Friday, September 22, 2017

"Don't try to Shoot the Hurricane"—Okay...But What if We Did?

Before the arrival of hurricane Irma in Florida, the Pasco county sheriff's department posted a tweet that quickly went viral, urging citizens not to try and shoot at the hurricane:

Thanks, Sheriff.
This wasn't apropos of nothing—the tweet was a response to an article about a (tongue-in-cheek) facebook event encouraging people to shoot the hurricane, which attracted about 40,000 "interested" fans. Hopefully nobody took it too seriously and went out to take a stand against Irma with their shotguns.

But, being the kind of people we are, this sparked a discussion at the office...COULD you stop a hurricane by shooting at it? Kinetic energy is kinetic energy, and—if you fire a bullet into the approaching side of a hurricane, the bullet isn't likely to make it out the other end before it stops. That means that all the kinetic energy of the bullet has been transferred to the air mass, and if they're heading in opposite directions, every bullet fired would reduce the intensity of the hurricane by a small amount.

This highly scientific diagram should help.
So, having gotten it out of the way that you ABSOLUTELY SHOULD NOT DO THIS, how many bullets would it have taken to kill Irma dead?

To find out, the first thing we'll need to know how much kinetic energy was in the storm. Fortunately, this data is pretty readily available—the integrated kinetic energy (or IKE) of a hurricane is regarded by most meteorologists as an important tool to assess the intensity of a storm and its potential for damage. Many even prefer it to the familiar Saffir-Simpson scale—when you hear a storm described as "Category 4" or "Cat. 5", this is the Saffir-Simpson categorization, which relies on top sustained wind speeds. But a big, spread-out storm with lower top wind speeds could do more damage than a tightly bound hurricane with higher velocities, so some argue that it's best to look at the IKE.

The neat thing is that the kinetic energy of a moving air mass is calculated in much the same way as the kinetic energy in something familiar like a car—it's simply half the mass times the square of its velocity. We know how much mass is in a cubic meter of air, and weather instruments in the path of a storm tell us how fast that air is moving at certain points, so by treating each cubic meter of air as a massive object moving at the measured windspeed, we can calculate the storm's IKE.

According to risk assessment firm RMS, Irma had an IKE of 125 Terajoules at landfall. For scale, a 60-watt lightbulb uses 60 joules per second, so the amount of energy in Irma would be enough to keep an incandescent lightbulb running for 66,000 years.

So now, in order to figure out roughly how much gunfire it would take to stop a hurricane of comparable size, all we need is the kinetic energy of a bullet. This varies greatly depending on the gun it's being fired from, size of the round, etc.—but we already know we're going to need a lot of ammunition here, so let's go with something fast-firing and high-impact.

This should do nicely.

That would be the General Electric M134 minigun, capable of firing 6,000 rounds per minute at a muzzle velocity of 850 m/s. Each bullet, according to the wiki specifications for the 7.62x51mm NATO ammunition that it uses, carries roughly 3,300 J of energy. In terms of kinetic energy transfer, this monster is pretty much unparalleled until you get into things like heavy ordnance, or absurd and unsustainable designs like the "million rounds per minute" Metal Storm.

Operating at maximum capacity, the M134 is capable of transferring nearly 20 million joules of energy per minute. This is a pretty extraordinary amount, but it's still many orders of magnitude shy of the amount of energy in a hurricane like Irma—a Terajoule is a trillion joules, i.e. a million million.

So how many bullets are we going to need? If there's 125 trillion joules in the storm, and 3300 in a bullet, we'd need 3.79*1010 rounds to completely undo Irma. 109 is a billion, so that means 37.9 billion rounds. A quick search around the web puts the estimate for the global number of bullets manufactured at 10-12 billion annually, so you're gonna need to save up.

So how much firepower are we going to need here? Let's say we've convinced the Commander in Chief that this is an issue of national security, and we've got the resources of the entire DoD at our disposal—all the manpower and miniguns we need—but we've only got 24 hours to disperse the storm.

To fire off the necessary 37.9B rounds from a single minigun operating continuously at 6,000 rounds per minute would take 4387 days, or twelve years. That's not going to do us much good—but it means that, if we want to get it done in 24 hours, all we need is 4387 miniguns!

Obviously there are logistical problems here—guns overheat and need to be cooled, need reloading, jam up, etc. If you spray enough lead into the approaching edge of a hurricane, some of it IS going to start coming back at you before the storm disperses, and could return at velocities of up to 70 m/s—like being shot at with an old school black powder musket. On the other hand, you probably don't need to counter ALL the energy in the hurricane to downgrade it to a non-destructive tropical storm, so let's assume (in the true spirit of Fermi Problem Friday) that those factors balance out.

So there you have it, folks: if you wanna kill a hurricane with bullets, all you need is 24 hours, a fortified brigade of heavy artillerymen, and $20B of ammunition. Of course, in the process, you'll probably put enough lead in the water to give the entire eastern coast of Florida heavy metal poisoning, so don't expect a medal.

Stephen Skolnick

3 comments:

  1. In the same tongue-in-cheek attitude as the original Facebook post that prompted the Sheriff's warning.....

    A hurricane is primarily ENERGY, so if you intend to affect it greatly you'll need an ENERGY weapon. An orbital high energy laser, fed by a few acres of solar panels, should do the trick. Fire the laser into the sea a few miles outward from the eyewall, and the heat generated in the ocean should cause enough heating to generate a counter-cyclone of its own, disrupting the eyewall and causing the storm to become disorganized. It may take several shots, but once the laser and solar cells are in orbit, the "ammunition" is free, and you can use the laser to disrupt every hurricane that approaches land.

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    1. Since they all rotate the same way (due to Coriolis forces, of course) any laser-induced cyclone OUTSIDE of the eyewall will inevitably disrupt the eyewall winds. Do that often enough, and you destabilize the entire central core of the hurricane.

      By the time we have orbital HELs, we'll have the computer power (which we currently do NOT) to calculate exactly where and for how long to fire the beam to disrupt the eye.

      "Equal but opposite"? Not what I wrote. You're just disorganizing the cyclone. And apparently you missed the "In the same tongue in cheek attitude" comment. And an HEL is at least theoretically possible, unlike battieries of miniguns....

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