Lightning (2024)

.
Copyright Cori Prazen (prazen.com/cori)Copyright Gene Moore (www.chaseday.com)

The History of Lightning:[Backto Contents]
(The information contained here was found at (thunder.msfc.nasa.gov/primer))

The first systematic, scientific study of lightning was conducted, aswe all know, by Benjamin Franklin during the second half of the 18th century.Before that, science had evolved to the point that we could separate positiveand negative charge. These charges could then be stored in “LeydenJars”. Leyden Jars were capacitors that could store the charge createdby a person rubbing two different materials together. Sparks couldthen be generated and observed.

Although others managed to make the connection between the sparks ofthe capacitors and lightning, Franklin was the first to actually designan experiment to prove the nature of lightning. He theorized thatclouds are electrically charged and, therefore, lightning must be electricalin nature. His original experiment was to stand on an electricalstand and hold an iron rod with one hand so that an electrical dischargebetween the free hand and the ground would be made. According tohis theory, an electrically charged cloud would create sparks that would“jump” between the iron rod and a grounded wire (which he held up withan insulating wax candle). Franklin never successfully completedthis experiment.

The experiment was successfully completed by at least two other men,however. Thomas Francois D’Alibard of France managed to see sparksjump from the iron rod he was holding in May 1752. A Swedish physicistby the name of G. W. Richmann was working in Russia during July 1753 whenhe managed to successfully complete this experiment as well. He didn’tget to enjoy his success long, however, as a bolt of lightning struck himdead moments after the success.

Benjamin Franklin, on the other hand, thought of a better way to provehis hypothesis. He used a kite in place of the iron rod, since itcould get closer to the cloud and it could be flown from anywhere.In 1752, Franklin tied a key to a damp kite string, which was then tiedto an insulating silk ribbon wrapped around Franklin’s hand. Sparkswere observed to be jumping from the key as Franklin’s grounded body provideda conducting path for the electrical currents that resulted from the strongelectric field buildup in the storm clouds.

Franklin was also able to measure the sign of the charge delivered throughthe kite. Thus, Franklin was not only able to show that thunderstormscontain electricity, but also able to infer that the lower portion of theclouds were negatively charged.

This was the largest step made in the understanding of lightning untilthe late 1800s. About this time, photography and spectroscopic toolsbecame available for lighting research.

Using more advanced equipment, the researcher Pockels, was able to analyzethe magnetic field induced by lightning. This allowed him to accuratelyestimate the magnitude of the current in a lightning bolt. Similarmethods were used by many experimenters throughout the late 19th century.

C. T. R. Wilson was the first to use electric field measurements toestimate the structure of the charges during lightning discharges.Wilson’s work made many contributions to our present understanding of lightning,as well as lending us tools to use for modern lightning research.

Lightning research was heavily increased in the 1960s. This increasewas caused by several things. First of all, lightning posed a greatthreat to aerospace vehicles and solid state electronic used in computersand other devices (which came into play in the 1960s). Secondly,the research was made easier and better by improved measurement and observationalcapabilities created by technology.
[Back to Contents]

An Explanation of Lightning:[Backto Contents]
(The information contained here was found at (thunder.msfc.nasa.gov/primer))

Inside of clouds are small particles known as “hydrometeors”.As these particles grow and interact, the collisions cause them to becomecharged. After studying these particles, researchers believe thatthe smaller particles tend to become positively charged while the largerparticles become negatively charged. Gravity pulls the larger, negativelycharged particles downward, and updrafts tend to send the smaller, positivelycharged particles upward. The result is that the higher portion ofthe cloud has a net positive charge while the lower portion of the cloudhas a net negative charge. The separation of particles causes a largeelectrical potential not only within the cloud itself, but also betweenthe cloud and the earth. This electrical potential can become millionsof volts in magnitude. Eventually, the electrical resistance in theair breaks down and lightning, the electrical discharge between the regionsof the cloud or between the cloud and the ground, is formed.

A single lightning “flash” is formed by a series of lightning “strokes”.Usually there are about four strokes per flash. An average durationof time for a stroke of lightning is about 30 microseconds. The averagepeak power of a stroke of lightning is about 10^12 watts.

The electrical discharge, lightning, results in heating up the atmosphereimmediately around the lightning strike. The lightning can actuallyheat the area in the general vicinity to 20,000 degrees C! (Thisis 3 times the temperature of the surface of the sun). The air thathas been heated by the lightning is then compressed. This producesa shock wave, which quickly decays to an acoustic wave as it flows awayfrom where the lightning struck.

The flash and the resulting acoustic wave (thunder) that was describedin the last paragraph both occur at the same time, so you may be asking“why do I hear the thunder so long after I see the lightning?” Thereason is because light travels at 186,000 miles per second, and soundonly travels at one one-millionth of this speed (approximately 331 metersper second). Thus, although the lightning and the thunder occur atthe same place and time, the thunder will be heard well after the lightningis seen. The further from the lightning you are, the longer the lagtime will be. In fact, one way to estimate the distance to a lightningstrike is by counting how long it takes to hear the thunder after you seethe lightning strike. If you take this result and divide it by 5,you will have an approximation to the distance to the strike (in miles).
[Back to Contents]

Most Common Types of Lightning:[Backto Contents]
(The information contained here was found at (thunder.msfc.nasa.gov/primer))

Cloud-to-Ground Lightning:

Cloud-to-ground lightning is by far the most dangerous form of lightning.Although this type of lightning is not the most common type, it is theeasiest to research. Therefore, we know the most about cloud-to-groundlightning. Most cloud-to-ground strikes begin near the lower portionof the cloud. As shown below, the lower portion of the cloud is negativelycharged. This results in the lightning delivering negative chargeto the ground. There are, however, a small percentage of flashesthat carry positive charge to the earth. These flashes do not typicallyoccur during the middle of a thunderstorm when there is a “bank” of negativecharge between the top, positively charge portion, of the cloud and theground. Usually, these types of strikes occur when the thunderstormis dissipating and there are not as many negatively charged particles builtup between the top of the cloud and the ground. These flash are alsomore prominently found during the winter months than during the summermonths. This is possibly caused because the cold winter air doesnot create as many updrafts to lift the smaller, positively charged particlesto the top of the cloud. A third way for lightning to cause a nettransfer of positive charges to the earth is when there is a tall Earthgrounded object. This will be explained in more detail under “Descriptionof Lightning Discharge Processes”.

Intra-Cloud Lightning:

Intra-cloud lightning is by far the most common type. This typeof lightning occurs between oppositely charged portions of the same cloud.The flash usually occurs between the positively charged top of the cloudand the negatively charged bottom of the cloud. When intra-cloudlightning occurs, an outside observer, such as you or I standing on theground, does not normally see more than a flicker of light. Thisis because the cloud obscures the lightning flash and makes it hard tosee. Sometimes, however, portions of the flash will leave the obscuringboundary of the cloud, and a bright channel of light, similar to a cloud-to-groundflash can be seen for miles.

Inter-Cloud Lightning:

This type of lightning is similar to intra-cloud lightning. Theonly difference is that it occurs between two separate clouds. Thedischarge is then bridging a gap of clear air, rather than charged particlesinside of clouds.

Description of Lightning Discharge Processes:

The strong electric fields created between the charged particles inthe clouds and the on the ground will cause the air to break down.With this initial breakdown, a stream of particles (known as a streamer) may begin tomove downward towards the earth. This streamer moves in “steps” ofapproximately 50 meters per step, and is called a stepped leader. As this streamer moves,it creates an ionized path depositing charge as it goes. As this leader nearsthe earth, a large potential difference between the leader and the earthis made. Typically, another streamer is launched from the earth whenthe initial streamer nears the earth’s surface. This second leadershoots up and intercepts the descending leader before it hits the ground.Once a fully connected path has been created by the two leaders colliding,a stroke of lightning returns up the already ionized path at speeds closeto that of light. The result of this return stroke is the release of an incrediblylarge amount of energy, a very bright light, and as explained above, thunder.Sometimes, when a thunderstorm is over a tall Earth grounded object suchas a radio antenna, the initial leader may come from the object ratherthan from the cloud. This then creates a “ground-to-cloud” flashand is the third way for lightning to deliver a net positive charge toEarth that was mentioned above in the section “Cloud-to-Ground Lightning”.This flash can easily be picked out from other flashes because it typicallyhas the branches pointing upward instead of downward.

Intra-cloud lightning is created similarly to cloud-to-ground lightning.The main difference between the two is that intra-cloud lightning doesnot have a return stroke of lightning.

Other Types of Lightning Activity:[Backto Contents]

Ball Lightning:
(The information contained here was found at (www.eskimo.com/~billb/tesla/ballgtn.html))

This phenomena does not look like "lightning," instead, it appears asa mysterious glowing sphere which drifts through the air. It is typicallythe size of a grapefruit, but sometimes appears as small as a pea, or aslarge as a bus. It usually lasts only a few seconds, but sometimes persistsmuch longer. Various colors of "BL" have been seen, sometimes it changescolors, and sometimes it has internal structure. Most researchers agreethat it is real, yet its nature is still highly controversial, and no sensibletheories yet exist to explain it.

Copyright National Geographic (www.nationalgeographic.com)

Red Sprites:
(The information contained here was found at (elf.gi.alaska.edu/#chrspr))

Sprites are massive but weak luminous flashes that appear directly abovean active thunderstorm. They occur at the same time as cloud-to-groundor intracloud lightning strokes. The structure of a sprite can be smallsingle or multiple vertically elongated spots, spots with faint extrusionsabove and below or bright groupings. Sprites can extend to altitudes ofabout 95 km and are most often red. The sprites are rarely seen singly.They usually occur in clusters of two or more.

Blue Jets:
(The information contained here was found at (elf.gi.alaska.edu/#chrjet))

Blue jets are a second phenomena that appear above thunderstorms. Theseare narrow cones which are ejected from the electrically active core regionsof a thunderstorm. Blue jets are typically emitted at speeds of approximately100 km/s (Mach 300). They then fan out and disappear at altitudes of 40-50km.

Copyright Solar Data Analysis Center (umbra.nascom.nasa.gov)

How to Look for Sprites and Jets:
(The information contained here was found at (elf.gi.alaska.edu/#chrjet))

A clear view above a thunderstorm is required. This generally meansthe thunderstorm activity must be on the horizon. Additionally, there mustbe very little intervening cloud cover.

Best viewing distance from storm is 100-200 miles (200-300 km). At thesedistances sprites will subtend a vertical angular distance of 10-20 degrees.This is 2-4 times the separation of the pointer stars in the Big Dipper.

For observing sprites, it must be completely dark. (i. e. no longertwilight)

Eyes must be completely dark adapted. Use same criteria for this asfor astronomical observing. If you can see the Milky Way, then it is probablydark enough and the eyes have adapted enough to see sprites.

Fix your gaze on the space above an active thunderstorm. Do not be distractedby underlying lightning activity in the storm. Block out the lightningif necessary using a piece of dark paper in such a way as to still beingable to view what is going on above the cloud.

Sprites will be very brief flashes just on the edge of perceptability.They occur too quickly to follow with the eyes, but their strange verticallystriated structure and dull red color may be perceived.

Patience will be rewarded. If the right kind of storm is present andone's viewing geometry is favorable, then there is a greater likelihoodof seeing a sprite than of seeing a shooting star or comet.
[Back to Contents]

Lightning Facts:[Backto Contents]

These facts are taken from the AutomatedWeather Source Online:
1)Average Lightning Stroke is 6 miles long.
2)The Temperature of lightning's return strokecan reach 50,000 degrees Fahrenheit. The surface of the sun is not eventhat hot! (around 11,000 degrees Fahrenheit).
3)Once the leading edge of a thunderstormapproaches to within 10 miles, you are at immediate risk due to the possibilityof lightning strokes coming from overhanging anvil cloud. Because of this,many lightning deaths and injuries occur with clear skies directly overhead.
4)On average, thunder can only be heard overa distance of 3-4 miles, depending on humidity, terrain and other factors.
5)Average thunderstorm is 6-10 miles wide.
6)Average thunderstorm travels at a rateof 25 mph.

This fact was taken from NationalGeographic Online:
7) Each flash contains about one billion joulesof electricity. That's enough energy to light a 100 W light bulb for threemonths.

[Back to Contents]

Lightning Safety:[Backto Contents]

Lightning Safety Rules adapted from the
National Oceanicand Atmospheric Administration (NOAA)

1)Stay indoors, and don't venture outside,unless absolutely necessary.
2)Stay away from open doors and windows,fireplaces, radiators, stoves, metal pipes, sinks, and plug-in electricalappliances.
3)Don't use plug-in electrical equipmentlike hair driers, electric toothbrushes, or electric razors during thestorm.
4)Don't use the telephone during the storm.Lightning may strike telephone lines outside.
5)Don't take laundry off the clothesline.
6)Don't work on fences, telephone or powerlines, pipelines, or structural steel fabrication.
7)Don't use metal objects like fishing rodsand golf clubs. Golfers wearing cleated shoes are particularly good lightningrods.
8)Don't handle flammable materials in opencontainers.
9)Stop tractor work, especially when thetractor is pulling metal equipment, and dismount. Tractors and other implementsin metallic contact with theground are often struck by lightning.
10)Get out of the water and off small boats.
11)Stay in your automobile if you are traveling.Automobiles offer excellent lightning protection.
12)Seek shelter in buildings. If no buildingsare available, your best protection is a cave, ditch, canyon, or underhead-high clumps of trees in open forest glades.
13)When there is no shelter, avoid the highestobject in the area. If only isolated trees are nearby, your best protectionis to crouch in the open, keeping twice as far away from isolated treesas the trees are high.
14)Avoid hilltops, open spaces, wire fences,metal clotheslines, exposed sheds, and any electrically conductive elevatedobjects.
15)When you feel the electrical charge --if your hair stands on end or your skin tingles -- lightning may be aboutto strike you. Drop to the ground immediately.

For more information on how to avoid being struck by lightning clickhere.
[Back to Contents]

Lightning Links:[Backto Contents]

The National Geographic Online: http://www.nationalgeographic.com/features/96/lightning
National Oceanic and Atmospheric Administration (NOAA): http://www..noaa.gov
The University of Alaska: http://sprite.gi.alaska.edu& http://elf.gi.alaska.edu
Global Hydrology and Climate Center: http://thunder.msfc.nasa.gov/primer