lightning.html"> Lightning


In the United States, between 75 and 100 Americans are hit and killed each year by lightning. Approximately 10,000 forest fires are started each year by lightning. Approximately $100 million in annual losses result from forest and building fires caused by lightning. The power of lightning's electrical charge and intense heat can electrocute on contact, split trees, ignite fires and cause electrical failures.

What Is Lightning And How Does Lightning Form?

It was in 1752 that Benjamin Franklin discovered that lightning was electricity by flying a kite in a thunderstorm. Definitely not . Lightning results from the discharge of electrical energy between regions of high positive charge and regions of high negative charge. In order for these regions to gain a high magnitude of positive or negative charge, there must be a charge separation process occurring within the cumulonimbus cloud of a thunderstorm.

But, how does this charge separation occur? One theory, although not the only one, involves ice crystals and a characteristic of ice called the thermoelectric effect - the tendency for electrons, loosely held by molecules, to migrate toward warmer regions of the ice. When an object gains electrons it becomes negatively charged and when it loses electrons it becomes positively charged. Both have become ionized by this process.

Typically, the surface of the earth is slightly negatively charged and the upper troposphere is slightly positively charged. Within a cumulonimbus cloud, processes occur which change this pattern.

One theory suggests an interaction between ice crystals and graupel and supercooled water droplets. Remember, graupel can serve as the nucleus for hail.

Supercooled water droplets can occur in clouds because water molecules can exist as liquid water at temperatures below freezing (32oF or 0oC).
However, when the temperature reaches -40oC, the water will spontaneously freeze.
When small graupel falls into a region of supercooled water droplets and strike the droplets, the water freezes on the outside of the graupel.
The latent heat released by the liquid water molecules makes the surface of the graupel slightly warmer than its surroundings.
As long as the graupel is gathering supercooled water droplets, the graupel remains slightly warm.
Also present are small ice crystals, water droplets which have frozen before being struck by graupel. These ice crystals are at the temperature of the surrounding air while the graupel is slightly warmer.
The striking of cooler ice crystals by warmer graupel results in electrons moving from the ice crystals to the graupel.
Thus, the small ice crystals become positively charged and the larger graupel becomes negatively charged.
The vigorous updraft and downdrafts within the cloud tend to carry the small ice crystals to the upper levels of the cloud while the larger graupel tends to remain at lower levels.
The cloud now becomes more positively charged at upper levels (called the P-region) and more negatively charged at lower levels (called the N-region).
Now that is
There also seems to be a small positive region in the lower part of the cloud near where precipitation is occurring (called the p-region).
Note: Charge separation by this process can only occur in clouds which extend above the freezing level. Temperatures must be colder than freezing.

What are other names for graupel?
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How large are typical graupel particles?
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When a charged cumulonimbus cloud moves across the ground or water surface, a positive charge is induced on the surface below the cloud. Remember that like charges repel each other. So, with the lower part of the cloud having a net negative charge, it repels the negative charge on the ground, leaving a net positive charge on the ground below the cloud. And also a net positive charge on anything on the ground: trees, buildings, people, CATS!!!!.

That can be an electrifying
We now have a charge separation between the cloud and the cat - errr, ground!

Exact values of charge strength in each of the regions varies, but a charge of +40 coulombs has been suggested as a typical value for the P-region (near top of cloud) and -40 coulombs for the N-region (near bottom of cloud) and +10 coulombs for the p-region (small p - near lower portion of rain area). One coulomb is the amount of charge which is moved past a given cross-section of wire when a current of 1 ampere flows for 1 second.

The charge within the cloud must be greater than the lightning flash. A typical lightning flash generally moves about 25 coulombs of charge in a cloud-to-ground lightning flash.

What is a coulomb?

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What is an ampere?

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Air is not a good conductor of electricity and the magnitude of the charge at the base of the cloud and on the ground must be great enough to overcome the natural resistance to electric flow of the air. Similarly, for an lightning strike to occur within a cloud, the resistance must be overcome. When the air, or a path through the air becomes ionized, the path becomes a much better conductor of electricity and the lightning strike can occur much easier.

This ionization process occurs in a chain reaction fashion. Starting from the N-region at the base of the cloud, electrons "jump" from air molecule to air molecule below the cloud extending a narrow path of ionized air downward toward the ground. These ionized pathways branch out as the paths are extended in steps fashion, each step about 50 meters long. Each step takes less than a millionth of a second and the time between steps is about a 50 millionth of a second. This stepped ionization process is called the stepped leader. The stepped leader moves toward the ground at about 75 miles per second. This stepped leader is a low-luminosity traveling spark and cannot be seen by the human eye, although it can be photographed with special cameras.

When the stepped leader nears the ground, within about 100 meters, the magnitude of the positive charge on the ground below the stepped leader and the charge of the stepped leader are so great that electrical breakdown of the air between these two regions occurs beginning at the ground (or an object on the ground) and moving upward toward the stepped leader. This upward moving charge is positive and is called simply a leader. There may be many leaders extending upward toward the stepped leaders. Whenever a connection is made between the downward moving stepped leaders and the upward moving leaders, a complete path of ionized air is made connecting the cloud to the ground, like a long thin wire.

When the connection is made, electric charge moves rapidly through the connection similar to overloading an electrical wire.

The "wire" becomes very bright and intensely hot. This first flow of current is called a return streamer, or return stroke and is the first lightning that we see. This return streamer propagates up the channel at a velocity of about 1/10th the speed of light, or 3 x 107 m/s, making the trip in about 100 millionth of a second.

That's a little slower than warp speed but if it gets you, you'll be just a little bit warped.

What is the typical current measured in the return streamer and what is the maximum current measured in the return streamer?

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The energy in this current produces temperatures in the channel greater than 50,000oF, in millionths of a second, too quick for the air to expand gradually.

The pressure produced along the channel by this intense heat is on the order of 10 to 100 times normal sea-level pressure. This high pressure region rapidly expands outward causing compression of the surrounding air. This region of compression propagates outward as a shock wave, initially traveling faster than the speed of sound but after about one-tenth of a second, it travels as an ordinary sound wave. The thunder you hear is the arrival at your ear of these pressure variations produced by the shock wave.

Now, after the initial return stroke, more negative charge from the cloud flows down the already ionized path in a smooth, not stepped, flow (called a dart leader) and initiates another return stroke in about 1/30 of a second. This dart leader - return stroke process may be repeated many times, making the lightning strike appear to flicker.

Almost all lightning discharges occur in basically the same manner, but the condition under which they develop and are viewed make the flash look different.

Here is a list of the commonly used names of the different appearances of lightning.