lightning

A piece of fossilized lightning

The lightning accompanying monsoon storms reminded me of a curiosity I have in my collection. It is a cylinder of fused soil about 6 inches long and 2.5 inches in diameter.

Fulgurite 1

Fulgurite 2

This structure is called a “fulgurite” and it is produced by a lightning strike – it is “fossilized” lightning. The material is also called “lechatelierite” which is a mineraloid of fused quartz. It can also be produced from meteorite impacts. Fused sand requires a temperature of at least 1,800 °C (3,270 °F) and it is estimated that peak temperature of a lightning bolt can be over 30,000 °C . This particular lightning strike occurred in New Mexico. A colleague of mine saw it happen and then collected some pieces of the resulting fulgurite.

What also brought this to mind was a new paper recently published in Nature Scientific Reports. (Read full paper: http://www.nature.com/articles/srep30586 )

Two researchers from the University of South Florida School of Geosciences decided to study fulgurites to see if they could developed a method to measure the amount energy expended by a bolt of cloud-to-ground lightning. Atmospheric physicists can approximate lightning bolt energy by measuring the electrical current and temperature of bolts as they occur. The numbers are usually approximations. The USF team is the first to investigate the energy in lightning strikes by using geology “after-the-fact” research, rather than measuring energy during a strike. By conducting this lightning strike “archaeology,” the researchers were able to measure the energy in a bolt of lightning that struck Florida sand thousands of years ago.

“The team collected more than 250 fulgurites – both recent and ancient – from sand mines in Polk County, Fla., at a site that is believed to have recorded thousands of years of lightning strikes, providing a way to measure the lightning strike history of what is today called the I-4 Corridor, a region near Tampa and Orlando. They analyzed the properties of the fulgurites, paying particular attention to the length and circumference of the glass cylinders because the amount energy released is revealed by these dimensions.” (By the way, the press release touts that Florida is the “lightning capital of the United States.” I wonder if they have ever been to Arizona.)

The researchers developed a statistical model based on the length, diameter, and composition of the fulgurite to help them estimate the energy in the lightning strike. You can read the paper to judge if their assumptions are reasonable.

In an earlier study, other researchers studied the gases trapped in glassy bubbles in fulgurite (see the journal Geology,) That study concluded that fulgurite gases and luminescence geochronology can be used in quantitative paleoecology. Thermoluminescence can be used to date the specimen. These researchers found that theSahel desert in northern Africa extended much farther north 15 thousand years ago.

Anatomy of a Thunderstorm

The summer monsoon is upon us and hopefully it will bring much needed rain. But it will also bring lightning and destructive winds. The term “monsoon” does not mean rain or storms, but a seasonal shift in wind patterns. During the winter, Arizona air flow is usually from the west and we receive the remnants of Pacific storms. During the monsoon, the air flows from a southerly direction bringing us moist air from the Gulf of Mexico, the Gulf of California, and the tropical Pacific. The desert heat and orographic uplift from mountain ranges turn that moist air into thunderstorms.

The graphic below from the National Weather Service shows the parts of a monsoon thunderstorm. The storm is moving left to right.

Anatomy-of-thunderstorm1

The first stage of thunderstorm development is the updraft where warm, moist air is lifted by columns of hot air rising in desert valleys or by air passing over a mountain range. This uplift causes water to condense to form cumulus clouds. Falling water droplets pull the air down with them to form the downdraft and rainy part of the storm. The strong downdraft causes a dust cloud, the Haboob, in front of the storm. Frequently dust devils dance in front of the Haboob. As the storm progresses, the downdraft can produce destructive microbursts of high wind. “Gustnadoes” are tornado-like vortex, similar to dust devils, but stronger, that appear to develop on the ground and extend several hundred feet upward. These vortices generally develop along the leading edge of an outflow boundary from a thunderstorm cell. Although generally of limited duration, the winds of gustnadoes can be strong enough to cause damage.

Some safety tips about lightning:

Lightning often strikes outside of heavy rain and may occur as far as 10 miles away from any rainfall. As a rule of thumb, if the time between lightning flash and thunder is 30 seconds or less, the lightning is about 6 miles away. Research has shown that the most successive flashes are within 6 miles of the first one, which means that you should have reached a safe place if lightning is less than 6 miles away.

If possible stay indoors or in an automobile. If outside, avoid isolated tall structures such as trees and power poles. Get off and away from open water. Avoid open metal vehicles such as tractors, farm equipment, motorcycles, and golf. Avoid power lines, wire fences, metal pipes and railings. If you are caught in an open area far from shelter, and you feel your hair stand on end, lightning may be about to strike you. Drop to your knees and bend forward putting your hands on your knees. DO NOT lie flat on the ground, that makes you a bigger target and a better conductor of electricity. Stay away from running water inside the house; avoid washing your hands or taking a bath or shower. Electricity from lightning has been known to come inside through plumbing. When inside, stay away from TV sets, electrical appliances, bathtubs and sinks, do not talk on the telephone, or play on the computer, don’t touch an electrical cord or outlet.

Be aware and be safe.