Lightning occurs as the result of powerful movements from within a storm cloud. Wind rapidly circulates fine droplets of rain and crystals of snow and as a result, electrons are wrenched free, building up large amounts of static electricity. The charge increases until it becomes so powerful that the air can no longer provide enough resistance to prevent an electrical breakdown, and a huge spark is born: lightning.

The lightning bolt can travel downward (from cloud to ground) or upward (ground to cloud) and has two forms - hot and cold. Hot lightning is so called because the flashes carry a continuing current; while cold lightning does not. Fires tend to be started by hot lightning, though cold lightning can be highly destructive as well. This is so because heat tends to be produced by the amount of resistance that a given object has to electrical current. Even though cold lightning is relatively brief in duration (thousandths of a second vs. tenths for hot), if the material has a high enough resistance to the flowing current, then it can be heated so rapidly that its mass vaporizes, building up extreme pressure, which can then blow the object apart. If resistance is low, however, the material experiences little heating and subsequent damage (materials with low resistance to current are called conductors, i.e. metal).

The Bolt of Lightning: a study in attraction

One simple principle governs the formation of a bolt of lightning: attraction. As can be seen with your typical battery, electricity has two charges - positive and negative. The opposite charges are drawn to one another because of their need to meet, or attraction. In fact, as the negatively charged electrons are torn free by the turmoil within the storm cloud and accumulate at its base (resulting in a net negative charge), a positive charge will actually gather beneath the storm, following closely as the cloud moves. The positive charge will build in particular on objects that protrude above the earth`s surface, such as trees or tall buildings. As the charges continue to increase, they move ever closer towards one another. At this point, a dim traveling spark, known as a stepped leader, begins to travel from the cloud to the ground in rapid steps around 50 yards in length. Each step, or the zigzag shape we commonly identify the shape of lightning with, takes less than a millionth of a second to complete, with the time between steps being some 50 millionths. This occurs without any "knowledge" of what structures or geography may exist below the leader, meaning the eventual strike point has not been determined until after the strike has already begun. In fact, "awareness" only occurs when the leader is anywhere from 10-100 yards from its final point of contact. The leader itself cannot be seen by the naked eye, but special cameras can capture it on film. According to Colorado lightning photographer Ken Langford, follow these steps when attempting to photograph lightning: Always shoot from as safe a location as possible! Know how to use the manual setting of your camera. Use a tripod. Use a slow film such as Kodachrome 64. Use the widest aperture setting. Focus to infinity. Shoot using the bulb setting. Once awareness takes place, an upward-moving spark originates from the strike point and jumps to the stepped leader, forging a finished path to the ground. This channel is an incredible conductor, so once the connection is complete, negative charges at the bottom of the channel flow forcefully to the ground, generating a large current that illuminates it. This brilliant flash, called the return stroke or channel luminosity, is generally a series of three to four separate strokes, separated by pauses of 40 to 50 thousandths of a second, which manifest too quickly for the eye to differentiate between. The flash then continues upward at a rate of 20,000 - 60,000 miles per second, causing the great boom of thunder. This noise results from the arc of energy rapidly heating the surrounding air molecules, which then cool quickly, creating a sound wave that covers 1 mile in around 5 seconds: Use this simple formula to determine how far away a lightning strike is to you (Time between flash & thunder) ___________________________    =    (Distance)       5 Example: 15 seconds _____________    =    3 miles       5 The majority of strikes to small structures and the ground happen in this fashion, with a strike being initiated by a stepped leader traveling to the ground. Tall buildings, however, are an exception to this rule. Due to their ability to collect a net positive charge, the structures themselves tend to send up their own stepped leader, creating a discharge that branches upward instead of down. This means that lightning can indeed travel in both directions.

The probability that lighting will strike a given structure hinges on two main factors: the height of the building and the frequency and severity of thunderstorm activity experienced in the surrounding area. As discussed in the article above about how lightning forms, oftentimes the strike point on a tall building is where the lightning bolt initiated from. For smaller objects and structures, such as trees and residences, the opposite is generally true, which means they are less likely to be struck (though several other elements also contribute, such as the size of the structure and relative location to other taller buildings or land forms). According to Martin A. Uman from the Department of Electrical Engineering at the University of Florida, Gainesville, “for a structure projecting above a moderately flat area in a region of moderate thunderstorm activity, say Pennsylvania or New York, a 50 ft structure will be struck about once every 4 - 6 years, and a ¼ acre of flat land – large residential lot – will be struck about once every 100 years or more”. Applied to a well known skyscraper, such as the Empire State Building in New York, this rule would indicate that it should be struck quite often, which it very much so is. In a given year, in fact, the Empire State Building is struck by lightning an average of 23 times, with the maximum number ever recorded for the same period being 48. During one thunderstorm alone, an incredible eight strikes hit the building within a span of 24 minutes! This chart reflects the ratio of strikes expected over the course of a year to the height of a given building:

Number of Strikes Height of Structure (in ft)
1 300
3 600
5 800
10 1000
20 1200

The material used in covering a roof can reduce the amount of damage sustained in the event of a direct lighting strike. Since plastic, wood, and asphalt all have a high resistance to electrical current, they are prone to catching fire or exploding upon being struck. Metal, on the other hand, “is both an electrical conductor and a noncombustible material, [so] the risks associated with its use and behavior during a lightning event make it the most desirable construction material available”, says the Metal Construction Association.

There is a large body of evidence to back this claim, as metal actually helps to protect occupants of cars during a thunderstorm, because should the care be struck, the current will tend to flow through the metal skin of the vehicle, as opposed to the interior. The same goes for modern commercial aircraft, which are almost entirely metal, so lightning current does not penetrate them often, and those that are hit continue to fly in almost all cases.

Click here to learn more about metal roofing.

In addition, metal plays a vital role in the only known method for protecting a building or tree against lightning – the lightning rod.

People have used a variety of methods over the centuries to attempt, sometimes with fatal results, to ward off lightning. Case in point, until as recently as the 18th century, church bells in Europe were rung during electrical storms because the “educated” believed that the noise made by the chiming bells broke the continuity of the lightning path, while the “ignorant” were convinced that the sounds only purpose was to repel evil spirits. This is why many Medieval bells can be seen to have the inscription Fulgura Frango, or “I break the lightning”. The results of this logic were nothing short of catastrophic: over a period of 33 years, 386 church towers were struck and 103 bell-ringers died while holding their ropes.

Other Interesting Facts about Lightning