Tuesday Noon Siren
Every Tuesday at noon, San Francisco tests the Outdoor Warning System. During the test the siren emits a 15 second alert tone. In an actual emergency, the siren tone will cycle repeatedly for 5 minutes. Should you hear the sirens at any time other than Tuesday at noon, go indoors and immediately tune to a news source such as KCBS 740 AM, or other local media stations.
OPWS 20150205.pdf - Updated 2015
In a major disaster it might be 3 days before vital services are restored. When you hear the Tuesday Noon Siren, it's your weekly reminder to make an emergency plan, build an emergency supply kit and be prepared to be self sufficient for 72 hours.