Lightning Facts

What causes lightning?
Lightning originates around 15,000 to 25,000 feet above sea level when raindrops are carried upward until some of them convert to ice. For reasons that are not widely agreed upon, a cloud-to-ground lightning flash originates in this mixed water and ice region. The charge then moves downward in 50-yard sections called step leaders. It keeps moving toward the ground in these steps and produces a channel along which charge is deposited. Eventually, it encounters something on the ground that is a good connection. The circuit is complete at that time, and the charge is lowered from cloud to ground. The flow of charge (current) produces a luminosity that is very much brighter than the part that came down. This entire event usually takes less than half a second.

Where does lightning usually strike?
Lightning comes from a parent cumulonimbus cloud. These thunderstorm clouds are formed wherever there is enough upward motion, instability in the vertical, and moisture to produce a deep cloud that reaches up to levels somewhat colder than freezing. These conditions are most often met in summer. In general, the US mainland has a decreasing amount of lightning toward the northwest. Over the entire year, the highest frequency of cloud-to-ground lightning is in Florida between Tampa and Orlando. This is due to the presence, on many days during the year, of a large moisture content in the atmosphere at low levels (below 5,000 feet), as well as high surface temperatures that produce strong sea breezes along the Florida coasts. The western mountains of the US also produce strong upward motions and contribute to frequent cloud-to-ground lightning. There are also high frequencies along the Gulf of Mexico coast westward to Texas, the Atlantic coast in the southeast US, and inland from the Gulf. Regions along the Pacific west coast have the least cloud-to-ground lightning. Flashes that do not strike the surface are called cloud flashes. They may be inside a cloud, travel from one part of a cloud to another, or from cloud to air.

Can lightning be detected?
Since the 1980s, cloud-to-ground lightning flashes have been detected and mapped in real time across the entire US by several networks. In 1994, the networks were combined into one national network consisting of antennas that detect the angle from ground strike points to an antenna (direction-finder antenna), that detect the time it took for them to arrive at an antenna (time-of-arrival method), or a combination of both detection methods. The network is operated by Global Atmospherics, Inc. Flashes have also been detected from space during the past few years by an optical sensor. This experimental satellite covers the earth twice a day in tropical regions. The satellite also detects flashes that do not strike the ground, but cannot tell the difference between ground strikes and cloud flashes.

How many flashes are there?
Over the continental 48 states, an average of 20,000,000 cloud-to-ground flashes have been detected every year since the lightning detection network covered all of the continental US in 1989. In addition, about half of all flashes have more than one ground strike point, so at least 30 million points on the ground are struck on the average each year in the US. Besides cloud-to-ground flashes, there are roughly 5 to 10 times as many cloud flashes as there are to ground.

What types of damage can lightning cause?
Cloud-to-ground lightning can kill or injure people by direct or indirect means. The lightning current can branch off to a person from a tree, fence, pole, or other tall object. It is not known if all people are killed who are directly struck by the flash itself. In addition, flashes may conduct their current through the ground to a person after the flash strikes a nearby tree, antenna, or other tall object. The current also may travel through power or telephone lines, or plumbing pipes to a person who is in contact with an electric appliance, telephone, or plumbing fixture. Similarly, objects can be directly struck and this impact may result in an explosion, burn, or total destruction. Or, the damage may be indirect when the current passes through or near it. Sometimes, current may enter a building and transfer through wires or plumbing and damage everything in its path. Similarly, in urban areas, it may strike a pole or tree and the current then travels to several nearby houses and other structures and enter them through wiring or plumbing.

How do you stay safe when lightning is around?
Everyone should remember the slogan – When Thunder Roars GO INDOORS! If you can hear thunder you are no longer safe outside. At the first sound of thunder, anyone outdoors should immediately seek shelter. The best defense is to plan ahead and avoid exposure to lightning when a thunderstorm occurs. Know where safe shelter is located and leave enough time to reach safe shelter before your danger level is high. Don't be an isolated tall object, and don't be connected to anything that may be an isolated tall object. The best shelter is a substantial building that is equipped with a lightning protection system. The next safest location is a substantial building that is equipped with plumbing and wiring--in other words, one that is used or lived in by people for a major portion of the day. A very unsafe building for lightning has only a roof and some supports, but no wiring or pipes extending into the ground. A vehicle with a metal roof provides good shelter, and is much better than being in the open or in an ungrounded building, but is not as good as being in a building. For more information on lightning safety visit www.lightningsafety.noaa.gov

How Effective are Lightning Protection Systems?
A great deal of data on lightning protection system performance was collected during the first half of the last century.

Farmers Mutual Insurance Companies of Iowa, studied lightning fire losses from 1905-1912. They determined that the efficiency of lightning rods installed can be estimated at nearly 99%.

Similar statistics were reported by Patrons Mutual Fire Insurance of North Western PA for 1910-1918 during which they paid 414 claims for lightning damage to unprotected buildings and no losses were reported in buildings protected by lightning rods.

In Canada the Ontario Fire Marshall’s inspected lightning protection systems and tracked lightning damage. During 1918-1939 they documented 17,982 lightning related fires of which just 17 involved structures with lightning protection installed. This is an efficiency of 99.999 percent. They also reported that “in no case has a building rodded under the Lightning Rod Act been destroyed by lightning after having been inspected by the Fire Marshall’s office.”

The US Dept of Defense tracks lightning damage at explosives storage facilities, which number in the 1000’s world-wide. In the 82 years from 1918-2000 they report 59 incidents of lightning damage. Of these, 4 were to structures that were equipped with lightning protection.

The FAA tracks lightning damage to its facilities, which are protected by lightning protection systems. FAA documents all lightning strikes within 20 nautical miles of Doppler radar installations. In July, 2000 alone they documented over 250,000 lightning strikes within a 20 nautical mile radius of their sites with just 3 lightning related radar malfunctions. In two Florida locations during this single month, 25,000 and 20,000 cloud to ground strikes were recorded with no equipment failures.

How to Pick a Lightning Protection System Podcast

From iivideo - Lightning can be a dangerous hazard to your home with a single bolt carrying as much as 30 million volts of electricity. Fortunately there are lightning protection systems available to keep your home safe. Find out how to choose the right system from the right professionals to protect yourself from disaster. Watch the video below: