Thursday, January 20, 2011

Steph’s Science Corner: The Lightning Mystery

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Photo courtesy of http://cozart.org/images/lightning.jpg We all know the famous story; Benjamin Franklin stands in a storm with a kite, a key attached to its base, and uses lightning to discover electricity. Or maybe he discovers lightning with electricity. Or perhaps he discovers that that lightning was made up of electricity? Most of us don’t actually know what happened. In 2006 Mythbusters ran an experiment on whether Franklin could have survived a direct lightning-strike to the kite, and found that he could not have. So what was the real story?

In whatever direction the truth actually leads, many of us are fascinated by this awesome, powerful, and deadly metrological phenomenon. The deafening crack of the ensuing thunder can rattle the most stoic nerves. Yet it is as commonly misunderstood as it is terrifying. Most of us are guilty of spreading at least one incorrect fact about its nature, its origin, its behavior, or how best to avoid it. But you might be surprised to discover that until this last decade, it was just as misunderstood by modern science. A staple of our physical world that has been around since the era of man (and not just on our planet), the mechanism of a lightning strike is poorly understood and highly debated, and it is still being studied at research centers across the world.

Some people choose to see this as the will of God, humiliating the efforts of mankind’s futile attempt to understand his divine creation. If this describes you, you probably found this blog by accident. But if you’re like me and you think thunderstorms are thrilling, then you might appreciate this week’s topic. We’re awfully close to a complete understanding. It’s just much more complicated than most of us realize.

The Ben Franklin story

From the Benjamin Franklin House in London, kid's page. In 1750, Franklin published a proposal for an experiment to show that lightning consisted of the same electricity that was produced by rubbing things together (like wool socks and carpet). The plan was to extract a spark during a storm using an apparatus made of a kite with a metal key attached to it by a piece of string. Franklin argued that the static charge in the air would transfer to the key. Then, upon touching the key to a finger or a Leyden Jar, a spark would leap out. Getting struck by lightning was not part of the plan. The speculation is that he conducted his experiment in 1752, but it wasn’t credited to him until 1767, and not by Franklin himself. He is credited with inventing the lightning rod, however.

Franklin often indicated that he was aware of the dangers of getting electrocuted, and made all attempts to remain well-insulated (perhaps by using a silk kite string, or by attaching it to the ground). During this time in history, scientists were electrocuted and killed, including one very famously and very gruesomely, but you’ll have to read to the bottom to find out about that.

Static shocks and the breakdown of air

imageWhen you drag your socks across the carpet, you rub electrons off it and onto yourself, building up a charge. This build-up creates an electric field between you and the other things in the room. But air is not a very good conductor, so electrons stay tightly on you. But as you decrease the distance between you and another object – the doorknob, the cat, your friends – the electric field (which depends on distance) becomes locally very strong. The closer you get, the stronger the field, and at microscopic distances, the field becomes so strong that it overcomes air’s ability to insulate. You experience an arc-discharge of all the electrons you’ve been carrying as they leap across the air to your target. This phenomenon is called dielectric breakdown, and every insulating material has a specific voltage at which this occurs. For air, it is three million volts per meter of distance that the arc has to travel. The picture above shows dielectric breakdown in a piece of insulating material, and you’ll notice that it looks an awful lot like a lightning strike. Problem solved? A lightning strike is just the dielectric breakdown of air when the field between the clouds and the ground gets too large, right? But my readers know me better.

The strike

imageIn a storm, clouds build up charge (we'll get to the how next). The earth is a repository of spare charge, and when a electrons build up on the bottom of a cloud, it repels negative charges away from the ground beneath it, causing a swell of positive charge at that spot. When the electric field between these two opposites grows too large, dielectric breakdown occurs, and the “strike” is a sudden, massive flow of current. Part of it comes down from the cloud, but there is also a simultaneous rising-up of positive charge from the ground to meet it, known as a streamer. Anything on the earth has the potential to make a streamer, and the higher up from the ground the object reaches, the shorter the path the electricity has to travel to discharge, making higher objects better targets.

intracloud lightningLightning doesn’t always strike the ground. It’s much more common to see lightning between two charged ends of a single cloud, or intercloud lightningbetween one cloud and a nearby neighbor. But regardless of the type of strike, the sudden, extremely high current generates a massive amount of heat, causing the air around it to expand. This forms a concussive blast that reverberates outward, which is how we hear thunder.

Charging up the clouds...but where’s the extra charge?

This is where the debate begins. We aren’t exactly sure by what mechanism clouds build up charge. The leading theory speculates that frictional transfer occurs between ice crystals and water droplets (just like static shock in the above analogy). But then how do we explain how positive and negative charges separate? One theory is that when neutral ice droplets fragment, the heavier bulk piece keeps the negative while the smaller pieces carry the positive, and separation occurs because of gravity. Another theory suggests that positive ions are pushed up from the earth by convective updrafts. The thermoelectric theory has to do with colliding droplets of different temperatures, and there’s another weird phenomenon that we’ve observed called Graupel melting, which shows that melting ice that contains air bubbles makes positively charged water. There are many other theories as well, and no one is certain exactly which one is correct.

Runaway electron avalanches and X-rays. Are aliens to blame for lightning strikes?

There’s still one huge problem. No measuring device that we’ve sent up into the clouds has even come close to measuring 3,000,000 volts per meter. At best, we’ve measured electric fields of about 200,000 volts, which is a little more than six percent of the field size that we need for dielectric breakdown (much less when you consider distance to the ground). This is where modern science has been stuck. Until recently, few explanations held merit, and they were weak at best. One idea was that higher field strengths existed, but in such tiny volumes for such brief moments that no one had managed to measure one. But that still left the question of how. The propagation of lightning adds even more curiosity – why the jagged path?

imageIn conventional discharge, electrons experience drag force, like when you stick your hand out a car window. The faster you go, the more air-resistance you feel. But electrons are special in that once they get really going (around two percent of the speed of light, or six million meters per second) that drag force starts to decrease. If something gave them a big enough push, they could gain an enormous amount of energy as they sped downwards, in a process called Runaway Breakdown.

So what pushes them? Current theories blame the high-speed cosmic particles that are bombarding our atmosphere from space. If a high speed proton collided with an air molecule, it could eject high- energy “seed electrons.” This small population could make more collisions, creating a cascade, and all these electrons would be further accelerated by the electric field within a cloud. A lead group of electrons would then emerge, colliding with the air, creating a step-by-step ionized channel with every collision, like the carving of a riverbed. This lead-group leaves in its wake a conduit through which current can flow when it reaches the ground.

How could we prove this? If you remember back to my post on X-rays, you’ll recall that high energy radiation forms when high-speed electrons collide with bulk materials. As recently as 2005, groups of scientists recorded X-rays and gamma rays that appear to have been emitted during the moments leading up to a lightning strike. Even with this discovery, however, the numbers still don’t quite add up. And taking these measurements isn’t exactly easy, considering that lightning strikes are difficult to predict (and to reproduce), and that a huge burst of energy is released, which could fry or at least mask the readings on plenty of devices. But it seems like cosmic rays from outer space might be the missing link in the question of lightning formation.

Types of lightning, how they hit, the sounds they make, and which ones kill you

imageThe most common form of lightning, responsible for 95% of all strikes, is negatively-charged lightning that arcs from the lower-half of the cloud. The current runs from 30-300 kiloamps, with a charge of five coulombs, a potential of 100 megavolts, and an energy dissipation of 500 megajoules (enough to power a 100 watt bulb for two months). They are followed by several secondary strikes in succession. They are hotter than the surface of the sun, at 30,000 degrees Celsius. The heat, not the electricity, is what’s responsible for the brilliant white-blue flash.

http://www.srh.weather.gov/srh/jetstream/lightning/images/lightning2.jpg It is also possible for the positively charged upper half of the cloud to form a channel to the ground. Positive lightning is rare, and significantly more deadly. It carries a current of at least 300 kiloamps, a charge of 300 coulombs, a potential of one gigavolt, and it dissipates enough energy to power a 100 watt bulb for 95 years. They are usually composed of a single strike that lasts for about 10-100 milliseconds. Modern airplanes can withstand a negative lightning strike, but not a positive strike. They tend to occur at the end of a storm.

Photo courtesy NASA and http://science.howstuffworks.com/lightning8.htmfrom www.stormchasing.nl/lightning.htmlOther types of lightning include ground-to-cloud lightning (left), bead lightning (a high-intensity form of cloud-to-ground lightning that leaves behind a visual effect that resembles a string of beads), ribbon lightning (high cross-winds make the channel straight, and multiple return strikes cause it to appear thick, see right), and staccato lightning (a rapid succession of single strikes).


Super-cool, unusual types of lightning

Remember that promise I made about stories of gruesome deaths? One of the strangest and most elusive forms of lightning (or whatever it is) is ball lightning. Doesn’t sound scary? Witness accounts vary, it has rarely been caught on film, and it is possible that several different phenomena are being incorrectly grouped together. But it’s still really cool.

Photo taken by student in Nangano Japan, sourced from http://en.wikipedia.org/wiki/Image:Ball_Lightning.jpgBall lightning is a giant ball of red or yellow flame-charge that floats through the air. They can appear during thunderstorms, but some have been recorded during clear weather. One of the earliest recorded events was during The Great Thunderstorm in the UK in 1638. Four people died and 60 were injured in what sounds like a ball lightning strike of a church, according to witness descriptions.

The most famous instance was in 1753 when Professor Georg Richmann of St Petersburg was attempting to recreate Benjamin’s Franklin’s kite experiment. He was attending a meeting at the Academy of Sciences when he heard thunder. He ran home to set up his experiment only to be struck in the head by a ball of lightning, killing him and leaving a large red spot. His shoes were blown open, his clothes singed, his door torn off its hinges, and his personal engraver (who was present to record the event) knocked unconscious.

Common misconceptions that you should stop spreading around

I’ll conclude with this list of corrections. If you take nothing away from this post other than these facts, I’ll count my job done:

1. Hiding under a tree is a bad idea. The higher the tree, the more likely it is to get hit. Make yourself as low as possible and try to spend time near other low-lying objects.

2. Yes, lightning strikes can stop your heart, but you’re in much greater danger from the heat and the concussive force. Some people survive direct lightning strikes for this reason. It’s worse to be right next to one than to be the object of one.

3. Cars are not safe because of rubber tires, they are safe because they act as Faraday Cages. If a conducting object is hollow, all charge resides on the conductor’s surface -- none of is exists in the hollow interior. If your car is hit by lightning while you’re inside of it, try not to touch the edges. Charge will dissipate to the ground through the metal body of your vehicle.

4. There is absolutely no reason why lightning can’t strike the same place twice. In fact, several tall buildings are struck repeatedly during storms. They’re designed to safely dissipate this charge into the ground without harming the insulated people inside

5. To calculate how far away you are from a lightning strike, count the seconds until you hear the thunder. Sound travels about 0.22 miles per second. Multiply the number of seconds by 0.22 (or just divide by 5) to figure out how many miles away it is from you.

Many of the graphics and facts used in this post come from a May 2005 Scientific American article called “A Bolt Out Of The Blue,” by Joseph R. Dwyer. It’s a great read, and he does good research.

...and that’s it for this week! I’ll be heading out of the country in about eight weeks, so get your post ides in now before my little sabbatical. Email me at science@charge-shot.com.