Researchers unravel mystery of lightning diversity

The type of lightning depends on where the charge builds and where the imbalance in charge exists in the clouds.

When it comes to lightning, most people see it and head for safety, choosing not to stop and discuss what type it is. But researchers say they have developed a technique that shows how diverse lightning can be.

For example, most people don't see lightning see lightning strikes that go from clouds to the ground, but some lightning goes upward, forming blue jets and gigantic jets. Perhaps the most dangerous lightning appears as "bolts from the blue" - lightning that begins upward, but then moves sideways and then downward to hit the ground as much as three miles from a thunderstorm, say researchers at New Mexico Tech and Penn State.

About 90% of lightning occurs inside clouds and is not visible to the casual observer, researchers said. The researchers wondered if lightning that appears within clouds and the lightning that escapes upward or downward shared the same development mechanisms, researchers said. Lightning forms in clouds when different areas of the cloud become either positively or negatively charged. Once the electric field near a charged area exceeds a certain propagation level, lightning occurs. The type of lightning depends on where the charge builds and where the imbalance in charge exists in the clouds. The mechanism behind different types of lightning is what the new model shows, researchers said.

"With the help of colleagues from New Mexico Tech, we were able to build a model of lightning and apply it to the various types of lightning," says Jeremy Riousset, graduate student in electrical engineering, Penn State. "Thanks to their observations and measurements, we know how lightning like 'bolts from the blue' happen. We know they develop like normal intracloud lightning before escaping the thundercloud at upper levels and branching toward the ground."

They also discovered that upward and sideward lightning events occurred shortly after normal downward lightning bolts occurred or intracloud lightning produced a local charge imbalance in the cloud.Using detailed pictures of "bolts from the blue" lightning from New Mexico Tech's Lightning Mapping Array, researchers created a three-dimensional lightning location system that uses multiple measurement stations to capture and time the VHF signal of the lightning.

The Lightning Mapping Array can map lightning within clouds, something that normal optical photography or videography cannot do, researchers stated.Researchers found that with intracloud lightning, the most common form of lightning, the transfer of charge occurs between the most negatively and most positively charged areas, the middle and upper parts of the cloud, respectively.

Lightning that strikes the ground does so because precipitation or the storm's progression creates an excess of net negative charge in the mid-levels of the cloud. This results in either a direct ground strike or a bolt from the blue, researchers saidAn alternative way to discharge a middle negative charge is through a gigantic jet, which propagates upward. The height of the clouds somewhat controls whether a gigantic jet or bolt from the blue propagates. The higher the top of the cloud, the more likely a gigantic jet will appear, researchers said. However, large positive charge in the upper levels of the storm causes blue jets.

In run-of-the-mill thunderstorms, blue jets are positive, originate in the uppermost part of the cloud and propagate continuously upward; while gigantic jets are negative, begin like a normal intracloud flash and propagate stepwise upward. Inverted polarity storms do exist and the charges of the various lightning types would then reverse, researchers said.The higher the cloud, the more likely either type of jet becomes.

Thunderstorms in the tropics form with very high clouds increasing the chances of jets forming. Thunderstorms in the temperate United States do not have clouds quite so high, allowing a great number of bolts from the blue to occur. Bolts from the blue are very common in continental mid-latitude storms, researchers said.

Such research could have a number of applications. For example, NASA is building an enormous lightning protection system at the Kennedy Space Center that promises to protect people and equipment but also collects strike information for analysis by launch managers.The structure will be the largest on the space compound and will feature large cables strung between three 594-foot-tall steel and fiberglass towers. Each tower is topped with a fiberglass mast and a series of catenary wires and down conductors designed to divert lightning away from the rocket and service structure. This configuration helps keep the vehicle isolated from dangerous lightning currents, NASA said.

The system will also include an array of sensors, both on the ground and the mobile launcher, will help determine the vehicle's condition after a nearby lightning strike. This can prevent days of delays, NASA said on its Web site.

In addition, NASA and the U.S. Office of Naval Research are using a new and growing set of networked sensors on land, sea and in space that use lightning strikes to help forecasters predict just how powerful an oncoming storm may be. Researchers are using data from a growing network of new, long-range, ground-based lightning sensors, a NASA satellite and aircraft-based sensors. Specifically they are looking at the relationship between hurricane eyewall lightning outbreaks and the intensity of hurricanes.

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