and 10 times during most storms that pass overhead.
During 2005 and 2006, Storm Highway undertook a 15-month long project to document strikes to the WVAH tower, using three video cameras carefully set up to capture finely detailed, close up images of the upward lightning events. Many of the photos on this page are a result of this project. Below are video images of 15 strikes to the WVAH tower taken during the Storm Highway project between April 2005 and July 2006. (Video from this project is avialable at StormScenes.com.)
 
       
   
   
SIDEBAR 1: Report and analysis of June 14, 2005 upward lightning video
SIDEBAR 2: Report and analysis of July 4, 2006 upward lightning video
>> Oklahoma City: Tower lightning playground
Oklahoma City, Oklahoma is home to a large number of tall towers clustered together in the northern section of the city. Strong to severe thunderstorms common in this area (known as the heart of Tornado Alley) frequently pass over the Oklahoma City 'tower farm', with staggering displays of ground-to-cloud lightning strikes to the structures. The following photos show two double tower strikes within five minutes of one another during a potent storm on May 29, 2001. Notice the left main channel in the first photo splits into two upward branches.
 
>> Chicago: Skyscraper lightning targets
Chicago's Sears Tower and John Hancock Center are no different from any tall broadcast tower in that they are both frequently hit by ground-to-cloud lightning. The following are from video of several upward-moving discharges to the structures in July of 2006:
    
  
Lightning and tall structures: Video Clips
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All of our video clips are hosted on StormScenes.com. To conserve bandwidth, free registration is required to view StormScenes video files.
Lightning strikes to WVAH Tower - July 4, 2006
Windows Media, 27.4MB
Video from three cameras showing simultaneous telephoto, extreme telephoto and wide shots of three strikes to the 1,500-foot high WVAH television tower near St. Albans, West Virginia.
Lightning strikes to WVAH Tower - June 14, 2005
Windows Media, 2.6MB
A split-screen video showing simultaneous zoom and wide shots of three more strikes to the 1,500-foot high WVAH television tower near St. Albans, West Virginia. Includes slow-motion replays.
Lightning strikes to WVAH Tower - April 17, 2006
Windows Media, 19.6MB
Video from two cameras showing simultaneous telephoto and extreme telephoto shots of yet another three strikes to the 1,500-foot high WVAH television tower near St. Albans, West Virginia.
Lightning strike to WVAH Tower - July 27, 2005
Windows Media, 982KB
A split-screen video showing simultaneous zoom and wide shots of another strike to the 1,500-foot high WVAH television tower near St. Albans, West Virginia. Includes a slow-motion replay.
Lightning strikes Chicago's Sears Tower
Windows Media, 989KB
A lightning bolt strikes the Sears Tower in downtown Chicago during a July morning thunderstorm.
Multiple strikes to towers in Raleigh
Windows Media, 3.2MB
Zoom and wide shots of five strikes to three towers in Raleigh, North Carolina, some two and three at a time.
Nighttime lightning strike to WVAH Tower - April 22, 2005
Windows Media, 1.9MB
A zoom shot of another strike to the 1,500-foot high WVAH television tower near St. Albans, West Virginia. Includes a slow-motion replay.
Ground-to-Cloud Lightning Links
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Charles Allison has taken numerous stunning photos and video of upward-moving lightning from towers in Oklahoma City and Tulsa.
Carter E. Gowl shot a photo of a purely natural ground-to-cloud lightning strike (that is, absent from any man-made structure) to Pilot Peak in Wyoming.
Mike Theiss has some impressive video of upward-moving lightning striking towers in Wichita Falls, Texas.
Sean Gautreaux has a photo of ground-to-cloud lightning strike to one of the former World Trade Center towers in New York City.
Jack 'Thunderhead' Corso has several photos of lightning striking the Empire State Building in New York City. |
