Not Aliens: How an 'Arc Flash' Turned the New York City Sky Electric Blue

"The one in New York is unusual because of how long it lasted."

A searing flash of fluorescent blue light triggered fears of an alien invasion in Queens, New York, late Thursday evening. As the dark sky over the borough suddenly glowed an extraordinary shade of Twitter-bird blue amid sounds of banging and crashing, terrified residents posted videos, like the one above, to social media. Scrambling to calm the chaos, New York City Mayor Bill de Blasio and Con Edison chalked the phenomenon up to a power surge at a local plant.

“Light was caused by electrical surge at a substation,” said de Blasio in a statement on Twitter. The New York Police Department tweeted: “The lights you have seen throughout the city appear to have been from a transformer explosion at a Con Ed facility in Queens.” Con Ed followed up by confirming there was a “sustained electrical arc flash.”

Online commentators scoffed at the seeming lameness of this “cover up,” but the science behind such “arc flashes” is sound, and it really does provide a natural explanation for the sky’s unnatural shade of blue.

“The one in New York is unusual because of how long it lasted,” Dan Robinson, a well-known storm chaser and photographer who has photographed arc flashes during storm events, tells Inverse. “Most arcs are just as bright, but only last a second or two before a circuit breaker trips and cuts power.”

What’s an Arc Flash Anyway?

An arc flash, sometimes referred to as a “power flash,” is simply an arc caused by a shorted-out power line, according to Robinson. The arc occurs as electrical energy flows through the air from one conducting material to another, resulting in the release of light (an arc flash) and a shock wave (an arc blast, which is very loud and can knock things over).

At a power station’s very high voltages (tens to hundreds of kilovolts), the jolt of electrical energy through the normally-nonconductive air produces an enormous amount of light and heat. The equipment at the Astoria Con Ed plant carried 138,000 volts.

As that insanely strong current ran through the air, it turned it into plasma — a state of matter characterized by ionized molecules that are very electrically conductive. Plasma, which can be thought of as a charged gas, is the reason for the otherworldly glow.

Why So Blue?

The blue hue of the flash comes down to the type of ionized gases in the plasma. In much the same way that lightning appears whitish-blue or whitish-purple, a power flash can appear blue because it involves a current passing through charged air, which contains ionized oxygen and nitrogen atoms and occasionally water vapor.

electrical discharge glow
Electrical discharge through air causes excited electrons to emit light. The color depends on the type of gas molecules in the air.

The well known bluish-purplish-white “ionized-air glow” is chalked up to the excitation of electrons in nitrogen atoms. When zapped with a strong current, nitrogen’s electrons transiently rise to a higher energy level, then fall back down to their normal state — emitting blue light in the process. (Other gases, with different arrangements of electrons, emit different colors: Electricity passing through neon gas produces its classic orange-red glow, while sodium gas makes yellow.)

atoms emitting light
A simplified diagram explaining how an excited electron emits light on its way back down to its original energy state.

“The vast majority of them are blue/turquoise,” Robinson says. “They can change color back and forth to orange and red. The arcs themselves are blue, other colors appear if something is being burned by the arc (wood, oil or other materials).”

Not That Weird After All

Arc flashes are actually pretty common, although not at the scale New Yorkers saw on Thursday night. A lightning strike constitutes an arc flash, as does the phenomenon of St. Elmo’s Fire, as well as the tiny spark that jumps from your hand to a metal doorknob when you experience a static shock. More often than not, they’re the same blue-white hue as the Astoria sky was on Thursday night.

Though the phenomenon is far from inexplicable, it can still be pretty jarring, especially when the voltages involved are high enough to power a busy borough. Still: It’s better than aliens.

Media via Wikimedia (1, 2), Roger Kleinman