In the unfortunate event of a nuclear strike against the United States, the Emergency Alert System will tell people what they can do to minimize their chances of being vaporized.
First, the FEMA National Radio System-headquartered here-will deliver a message from the White House to the National Public Warning system, composed of 77 radio stations across the country. Then, these stations deliver the message to nearby radio and television broadcasters, who are required by the FCC to have special equipment to pick up and deliver these messages immediately after receiving them… unless the alerts are weather or child-abduction related, in which case they can opt out if they decide the weather's not that bad or the kid's not that cute.
One of the interesting things about this warning system is the alert signal that plays on phones, radios, and TVs. "It's composed of two tones being played at the same time: 853 hertz and 960 hertz" and is illegal to play the signal. You can download a sample at the Wikipedia entry for the Emergency Alert System for private listening, but don't play it on a podcast or tweet the sound file — many TV networks and radio stations have been fined for using the signal for non-emergencies.