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Lightning

Page history last edited by Eva 9 years, 2 months ago

History



Benjamin Franklin endeavored to test the theory that is about the similarity between lightning and sparks. He did his testing in a spire in Philadelphia, United Sates. Before the spire completed done, he got an idea what invited kite into his lab. Therefore, in the next thunderstorm in June 1752, he raised a kite. During this observation, his son was Franklin’s assistant. On his end of the string he attached a key, and he tied it to a post with a silk thread. During this observation, Franklin notice that the loose fibers on the string stretching out, and then he put his hand close to the key and the spark jumped. During the storm had soaked the line and made it conductive the rain had fallen. Even thought after the lab, Benjamin Franklin showed that lightning was a discharge of static electricity, there was little improvement in theoretical understanding of lightning for more than 150 years.


Properties

 

1. Lighting can occur with both positive and negative polarity. An average bolt of negative lighting carries and electric current of 30,000 amperes (“amps”) – 30 “kiloamps” (kA), and transfers five coulombs of electric charge and 500 megajoules of energy.

2. An average bolts of positive lighting carries and electric current of about 300 kA – about 10 times that of negative lighting.

3. The voltage involved for both is proportional to the length of the bolt. However, lighting leader development is not just a matter of electrical breakdown of air, which occurs at a voltage gradient of about 3 megavolts per metre (MV/m). TH ambient electric fields required for lightning leader propagation can be one or two orders of magnitude less than the electrical breakdown strength.

4. Lighting rapidly heats the air in its immediate vicinity to about 20,0000 °C (36,000 °F)— about three times the temperature of the surface of the Sun. The sudden heating effect and the expansion of heated air give rise to a supersonic shock wave in the surrounding clear air. It is this shock wave, once it decays to an acoustic wave, which is heard as thunder.

5.NASA scientists have found that electromagnetic radiation created by lightning in clouds only a few miles high can create a "safe zone" in the Van Allen radiation belts that surround the earth. This zone, known as the "Van Allen Belt slot", may be a safe haven for satellites in "middle Earth orbits" (MEOs), protecting them from the Sun's intense radiation. 

 


Formation

Electrostatic induction hypothesis

According to the electrostatic induction hypothesischarges are driven apart by as-yet uncertain processes. Charge separation appears to require strong updrafts which carry water droplets upward, supercooling them to between -10 and -20°C. These collide with ice crystals to form a soft ice-water mixture called graupel. The collisions result in a slight positive charge being transferred to ice crystals and a slight negative charge to the graupel. Updrafts drive the less heavy ice crystals upwards, causing the cloud top to accumulate increasing positive charge. Gravity causes the heavier negatively charged to fall toward the middle and lower portions of the cloud, building up an increasing negative charge. Charge separation and accumulation continue until the electrical potential becomes sufficient to initiate a lightning discharge, which occurs when the distribution of positive and negative charges forms a sufficiently strong electric field. 

Polarization mechanism hypothesis

     The mechanism by which charge separation happens is sill the subject of research. Another hypothesis is the polarization mechanism, which has two components:

  1. falling droplets of ice and rain become electrically polarized as they fall through the Earth's natural electric field.  
  2. Colliding ice particles become charged by electrostatic induction.

Lightning initiation

    Even assuming an electric field has been established, the mechanism by which the lightning discharge begins is not well known. Electric field measurements in thunderclouds are typically not large enough to directly initiate a discharge. Many hypotheses have been proposed, ranging from including runaway breakdown to locally enhanced electric fields near elongated water droplets or ice crystals. 

Leader formation and the return stroke 

File:Leaderlightnig.gif    

 As a thundercloud moves over the surface of the Earth, an electric charge equal to but opposite the charge of the base of the thundercloud is induced in the Earth below the cloud. The induced ground charge follows the movement of the cloud, remaining underneath it.

Illustrating of a negative streamer (blue) meeting a positive counterpart (red) and the return stroke.

see caption

Discharge

    When the electric field becomes strong enough, an electrical discharge (the bolt of lightning) occurs within clouds or between clouds and the ground. During the strike, successive portions of air become a conductive discharge channel as the electrons and positive ions of air molecules are pulled away from each other and forced to flow in opposite directions.

The electrical discharge rapidly superheats the discharge channel, causing the air to expand rapidly and produce a shock wave heard as thunder. The rolling and gradually dissipating rumble of thunder is caused by the time delay of sound coming from different portions of a long stroke.


Re-strike 

   High speed videos (examined frame-by frame) show that most lightning strikes are made up of multiple individual strokes. A typical strike is made of 3 or 4 strokes, though there may be more.

    Each re-strike is separated by a relatively large amount of time, typically 40 to 50 milliseconds. Re-strikes can cause a noticeable "strobe light" effect.

    Each successive stroke is preceded by intermediate dart leader strokes akin to, but weaker than, the initial stepped leader. The stroke usually re-uses the discharge channel taken by the previous stroke.

    The variations in successive discharges are the result of smaller regions of charge within the cloud being depleted by successive strokes.

    The sound of thunder from a lightning strike is prolonged by successive strokes. 


Types

1) Cloud- to cloud lightning

A lightning discharges between cloud and without contacting the ground. When the lighting occurs between two different clouds, it is called inter-cloud lighting. This type of lightning is the most common form of lightning. It usually occurs within a cloud with electricity passing between the negative charge at the bottom of the cloud and positive charged at the upper levels. Sometimes, cloud-to cloud lightning discharges between two opposite charges of clouds. It will occur often when the top of cloud with positive charge and negatively charged on the base of the cloud. Because the type of lightning normally happen at higher altitudes than cloud to ground lightning so it may be seen from some distance away (especially at night). If there is a large cumulonimbus cloud than it will be visible up to 320 km away.

 

2) Cloud-to ground lightning

This is the other common type of lightning. It occurs when there is an discharge within a thunderstorm. It occurs when the electrical charge travels between negative charge cloud base and ground with positive charge. From the all different types of lightning, this one poses the greatest threat to life and property since it strikes the ground. So the object what connect with the ground will conduct electricity more effectively than air which means that the elevated landmasses and tall objects are prone to strikes (such as building and tree)

 

3) Bead lightning

 

Bead lightning is a type of cloud to ground lightning which appears to break up into a string of short, bright sections which it will be last longer than the usual discharge. In the bead lightning, the width of the lightning channel varies, if the lightning channel cools and fades the wider section will be more slowly and remain visible longer.

 

4) Ball lightning

Ball lightning is very rare electrical storms. Sometimes it manifests as a glowing ball about the size of a basketball but sometimes, it is as small as a golf ball or as large as a small car. It hovers in the air between few seconds and couple minutes. The average is 25 seconds. Because ball lightning is a proposed electrical phenomenon of which little is known but there are certain common phenomenon occurs during ball lightning. During the ball lightning the temperature would be high enough to get fire; it hovers in the air usually short. And usually this type of lightning closes to the ground.

 

5) Dry lightning

This type of lightning is the most common natural cause of wild fires. Dry lightning is a term that occurs without precipitation at the surface. When the higher levels of the atmosphere are cooler and surface is warmed to extreme temperatures it will occur and produces lightning. So fire can beget dry lightning through the development of more dry thunderstorms which cause more fires.

 

6) Positive lightning

Positive lightning is far more powerful lightning and in which positive charge flows instead of electrons. There is only about 5% of lightning is positive in nature. Its only occur during the peak of a severe thunderstorm or as the storm is decaying. Instead of discharging with the negative charge at the bottom of cloud, it travels outside the cloud and strike the ground where is pool negative charge. People do not know what cause a positive strike, but they observed that positive lightning is about 5 times more powerful than negative strike and less 10 times longer. And most people believe that positive strikes are responsible for most forest fires and damage to electrical grids.


Damage

 

With population growth, lightning damage to property increases. The US National Weather Service (NWS) estimates an annual cost to US society of A$70 million. The National Lightning Safety Institute, however, takes into account the indirect costs of bush fires, power blackouts, aircraft mishaps, and estimates a damage bill of around A$10 billion per year.

The US weather service reports approximately 100 deaths and 500 injuries per year from lightning strikes but other agencies believe that, due to under-reporting, the number of fatalities should be around 450. Australia has up to 10 deaths and well over 100 injuries per year. Conservative estimates put the worldwide toll at 1000 deaths and 5000 injuries annually. This makes lightning deadlier than tornadoes.

 

 Evidence of a lightning strike.    

Typical tree damage caused by lightning strike. 


 Cattle struck dead by lightning.

  

Lightning traveled along this underground metal pipe. 


 


World Lightning Map 

 Where Does the Most Lightning Occur?
The area on earth with the highest lightning activity is located over the Democratic Republic of the Congo in Central Africa. This area has thunderstorms all year round as a result of moisture-laden air masses from the Atlantic Ocean encountering mountains.
 

Where Does the Least Lightning Occur?

Lightning has its lowest frequency in polar areas. It is rarely seen over the Arctic Ocean or Antarctica. These areas are too cold to host the types of storms that produce lightning. Lightning also has a very low occurrence over the oceans.


Problem

How to estimate lightning's distance?

 

Since light travels at a faster speed than sound, the flash of lightning is seen before the noise of thunder is heard. Sound typically takes five seconds to travel a mile, and this fact makes it simple to determine the approximate number of miles away the storm is:
 

1) Look for a flash of lightning. Using a stopwatch or the second hand on a wrist watch, measure the number of seconds that elapse from the moment you see the lightning until the moment you hear the thunder.

 

2) Divide the number of seconds you counted between the lightning and thunder by 5. For example, if you counted 10 seconds in between, the storm is approximately 2 miles away. This works because sound can travel roughly 1 mile in 5 seconds.


Video

 


Source

http://en.wikipedia.org/wiki/Lightning

http://science.howstuffworks.com/nature/natural-disasters/lightning.htm

http://www.windows2universe.org/earth/Atmosphere/tstorm/lightning.html

http://geology.com/articles/lightning-map.shtml

http://www.manfredkaiser.com/lightning_damage.html

http://www.weatheroffice.gc.ca/lightning/index_e.html

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