Anyone who’s listened to television or radio over the past five decades is intimately familiar with that horrible, chill-inducing noise of the Emergency Alert System. Aside from catching your attention, that nails-on-a-chalkboard screeching serves a useful purpose that calls back to the days of dial-up internet.

The tense atmosphere of the Cold War left the United States scrambling to prepare itself for the not-insignificant threat of an all-out nuclear war with the Soviet Union. Among the many ways the country prepared for the possibility of getting wiped out in some radioactive hellscape involved an intricate network of warning systems that spanned from coast to coast, quickly alerting residents to seek shelter and brace for impact.

Thankfully, though we came pretty close, the development of these systems proved unnecessary for their intended use. Many of the civil defense sirens set up around the country were repurposed and extended to use as tornado sirens (which are extremely unreliable, by the way), and bomb shelters are used for everything from tornado protection to swanky living quarters. (People still say ‘swanky,’ right?)

One of the safety initiatives developed during this uncertain time was a program called “Control of Electromagnetic Radiation,” or CONELRAD, because everything in the 1950s had to have a ridiculous name. Printed on every radio receiver manufactured during the 50s and early 60s, the recognizable “CD” triangle let you know where to tune in the event of a nuclear attack or invasion.

The system is pretty straightforward: in the event of a national emergency, the U.S. Government would order all radio and television stations to immediately stop broadcasting, and select sources would begin transmitting emergency messages over 640 AM or 1240 AM.

CONELRAD, its successor the Emergency Broadcast System, and the current Emergency Alert System are all designed as a way for the president to quickly address the public in the event of a catastrophe like a nuclear attack or imminent meteor strike. Since these very rarely happen, we use the Emergency Alert System for more common things like tornadoes, floods, and “civil emergencies” like a 911 telephone outage or widespread road closures due to a blizzard; in other words, things that have an actual impact on your life.

For the most part, activation of the Emergency Alert System is automated, with alerts broadcasting from a central source (like the National Weather Service) and quickly filtering through different points until it reaches your ear holes as the blood-curdling, demonic scream of a far away computer program. All over-the-air television and radio stations, in addition to satellite and cable companies, are required to have special equipment that receives the tones produced by the originating source and uses these tones to decipher what’s going on and what to do next.

Those awful screeches you hear at the beginning of the Emergency Alert System are digitized codes that communicate the type of threat, area (counties) threatened, and how long the threat is in place. If you were cool enough to have dial-up internet back in the day, that iconic noise you heard during your suspenseful wait to connect to AOL basically did the same thing—your computer communicated with the servers through a series of chirps, beeps, and tones.

The digital header, as it’s called, is repeated three times as a redundancy so the system can catch any mistakes lest it transmits the wrong message and whips everyone into a panic that Kim Jong-Un spilled his Rice Krispies and declared war on the entire Northern Hemisphere. This header message is followed by a loud, prolonged beep that’s designed to get your attention (it works!), which is then followed by an announcer or computerized voice informing you of what’s going on. The transmission ends with a quick series of tones that lets the system know it’s the end of the message, and that the station can resume normal programming.

In fact, this is pretty much how modern NOAA Weather Radios work; Specific Area Message Encoding (S.A.M.E.) technology is embedded in the weather radio feed, with radio devices automatically activating when they receive a warning transmission for the counties you programmed into the receiver. During weather emergencies, the rapid digitized beeping you hear on the weather radio feed is the same as what the Emergency Alert System is listening to, which is why you’re usually able to hear Tom on television and radio when your town goes under a tornado warning.

Even though the system is designed to let the president address the country during a national emergency, they’ve never used the system during an actual nationwide emergency. The only national emergency that could have required its use was 9/11, but news organizations were so quick to televise the event that activating the Emergency Alert System would have been counterproductive, and even so, the president wouldn’t have had a chance to address the nation until the attack was almost complete.

We know that the system works for local emergencies like tornadoes, but since it’s never been used on the national level during an actual emergency, we don’t know how well it would really work.

That being said, it has been activated on the national scale a couple of times. Back in 1971, NORAD tried to test the Emergency Broadcast System but accidentally transmitted the wrong message, leading some stations to go through the motions as if there were a real national emergency. Other than that, the federal government conducted a nationwide test of the Emergency Alert System in November 2011 to see whether or not an activation on the national level would actually work in an emergency. While a majority of television/radio stations received the alert and transmitted it to the public, there were several glitches along the way that prevented the test from succeeding.

The next time you’re falling asleep at 2:30 in the morning and the horrendous screeching of the Emergency Alert System jars you awake, remember that the resulting nightmares were caused by a bunch of computers talking to each other out loud.

[Top image via Paul Silva on YouTube, CONELRAD billboard via]

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