Lightning & Thunder

Both lightning and thunder are an aspect of meteorology that even young children are aware of. Almost everyone knows lightning is a discharge of electricity, thanks to Benjamin Franklin, who back in 1752 flew a kite attached to a wire and a key to prove this hypothesis. But there's a lot more to know about lightning and thunder, and I hope to expand the current knowledge you have with some information about the science behind it all. I'll begin with a picture of lightning I happened to capture in the middle of summer in 2008. It took a lot of time and patience, but I can finally say I have photographed lightning.



Lightning
Most of the lightning we are familiar with occurs within a mature thunderstorm, which is composed of cumulonimbus clouds. Cumulonimbus clouds are the big, towering clouds that grow in size vertically rather than horizontal. However, many people are unaware of other places lightning can occur. Volcanic eruptions, intense forest fires, nuclear detonations, heavy snowstorms, and some dust storms are also capable of displaying this phenomena. Below is a picture of lightning occurring in conjunction with the eruption of Eyjafjallajokul, the volcano that spewed lava in Iceland in the Spring of 2010.


Photo Courtesy: http://www.boston.com/bigpicture/2010/04/more_from_eyjafjallajokull.html

So we've all heard the saying "opposites attract", and this is in fact the main reason lightning forms. But how do the particles obtain a positive and negative charge? There are several theories that answer this question. One of them claims that when the tiny, colder ice crystals in a cloud come in contact with the larger, warmer hailstones that the ice crystals gain a positive charge and the hailstone a negative one. In other words, there is a net transfer of positive ions from the warmer object to the cooler object. The updrafts within in a thunderstorm could be responsible for carrying the tiny ice crystals to the top of the cloud and allowing the heavier hailstones to fall to the lower region of the cloud. Now we have a complete picture of the typical charges present within the cloud. Positive charges exist near the top, and negative charges are present near the bottom. A positive charge forms near the ground under the cloud, and this region of electrification actually moves along with the cloud. The most common type of lightning is cloud-to-cloud lightning. Only 20% is actual cloud-to-ground lightning.


Photo Courtesy: http://severe-wx.pbworks.com/f/charge%20separation.gif

Cloud-to-ground lightning starts off with a stepped leader that approaches the ground from the cloud base. The stepped leader is very faint and not visible to the human eye. At the same time, a current of positive charges flows upward from the ground. When the two meet, a return stroke travels upward to the cloud along the stepped leader's path at nearly 60,000 miles per second! This process can be repeated rapidly in the same lightning bolt creating a flickering effect we're all familiar with. There are two different cloud-to-ground lightning variants. Negative cloud-to-ground lightning is the more common of the two, occurring 90% of the time. This type is experienced when the negative cloud base is attracted toward the positive ground. The situation is reversed for positive cloud-to-ground lightning, as the positive charges present in the top of cloud reach negative charges on the ground. This creates a higher current level and can cause more damage. The average lightning bolt is 6-8 miles long and can travel 25-40 miles horizontally before turning down toward the ground. In October 2001, a visual lightning detection system measured a single bolt that traveled from Waco to Forth Worth to Dallas; that's around 110 miles!

The color of the flash of lightning can indicate different conditions. If a flash appears red, that typically means rain is present in the cloud. A blue flash indicates hail, and a yellow flash means there is a lot of dust in the atmosphere. White lightning conveys low humidity, which is a bad thing because fires are more likely to begin in these conditions. The lightning process continues in a cloud until all the charges in the cloud have dissipated.

There are 100 lightning strikes a second around the earth, that's nearly 8.64 million times a day that lightning strikes. The Empire State Building in New York City alone is struck about 500 times each year. So we always hear reports of fatalities, fires, and other bad situations arising from a lightning strike. However, few people realize the benefits lightning provides us. Lightning ionizes the air and produces nitrogen oxide. Studies suggest that this process could generate more than 50% of usable nitrogen in the atmosphere and soil, and nitrogen just happens to be an essential plant fertilizer. Lightning also plays a critical role in the natural cycle of forests by helping generate new growth. Areas burned by lightning triggered fires clear land of dead trees so that seedlings have space and soil to take root.

Thunder
In Norse mythology, the god Thor was said to have carried a hammer around, and every time he was angered he would make thunder and lightning strike. Ask many children today what thunder is and chances are you will hear a response similar to this.


The Norse God, Thor.
Photo Courtesy: http://www.theepochtimes.com/n2/images/stories/large/2008/10/21/thor_web.jpg

The true scientific answer behind it all has to do with the heat generated by the lightning strike. A single lightning bolt can heat the surrounding air up to 50,000 degrees Fahrenheit; that's five times hotter than the surface of the sun. The heat causes the rapid expansion and contraction of the air surrounding the bolt, and consequently results in a shock wave we hear as a rumble or a boom. The reason we see the lightning bolt before we hear the thunder has to do with the speeds at which each travel. The speed of light travels at 186,282 miles per second, but the speed of sounds only travels at about .211 miles per second. A common method for judging how far away a lightning bolt is counting the number of seconds between the flash and the boom. For every five seconds, the strike is a mile away. So if you counted to 20, the lightning strike is 4 miles from where you are located. Thunder is inaudible farther than 20 miles away, so no need to count past 100 seconds.


Sources:
Ahrens, C. Donald. "Thunderstorms and Tornadoes." Meteorology Today: an Introduction to Weather, Climate, and the Environment. Belmont, CA: Brooks/Cole, CengageLearning, 2009. 389-95. Print.
Burroughs, William James., and Richard Whitaker. Weather. San Francisco, CA: Fog City, 2007. 50-51+. Print.
"Facts About Thunder - Weather Imagery." Weather Imagery - A Little Mix of Everything. Web. 23 Aug. 2010. http://www.weatherimagery.com/blog/facts-about-thunder/.
"Flash Facts About Lightning." Daily Nature and Science News and Headlines | National Geographic News. Web. 24 Aug. 2010. http://news.nationalgeographic.com/news/2004/06/0623_040623_lightningfacts.html.
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Morgan, Sally, and David Ellyard. "Weather Myths." Weather. [Alexandria, Va.]: Time-Life, 1996. 32. Print.
"StrikeAlert: Lightning Facts." Outdoors Technologies: Strike Alert Personal Lightning Detector. Web. 24 Aug. 2010. http://www.strikealert.com/LightningFacts.htm.