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- Dramatic rooftop footage shows lightning striking the 550-foot-tall Washington Monument.
- Lightning causes a huge amount of damage every year, mostly to property much closer to the ground.
- The marble Washington Monument is topped with a set of lightning rods.
A Washington, D.C. news channel captured astonishing footage of lightning repeatedly striking the Washington Monument on Thursday night.
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How can this happen without wrecking the monument? How often does lightning strike the historic structure? We have questions.
First and foremost, the Washington Monument is made of white marble, but marble is a varying natural rock that comes in endless mixes and colors. And that’s just the science side: “Commercially, it includes all decorative calcium-rich rocks that can be polished,” Britannica explains.
Beginning in 1848, the monument was built over a long period of time with marble from different sources, which is why there are noticeable color differences—the same as you’d get with different batches of colorful yarn.
The capstone is a single, pyramid-shaped piece of marble, and at the very top of that is a tiny aluminum pyramid, called a pyramidion, from the same term used with the Egyptian obelisks (and their toppers) that inspired the Washington Monument.
The aluminum cap aimed to draw lightning that was then channeled through the center of the monument and into the ground. But, as one engineer told Atlas Obscura, the pyramidion had been struck by lightning and melted almost half an inch within six months. It was immediately supplemented by a “crown” of metal spikes and, in 2011, a new set of two parallel rods.
Underwriters Laboratory, a global safety science company, recognizes copper and aluminum as the best materials for lightning rods. Aluminum has the lower melting temperature at about 1,200 degrees Fahrenheit, but copper’s melting point of about 1,900 degrees is still dwarfed by lightning’s ability to heat materials to up to 50,000 degrees. The current from a lightning strike, ideally, passes rapidly through a lightning rod and is terminated into the ground.
In the Washington Monument, the aluminum pyramidion was linked with a cast iron frame that also supported the elevator. That’s a clever two-for-one design idea, but cast iron is a much less effective conductor than aluminum or copper. The more heat that lingers for even a split second in the aluminum pyramidion, the more it will show melting and wear over time.
If lightning were to strike the marble monument directly, that could immediately melt the strike site, and scientists find colloquially named fossilized lightning from strikes that hit materials like sand. But the Washington Monument’s rod system works well, despite times when the obelisk has been hit as many as six times during a single storm. (They turn off the elevator that uses the lightning rod structure when there’s lightning.)
The Washington Monument was built during a key time in the development of lightning rods, which the first secretary of the Smithsonian, Joseph Henry, had studied and brought with him to Washington. He advised homeowners and church builders like a de facto Dear Abby for lightning solutions.
And when a cutting-edge gas lighting salesman wanted to put a huge light on an 80-foot metal pole on top of the U.S. Capitol, Henry tried to ensure the huge mast was grounded, at least. It was removed after less than a year. (The weather-obsessed Henry also began the network of civilian observers that eventually became the National Weather Service.)
If lightning hits wood or concrete construction, it can cause fires, explosions, and more. The Washington Monument’s tough marble wouldn’t ever catch fire, but repeated strikes would likely damage the structure, and repairing the narrow apex of a 550-foot-tall obelisk is a costly and dangerous job no one wants to have to do.
In fact, the lightning crown was replaced during a convenient moment after a 2011 earthquake shook and damaged the monument. Expect the unexpected!
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