Weather Library > Artificial and natural lightning: Practical comparisons http://stormhighway.com/labsparks.php
Lightning can be artificially generated on a small scale, either by electrostatic machines, impulse generators or even the simple scuffing of one's feet on carpeted floor on a winter day. These man-made sparks have played an important role in what we currently know about lightning, and continue to be useful in research applications. However, there are vast differences between small sparks only inches or feet in length and the miles-long lightning created by thunderstorms. While artificial sparks can be used to reproduce and demonstrate a few properties of lightning and electrostatics, they are not reliable duplicates of full-sized natural discharges. The extreme difference in scale of artifical and natural discharges, including their associated environments and influences, yield vastly different behaviors. Therefore, the researcher must be aware of the limitations of artificial sparks in drawing practical parallels with full-sized lightning phenomena. This article will examine the differences between artifical and natural lightning, and what man-made sparks can and can't tell us about full-sized lightning behavior.
In This Article:
- Charge generation and scale
- Spark propagation
- 'Target' objects
- Case studies and examples
Charge generation and scale
A major difference between artifical sparks and natural lightning is the process used to generate each. A spark-generating machine normally consists of only two 'terminals' between which sparks are induced. This configuration creates a rather simple, symmetrical electric field that produces discharges which are mostly featureless, nearly identical and somewhat predictable. The sparks' energy source and origin points never change, and consequently the sparks themselves vary little in intensity and length. A thunderstorm's electrical structure is much more complex and asymmetrical, resulting in a nearly infinite number of discharge configurations impossible to predict in detail with our current technology.
Furthermore, the reality of scale sets a thunderstorm apart from its significantly smaller man-made counterparts. Scale is an important consideration in most studies of nature, not just in the area of electrostatics and lightning. We can illustrate the significance of scale by considering other areas of science and engineering:
In essence, it is impossible to truly scale down any environment in its entirety without some components remaining the same size. For instance, in the surface tension example, we can increase the amount of water and the size of the suspended object, but the surface tension always remains the same no matter how much we increase the water's volume or surface area. In the case of small sparks versus lightning, we're decreasing the length of the spark gap tens of thousands of times, but the air molecules remain the same size. To recreate natural lightning accurately on such a small scale, you would need to find a way to shrink everything, including the air itself, on a molecular level.
- Strength of materials: The strength of most materials does not remain the same as the dimensions of the object increases. For example, the strength of a steel truss bridge cannot be precisely tested by using a small model of the bridge made out of the same materials. You couldn't triple the size of a bridge and keep the same materials and proportions. Imagine a styrofoam cup, four inches high and three inches in diameter, full of water. If you were to increase the size of the cup, keeping the proportions exactly the same, you would soon reach a point in which the styrofoam would not have sufficient strength to contain the water inside. In essence, you couldn't build a 50-foot tall industrial-sized water tank out of the same styrofoam that the cup is made out of, even if you kept the exact proportions (thickness, density, etc) of the smaller cup. The styrofoam exhibits different structural integrity based on the scale of the object made with it.
- Surface tension of water: Very small objects, such as insects, can suspend themselves on top of water without the aid of bouyancy due to the water's surface tension. If the size of the object increases, even if the proportions remain exactly the same, its weight will eventually overcome the surface tension and the object will sink.
The difference in scale between artificial and natural lightning introduces another fundamental discrepancy between the two in the subject of leader propagation. This is particularly true with cloud-to-ground lightning strokes, for instance, where the stepped leader's 'steps' themselves are on the order of 30 feet or more. This means that an artifical spark less than 20 feet long hasn't even approached the length of a single 'step' of the typical full-sized lightning leader.
The size of the 'target' object is another aspect of the difference in scale between artificial and natural lightning. A natural cloud-to-ground lightning strike can range from 2 to over 6 miles in length, while even the largest man-made sparks are less than only several yards long. This is an important consideration when testing the susceptibility of specific items or materials to artificial sparks. Consider a 2-foot long metal pipe tested in a laboratory with an impulse generator producing sparks a few feet long. When the whole test environment is scaled up so that the length of the artifical spark matches the proportions of natural lightning, the setup would actually be simulating a super-tower or mega-skyscraper on the order of some 10,000 feet or more high! The same 2-foot pipe tested with a 5-inch tabletop Van de Graaff generator spark would be equivalent to a tower or skyscraper over 50,000 feet tall, which is taller than most thunderstorms themselves. Of course, no metal structures of that size exist on earth. Any results of such a test (including for example the conclusion that lightning is 'attracted' to the metal pipe over a wooden pole) could not be extrapolated to the behavior of natural lightning.
Case studies and examples
The Umbrella Test
Let's say we want to simulate the effects of an ordinary umbrella on a lightning channel using a five-inch spark from a tabletop Van de Graaff generator. The length of the natural lightning bolt that we want to simulate is two miles (3.2 kilometers) long, or 10,560 feet (3,219 meters). The length of the umbrella is 3 feet (about 1 meter) long. In this test, we are reducing the two-mile (3.2 km) long lightning bolt to a five-inch (12.7 centimeters) long spark. That's a reduction in scale of a factor of 25,344 - the Van de Graaff spark is 0.00004 of the size of the natural lightning strike. You can see that we clearly can't use the full-size umbrella (or even a tiny toy model) and get anything resembling an accurate test result!
To keep the scale of the test subjects correct, we will need to find a suitable replacement object for the umbrella that is 25,344 times smaller than the full-size one. To be specific, we need an umbrella or metal rod that is 0.0014 inches (0.036 mm) long! That's an object we couldn't even see without a microscope. Even a tiny iron filing particle would be too large.
Since we can't realistically perform our test with microscopic particles, what if we just decided to use an iron filing? An iron filing that is about 0.5mm (0.02 inches) long is equivalent to a 42-foot (13m) tall pole, when scaled up for consideration in the same test with the five-inch Van de Graaff spark.
A recent episode of the Mythbusters television show featured a debunking of the myth that small metal objects, such as jewelry, can attract lightning. To test the myth, two ballistics-gel dummies were targeted with large laboratory-generated sparks. Mythbusters found that the only object that consistently 'attracted' the lightning was a doorknob placed on the head of one dummy. However, the critical difference between these tests and natural lightning was once again, the scale. The spark-generating terminal was aimed at the dummies at point-blank range, directly above the dummies' heads by only a few feet. In reality, this setup was simulating a stepped leader at its final stage toward the ground - a stepped leader that, every time, was ending up a few feet above the dummies. If this were a real-world scenario, one would expect the chances that the dummies would be hit to be very high, since the lightning was getting ready to strike only inches from them to begin with.
A natural lightning bolt is starting out at high altitudes, traveling through miles of air before it reaches the ground. If lightning is coming down and happens to end up a few feet over a person (like the Mythbusters' test sparks simulated), only then may something metal on your person have some influence. It's similar to the difference between firing a gun at a target from a distance of three feet to a shot from a distance of two miles - the target will get hit more often when the gun is fired at close range.
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