Outdoor warning sirens, most commonly referred to as ‘tornado sirens,’ are signaled to go off when severe weather strikes, specifically when a tornado warning is issued, though there is a lot of variance from place to place.
By Hope Kleitsch
The biggest misconception is that tornado sirens are solely there to warn you of imminent tornadoes, no matter where you are. However, this is not true. Going back to their true name, outdoor warning sirens, these sirens are meant to be heard only when outside. They are not meant to be heard inside. This is why you should never rely on tornado sirens as your way to receive warning of a tornado.
Tornado sirens also vary from county to county. When a tornado warning is issued by the National Weather Service, that message will be sent to the affected county Emergency Management Agency (EMA). It is then up to the specific county to sound the sirens. 
Every county has different policies when it comes to sounding off the sirens. All counties will activate the sirens when a tornado warning is issued. Some counties, including Marion County in Indiana, will activate the sirens if there is a severe thunderstorm warning within a tornado watch box.
It can also be confusing for people when the sirens are going off in their town, yet the actual tornado-producing cell is not immediately near their area. This happens sometimes because the sirens will go off for an entire county, not just specific towns.
When sirens sound in your area, they will go off for approximately three minutes. When the sirens stop, that does not mean that the threat has passed! Never get out of your safe place until you get the all-clear from your local meteorologists.
For all of these reasons, it is important to stay weather aware when severe weather moves in. Be sure to stay tuned to your local weather stations and to have your severe weather plan in place.