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Why the Beirut explosion caused a huge mushroom cloud

Shockwave from blast was likely due to humid conditions—and not a nuclear attack, experts say.

 A large explosion rocked the Lebanese capital Beirut on August 4. (ANWAR AMRO/AFP VIA GETTY IMAGES)

Amid the physical smoke above Beirut, and the hot air of conspiracy-riddled social media platforms, it can be hard to be sure of much when assessing the cause of the huge blast that caused a towering mushroom-like white cloud, and led to widespread destruction and at least 135 deaths, in the Lebanese capital on Tuesday.

However, while experts have ruled out a nuclear explosion—attack or otherwise— evidence is mounting that for the past six years Beirut’s port, where the blast occurred, has been harboring what was effectively a gigantic bomb.

This “bomb” appears to have consisted of a shipment of 2,750 metric tons of ammonium nitrate, which Lebanese authorities say was being stored in a warehouse at the port. Though ammonium nitrate is typically used as an agricultural fertilizer, it’s not yet fully clear why it was in the port in the first place, or what the shipment was originally intended to be used for. But as Nick Waters, a former British army officer and senior investigator for the open-source analysis platform Bellingcat, stresses, ammonium nitrate formed the base of the bomb that was used by Timothy McVeigh in the 1995 Oklahoma City terror attack that killed 168 people.

“This isn’t just a chemical that happened to explode; this is a chemical that forms the basis of an industrial explosive,” he says. The fact that such a huge quantity of the chemical was just sitting in the middle of Beirut port—if the authorities are to be believed—is “just insane,” Waters adds.

Despite alternative theories popping up online that appear to contradict such an explanation, including that the blast was a nuclear explosion, Waters says the reams of footage and images—mostly taken on mobile phones by Lebanese onlookers—are in fact “very consistent.”

“They show what appears to be a fire at a warehouse… there are several smaller explosions and then there is one absolutely gigantic explosion at the warehouse,” he says. “So that immediately rules out a lot of possibilities. There’s no evidence that there was, for example, an air strike or anything; there’s no evidence there’s a ship that exploded in the port; and also there is zero evidence that this was a nuclear detonation.”

Brian Castner, a weapons investigator for Amnesty International, agrees that the idea of it being a nuclear explosion is “absolutely rubbish,” citing the lack of radiation and fires elsewhere.

Rather, he says the information coming from the Lebanese authorities, coupled with geolocated images that appear to show the ammonium nitrate store before the fire, and his own rough calculations about the radius and extent of the destruction, all seem to correlate. For him, the biggest question marks remain hanging over what else was in the warehouse and what caused the initial fire; in other words—“what was the train of events?”

“There are some videos that show flashes and bangs, which is consistent with fireworks ‘cooking off’, we call it, but it’s also consistent with small arms ammunition cooking off. I don’t know what that was for sure—it could have been yet a third option,” he says, warning against too much speculation.

“Here’s a good example [of such speculation]: the smoke plume from the warehouse was red. And so people have said, ‘Oh, this shows it’s ammonium nitrate’, Well, it’s consistent with ammonium nitrate (but) other chemicals burn red. So it’s one of those things that’s a confirmation but it’s not dispositive or unique.”

What is clear is that a fire started, “and if you get that much ammonium nitrate burning… it’ll start by burning and then at a certain point, if there’s enough heat and pressure that’s enough to cause the detonation that we saw,” Castner says.

As for the mushroom-like cloud that followed the biggest explosion and which has led some armchair detectives to conclude that it was a nuclear blast, Castner says that aspect is also explainable.

“When there’s a detonation, what’s caused is a shockwave and that shockwave is the air pushing itself into itself, essentially, and it sets up a front—kind of like a storm front [although] this is obviously many, many, many times more than that. But as the air pushes into itself, when the air is really humid, you can see the shockwave, and it forms that cloud,” he explains.

“So there’s always a shockwave (with detonations), the only difference is I think because it was warm, and because it was by the ocean, and because it was humid the conditions were right for you to see the shockwave this time—when oftentimes you don’t see it.”

But what about Donald Trump’s claim that the blast was an attack, and other persisting allegations of foul play?

One piece of footage making the social media rounds and bolstering such suspicions appears to show something flying into the scene of the initial fire, just before the huge explosion. Waters believes the footage is real—“in these kinds of situations you don’t actually tend to come across doctored footage,” he adds—but that the apparently flying object is most likely something incredibly innocuous: a bird.

“You can see it flapping. And then there are lots of other videos [much, much closer] from other angles which don’t show anything like that… which show no munition impacting,” he says.

Castner, meanwhile, says he’s not yet willing to completely rule out the incident being an attack of some kind. “I don’t know how the fire started,” he says. “If it was a small explosive device placed as an attack, it could have been.”

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