Tuesday, June 1, 2010

Lightning - An Overview

[Download pdf file of this article here]

Lightning has always evoked fascination. Recently I spent some time contemplating and perusing through lots of resources on the internet and in print about lightning and the possibility of harnessing energy from it. Here's a brief overview of what I gleaned from various sources.

The important figures associated with lightning (typical values):
  1. Voltage between cloud and ground ≈ 10^8 to 10^9 V (upto a billion volts)
  2. Current during lightning discharge ≈ 10 to 200 kA (hundreds of kilo-amperes)
  3. Peak power involved ≈ 10^10 to 10^12 W (several giga-watts)
  4. Energy discharged ≈ 250 kWh (hundreds of units)
  5. Temperature ≈ 30,000 ºC (about five times the surface temperature of the sun!)
  6. Duration of discharge ≈ 0.2 s
A collection of interesting facts about lighting:
  1. On an average, the earth receives 3 million lighting flashes a day i.e. about 40 flashes every second.
  2. Tropical Africa is the most struck by lightning, with up to 295 days a year subject to electrical storms. Lightning strikes most frequently in the Democratic Republic of the Congo (formerly Zaire)
  3. 90% of all lightning strikes are cloud-to-cloud.
  4. Thunder produced by a lightning strike travels one mile (1.6 km) every five seconds. Thus, counting the number of seconds between the visible "flash" and the audible "bang" and dividing by 5, provides the distance in miles at which the strike has occurred.
Types of lighting: 
    1. Cloud-to-ground
        - Negative lightning
        - Positive lightning (bolt from the blue!)
    2. Cloud-to-cloud or inter-cloud
    3. Intra-cloud
    4. Cloud-to-air

How it happens:

Certain types of clouds - known as cumulonimbus clouds or thunderclouds - develop a strong charge separation with large negative charges at the bottom and large positive charges at the top possibly due to the friction (tribo-electric effect) between upward rising air and falling water droplets and ice.  A large “shadow” region of positive charge is created under the cloud due to electrostatic induction. When the electric field between the bottom of the cloud and the surface of the earth becomes strong enough, streaks of ionized air (negatively charged stepped leaders) reach out downwards from the clouds while positive streamers grow upwards from objects on the earth. When the two meet, a short circuit is created for humongous amounts of charge to be transferred from the cloud to the earth within a fraction of a second and that is what we see as lightning. The huge current through the conducting channel creates large amount of heat, light,  sound and radio waves.

Keywords: cumulonimbus clouds, thunderclouds, triboelectric effect, electrostatic induction, negative stepped leaders, positive streamers, return stroke, dart leaders, secondary strokes

Lightning Effects:
  • Lightning contributes to nitrogen fixation.
  • If the lighting current path passes through rock, soil or metal these materials can become permanently magnetized due the fleeting but very strong magnetic field induced by the large current. This effect is called Lightning Induced Remanent Magnetism (LIRM).
  • The heat of lightning which strikes loose soil or sandy regions of the ground may fuse the soil or sand into glass channels called fulgurites.
  • Trees are common lighting targets. Of all common trees, the most frequently struck is the oak.
  • The electrical discharge during lightning rapidly superheats the discharge channel causing the air to expand rapidly and produce shock waves 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.
Lighting Safety:
  • Lighting can even reach you indoors of you are in contact with the telephone (landline) or plumbing – basically anything that has a continuous conducting path extending to a place outdoors where the lighting can strike.
  • Water is an excellent conductor, so it’s wise to stay away from lakes, ponds and pools during a lightning storm.
  • Even if the bolt strikes an object near you, the ground current that flows from the point of strike in all directions can easily flow through a person standing with his feet part. So during a thunder storm it’s advisable to (1) stay low, in a crouched position (to reduce the probability of the bolt striking you) especially if you are in a flat region with no taller structures than yourself  (2) Keep your feet together and have as little of your body touching the ground as possible (to reduce the ground current flowing through you)
  • Rubber tires aren’t why you are safe in a car during a lightning storm. In strong electric fields, rubber tires actually become more conductive than insulating. You are safe in a car because the lightning will travel around the surface of the vehicle and then go to ground. This occurs because the vehicle acts like a Faraday cage.
  • Lighting arrestors and lightning rods
  • Static discharge wicks in airplanes
Problems with harnessing energy from lightning:
  1. The discharge of extremely large power concentrated within a very small span of time would require a very very huge and costly device to tap the energy.
  2. Even if the device is somehow built, lightning itself is highly infrequent, inconsistent and unpredictable. We never know the exact time and place where it will strike.
  3. Even if you manage to find a place that receives lighting consistently, a lighting strike dissipates so much energy in heat, light, sound and radio waves that there is not enough left to be worth collecting.
Possible ways to overcome these problems:
  • Do not let the lighting discharge happen as a sudden flash. Find a way to slowly drain the stored charge in the cloud-to-ground capacitance and store it somewhere for being tapped later on.
  • As most of the lightning strikes are cloud-to-cloud and also because much of the energy in cloud-to-ground discharges are dissipated in the intervening medium as heat, light, sound and radio waves, the only way to harness energy from lightning is to go to the clouds and do something up there. To put it in Dr. Martin A. Uman’s words, “trying to harvest the energy of lightning from the ground is hopeless”.
References and useful links for information on lighting:
  1. Lightning Primer from NASA http://thunder.msfc.nasa.gov/primer/index.html
  2. NASA answers on lightning energy harvesting http://www.nasa.gov/centers/kennedy/about/information/science_faq.html#1
  3. Wikipedia article on lighting http://en.wikipedia.org/wiki/Lightning
  4. Good lighting explanation and videos http://science.howstuffworks.com/lightning.htm
  5. Can lightning be tamed? http://www.planete-energies.com/content/sky.html
  6. Harnessing lightning power http://www.weatherimagery.com/blog/harnessing-lightning-power/
  7. Harnessing lightning power http://www.science.edu.sg/ssc/detailed.jsp?artid=5001&type=6&root=5&parent=5&cat=57
  8. Harnessing lightning power http://peswiki.com/index.php/Directory:Lightning_Power
  9. Discussion forum http://answers.google.com/answers/threadview/id/765667.html
  10. Lightning Farms http://www.nytimes.com/2007/12/09/magazine/09lightningfarm.html?_r=3
  11. A detailed article on lightning http://www.electricalfun.com/lightning.htm
  12. Nikola Tesla: Power from Lightning http://www.mikebrownsolutions.com/tesla-lightning.htm
  13. Lightning”- A book by Dr. Martin A. Uman, a leading authority on lightning.