Frequently Asked Questions
Does lighting make a sound before it hits the ground?
To answer your question specifically, the sound of thunder is the superheating of molecules along the lightning channel. This doesn't happen until the cloud to ground path or "visible" path is completed. The sound is not lightning hitting the ground, but there must be a complete path from cloud to ground for the thunder to sound.
Where does lightning come from? Does lightning start from the ground and travel upwards, or does it start from the sky and travel downwards? And why does that happen?
>The answer to your question depends on how you look at things, and that’s where a lot of confusion lies. Clouds become charged through the separation of ions created by dust particles and ice crystals "rubbing" together as winds and updrafts circulate around. These ions generally accumulate in different portions of the cloud, and once they reach a level of intensity that allows for the breakdown of a very good insulator (air) they begin to reach down from the cloud, approaching earth or anything ground mounted. These invisible "feelers" reaching toward earth are called step leaders (they can be viewed and studied with an infrared lens). When they reach a certain point above earth or a ground-mounted object dependent on the intensity of the step leader, they pull ions of opposite charge up from ground-mounted objects, called streamers. Pointed objects or edges release ions more easily, so a flagpole, roof ridge, building corner, or the branches of a tree can provide an improved attachment point. Once the ions pulled up meet the step leader from the cloud, you have a complete circuit and the visible lightning bolt, or interchange of ions to neutralize the electrical imbalance. Several return strokes may follow the same path causing the "flickering effect". Ions located in the base of the cloud are generally negatively charged, so most lightning strikes (around 92%) are negative cloud to positive up from the ground. Positively charged sections of the cloud, normally near the top ("cloud anvil"), can also pull negative ions from below, generally from tall radio or TV towers, and account for the balance of strikes.
How far can lightning travel in salt water? A friend told me he heard on TV that it can travel 5 miles.
Lightning is trying to reach "ground" or enough oppositely charged ions to neutralize the lightning event. Lightning may travel through water to seek "ground" (possibly the earth below the surface or the shoreline), and the distance is variable based on numerous factors. If you will recall from junior high science, pure or distilled water is not a conductor of electricity. It is the mineral content of water that really makes it a better or worse conductor. Therefore salt water is a better conductor than fresh water, although both are conductive to some extent because they are not "pure." The questions is really how far any body of water will conduct and in what direction or directions? When lightning strikes in the middle of the ocean, is everything "electrified" for miles and miles in every direction? No, probably not - the charge we assume will seek the ocean floor. Exactly which direction will the charge go and how far would be a safe distance if a human were in the water? That is impossible to predict, so it is best to not be in the water when there is lightning in the area.
I work for the Safety Department and am concerned about lightning striking close to our building that could travel through the ground and into the building potentially affecting an employee. Will the effective ground provided around the building prevent such an occurrence?
Step potential deals with the effect of lightning spreading out over the surface of the earth to neutralize its charge imbalance. In soils with good electrolytic or high moisture content (clay, loam soils) it spreads out over a smaller area than in poorer soil conditions (rocky or sandy soils). This situation leads to an "electrified grid" of earth for a short duration. Someone standing in the affected area would have a difference of potential between their feet, dependent upon how far they are spread apart, and thus a current flows in the body. This is why many times a person in the vicinity of a strike gets knocked off their feet. A person lying on the ground may suffer more severe consequences, since more critical body parts may receive this current flow. Because of this same effect, isolated grounds for systems may pick up the ground charge at different time intervals, causing a difference of potential within a structure. If any electrical ground and a communication ground are connected to the same internal piece of equipment, the difference of potential will cause a current flow within the equipment, which may lead to damage in equipment or a hazard to personnel operating the equipment. This is the reason for interconnecting all grounded systems that enter a structure, along with structural framing and things like metallic water or gas piping to create a common ground. Surge suppression is also used to provide an equipotential bond for active service lines entering a structure that cannot be directly connected to ground. Besides common grounding of internal systems, two methods prove effective in creating a safer interior workspace for personnel. A ground conductor loop can be installed encircling the structure with connections to the grounded systems, incoming piping, and structural framing members, whether they are steel columns or reinforcing steel in poured columns. A structural lightning protection system would terminate at this ground loop. In this way all direct strikes and strikes outside the area would cause the loop conductor and any items attached to see the potential rise and fall at the same time casuing little effect internal to the loop. In a situation where a concrete slab is poured under the structure, the rebar can be interconnected to the loop at 100 ft. intervals around the perimeter, which will enhance the total grounding of the systems and provide equal potential for personnel standing on the floor. Most other forms of flooring are insulating by nature. In areas where there is only an earthen or gravel base, the loop is still fairly effective at maintaining potential within its boundaries. In applications where major fault currents could be expected, such as at a transfer station for an electric utility, a grid of conductors is laid below the earth to provide additional personnel protection. Again this grid allows for the rise and fall of potential equally, creating additional personnel safety. We often see this method used on small prominent structures where people go to avoid storms, like golf or picnic shelters.
"...a lightning protection system will not attract a bolt of lightning." Does this mean that if you have two arrestors on your roof, 10 to 20 feet apart, that the lightning bolt could come down right between the two arrestors?
Zone of protection in NFPA 780 is described by a 150 ft. radius sphere model. The air terminals or lightning rods are designed to support this "ball" off the insulating material of the structure, and therefore take the lightning strikes rather than the structure. If you consider this "attracting" lightning for a 10 ft. radius in certain instances, then I guess the strike termination devices do attract slightly. Many people have the idea that lightning can be attracted over some great distance, thinking that lightning protection "attracts" a large number of strikes to your property or causes lightning to strike your house rather than the neighbors. This concept of attraction is not true. Lightning protection generally can be thought of as protecting against lightning strikes that were going to occur to your house anyway.
On an overcast day last year, we observed from the safety of the house, a ball of light about 15 feet above the ground. It appeared to be a gigantic flash bulb, about 2 basketballs in size. There was no sound and no apparent damage anywhere. Is this lightning?
There are many "permutations" of lightning that have been reported. Everything from "St. Elmo’s Fire" that is ions being pulled up from the masts of ships, to "ball" lightning which has been reported to flash through a window, dance around the metal framework at the foot of a bed, and exit on to the next room. Lightning is normally looking for earth ground to neutralize the charge imbalance that has created it, so we expect it to strike a tall object and follow the structure (building or tree or whatever) into the ground. But that doesn’t always hold true. Obviously if lightning can jump 1 1/2 to 3 miles from a cloud base to ground through a very good insulator (air), it can move through the air sideways as well. In cases where the earth’s structure doesn’t contain an adequate electrolytic content to effectively "ground" a stroke, then the lightning can jump from item to item "looking" for a better pathway to a better ground. We cannot confirm your "apparition" was lightning, but it is possible.
I've always been worried about flashovers. Can you tell me more about them?
The major factor involved in flashover or side flashing within a structure is lightning seeking a "better" path to ground through other building grounded systems. U.S. Standards like NFPA 780 require the interconnection of all building grounded systems at grade level to equalize potential. On buildings 40 ft. (13m) tall or taller, all building grounded systems should be interconnected at or near roof level as well. This assures there are no better paths to ground than the lightning protection system components, which are properly designed to provide the best most direct path with no damage to the structure or contents. For very tall structures, intermediate equipotential loops are required at 200 ft. (60m) vertical intervals.There is also a formula shown in NFPA 780 for calculating side flash distance based on the distance from the closest bond between grounded systems, the number of downleads to ground in the area under consideration, and whether the sideflash would occur through air or building materials. If a grounded body lies within the sideflash distance it is bonded to the system at that vertical height, if not, it would not need a bond. Isolated or ungrounded metal bodies are only considered if they may help create a short-circuit path for lightning to follow between grounded metallic systems, since lightning is seeking ground and wouldn't leave a grounded path for an ungrounded one. You may wish to get a copy of NFPA 780 or LPI 175 for a full description of these requirements.
How could lightning have hit a home not as tall as surrounding homes and/or trees and have blown out roof sections near the ventilation jacks?
Lightning is a fairly random occurrence. It may not strike what you would consider the most likely target, but there are some parameters that we use. If we provide a complete lightning protection system on a structure, strike termination devices and conductors to a grounding system, the Standard consider this to provide a "protected zone" based on a 150 ft. radius sphere model. A 150 ft. radius curve, tangent to a lightning rod at the top and to earth at grade, provides shielding to items under the curve. To give you a simplified idea of distance, at lower elevations this replicates a 1 to 1 relationship, so a lightning rod mounted on a 30 ft. tall roof protects about 30 ft. out on the ground. It is important to understand that height may break the curve, so if you are 6 ft. tall and standing on the ground, you would need to be within 8 ft. of the 30 ft. tall building to be below the curve and not subject to a direct strike.You may have the idea that tall items within every square mile will take all strikes, but this is obviously not true from the zone of protection parameters used in the Standards. Protecting a structure really protects only that building and a small surrounding area. If there is not protection in place, then height, the ability to release ions, and conductivity give clues to lightning attachment probability. Trees may be tall, and have pointed appendages to release ions, but they are not very conductive. There are many items in residential construction that can provide superior paths to ground - like the electrical service, plumbing piping, gas systems, and phone systems - so a structure may be a better candidate for a strike. We would also note that two houses 30 ft. apart have little effect on each other. Again, the randomness of attachments comes from the fact that lightning cannot be attracted to anything over any great distance, even a complete protection system.
What are the consequences of indirect lightning?
Indirect lightning may be the name used for a variety of situations. A direct strike is fairly straight forward, but associated consequences may not be. There can be side flashing within a structure between separately grounded systems if they are not interconnected. The massive electromagnetic force of a lightning strike may cause inductance to wiring for any system in close proximity to the path of the lightning between a roof and the ground. Once lightning reaches the ground, it spreads over the surface of the earth for a distance to neutralize the charge with the distance dependent upon the electrolytic content of that particular soil structure. This causes an "electrified grid" effect, which may impact anything in the vicinity - people standing on the ground, earthing or grounding devices buried in the ground, or metallic piping systems. Lightning may strike utility lines and be carried into the structure if surge suppression is not applied at service entrances. This is why bonding of all building systems at grade, interconnecting long grounded systems near their top within a structure, and surge suppression is so important – all parts of a complete lightning protection system along with the direct strike protection.
Where can I find the best photo gallery and information on lightning?
We do not have a photo gallery on this site. When looking for things like this, we normally go to the website [www.sirlinksalot.net/lightning.html], which has lots of different sites that you can access on lightning. There are several sites to choose from showcasing lightning photos. Another good site for personal safety information is the government sponsored National Oceanic and Atmospheric Administration (NOAA) website at [www.noaa.gov].