Lightning Strikes

A look at how they affect aircraft and avionics.


When it comes to natural phenomenon, lightning has probably been the most observed throughout the ages. It has been worshiped, studied, and feared. Lightning is a result of static electricity and has been seen in volcanic eruptions, intense forest fires, surface nuclear detonations, heavy snowstorms, in large hurricanes, and even in the exhaust of large turbine engines.

However, it is most often seen in thunderstorms.

Ben Franklin usually gets the credit for concluding lightning is based on the theory of static electricity.

The Franklin experiment is as follows: He got the idea of using a flying object, such as a kite. During the next thunderstorm, which was in June 1752, accompanied by his son as an assistant, he raised a kite. On his end of the string he attached a key and tied it to a post with a silk thread. As time passed, Franklin noticed the loose fibers on the string stretching out; he then brought his hand close to the key and a spark jumped the gap. The rain had soaked the line and made it conductive.

Please do not attempt this at home!

At any given moment, there can be as many as 2,000 thunderstorms occurring across the globe. This translates to more than 14,500,000 storms each year. The National Aeronautic and Space Administration (NASA) satellite network indicates they produce lightning flashes about 40 times a second worldwide.

Positive and negative charges

A thunderstorm gathers a pool of positively charged particles as it moves along the ground and they travel with the storm. As the differences in charges continue to increase, positively charged particles rise up taller objects such as trees, houses, and telephone poles. A channel of negative charge, called a "stepped leader" will descend from the bottom of the storm toward the ground. It is invisible to the human eye, and shoots to the ground in a series of rapid steps, each occurring in less time than it takes to blink your eye. As the negative leader approaches the ground, a positive charge collects in the ground and in objects on the ground.

This positive charge "reaches" out to the approaching negative charge with its own channel, called a "streamer." When these channels connect, the resulting electrical transfer is what we see as lightning. After the initial lightning strike, if enough charge is leftover, additional strikes will use the same channel and will give the bolt its flickering appearance.

Thunder is a product of lightning and is a direct result of the shockwave generated by the rapid air movement caused by the streamer. As light travels faster than sound, we will observe the flash of lightning prior to the rumble of the resulting thunder. A general precaution is to consider lightning to be a local hazard if the thunder occurs within 30 seconds of the flash.

Some lightning originates in the cirrus anvil or upper parts near the top of the thunderstorm, where a high positive charge resides. Lightning that forms in this region follows the same situation as previously described, but the descending stepped leader will carry a positive charge while its subsequent ground streamers will have a negative potential. These bolts are known as "positive lightning" because there is a net transfer of positive charge from the cloud to the ground.

Positive has negative effect

Positive lightning makes up less than 5 percent of all strikes. However, despite a significantly lower rate of occurrence, positive lightning is particularly dangerous. Since it originates in the upper levels of a storm, the amount of air it must burn through to reach the ground is usually much greater. Therefore, its electric field typically is much stronger than a negative strike. Its flash duration is longer, and its peak charge and potential can be 10 times greater than a negative strike; as much as 300,000 amperes and 1 billion volts!

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