As I mentioned previously, I attended an all-day seminar on the subject of Lightning, presented on-site at AccuWeather by Vaisala, the Lightning Experts. (Vaisala is a private company based out of Finland that owns the original network of lightning sensors here in the United States). Even as a meteorologist and storm enthusiast, I learned a lot of fascinating things during the seminar, and I'll be bringing that information to you this week, in a 4-part blog series:
|READ THE COMPLETE SERIES: · Lightning Seminar Part 1: Lightning 101, Lightning Lingo · Lightning Seminar Part 2: Oddities and Questions 1/2 · Lightning Seminar Part 3: Oddities and Questions 2/2 · Lightning Seminar Part 4: History of Vaisala|
DISCLAIMER: The information presented here was culled from the seminar based on the talks that they gave, but nothing here should be considered the opinion of Vaisala or AccuWeather and this information should not be used for protection of life or property.
Today, in Part 1, I'd like to cover some lightning basics. In the subsequent parts this weekend, I'll cover additional related information.
WHAT CAUSES LIGHTNING?
In short, differing areas of positive and negative charge in a thunderstorm (and the ground below) is what causes lightning. The charges are created when ice crystals in clouds bounce off of graupel (hail) in clouds. It depends on the temperature as to which polarities they have.
The setup of "regions of charge" in a storm are not as simple as educational diagrams may have shown to you; there are actually several different regions of charges, including that of the ground. When the charges are uneven enough... the charges are transferred around in the cloud, or from the cloud to the ground. (Researchers in England and Japan have studied lightning by inserting a probe with a graupel pellet into cloud chambers with ice crystals.)
Before reading on, you need to get hip to a number of definitions of words that I'll mention frequently.
CG: A Cloud-to-Ground lightning Flash
Flash: A CG lightning event or "strike", containing one or more Return Strokes.
Return Stroke: An individual CG lightning strike, seen as a "branch" that touches the ground in photos.
There are typically 3-4 "Return Strokes" Per CG Flash. Usually the first Return Stroke is the strongest, and it is used to place the location of a CG strike in lightning data such as we have on AccuWeather.com. Strokes can be as far as 10 kilometers apart but are generally much closer.
Cloud Flash: An in-cloud / cloud-to-cloud / cloud-to-air lightning event that does not hit the ground.
These make up 60-100% of lightning in any given storm (stronger storms have even more*). Cloud Flashes precede CG strikes in 70% of storms, usually by several minutes. Rarely (10% of the time) the CG strike is first, but in terms of lightning safety, that first strike could be the most deadly. That's why Forecast Lightning alerts, such as those offered by ADCRP, are important to the protection of life and property.
Polarity: CG strikes are either positive or negative. Negative CG lightning strikes make up for 90% of flashes.
DOES LIGHTNING GO UP OR DOWN?
Lightning goes both "from the ground up" and "from the cloud down". The first part you can't see starts up and goes down and is called a "Leader", leading the trail of the transfer of charge. Special cameras can track these. The Return Stroke actually propagates up the channel, and is the light that we see.
The "decision" that lightning makes as to what to strike is made in milliseconds when fields of charge reach upwards (tens of meters) from all ground objects; the best upward connecting charge wins, be it the highest, or most conductive (metal).
When lightning hits an object, enough electricity is in the air that you may be injured or killed nearby, even if you are not directly struck.
There is little difference in lightning strikes when a storm moves from land to water or vice versa (it prefers neither surface to discharge).
ABOUT VAISALA'S LIGHTNING DETECTION NETWORKS:
Vaisala's U.S. lightning network contains 114 ground-based sensors (there are also 85 in Canada and some in other nations including Brazil, South Africa, Australia, and several European nations) detecting mainly VLF (very low frequencies) of electromagnetic waves that are produced by lightning and travel up to 1000 kilometers. The sensors are housed at various places where they pay for rent, uninterruptible power and Internet feeds.
Based on knowledge of magnetic fields and the data received from multiple sensors, location can be determined. The data flows into a Control Center in Arizona and then sent out to clients.
The NLDN (National Lightning Detection Network) Control Center (Arizona)
The network's accuracy for CG strikes has been determined through rocket-triggered lightning in Florida (within 450 meters), and video camera (279 meters). (Which means, in general, you can tell if lightning struck on one side of the street or the other, but you might not be able to pinpoint a certain house or building that was struck)
Near 100% of CG lightning strikes are detected in the Continental U.S, southern Canada, and the Caribbean, but only 10% of Cloud Flashes are picked up due to their weakness.
A special 3-D lightning network using VHF (very high frequency) sensors exists in the Dallas,Texas area, but these sensors are expensive and need to be line-of-sight so they have not been deployed nationwide.
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