Weather alerts are used for tragedies like the May 12, 2008 earthquake in China.

Wang Jian/China Fotopress/Getty Images

In China, after an earthquake measuring 7.9 on the Richter scale caused more than 55,000 deaths and billions of dollars worth of damage in May 2008, rivers blocked by landslides threatened to worsen the tragedy with torrential flooding. To prevent further loss of life, local government offices sent emergency text alerts to residents downstream, warning them of the rising waters and urging them to leave [source: Yahoo News/AP].

In Boone County, Ill., an unseasonal tornado surprised Poplar Grove residents in January 2008, flattening four houses and spurring neighboring Winnebago County to adopt a Reverse 911 system that now sends residents alerts of bad weather and other emergencies via landline phones, cell phones, e-mail and voice-over Internet protocol (VoIP) numbers [source: Rockford Register Star].

Loud sirens are increasingly being replaced by the use of digital and Web technology for emergency alerts. Weather alerts provide the warnings we need to head to shore, batten down the house or simply seek shelter. And while a siren may be the first thing that comes to mind as a bad weather alert, emergency alerts can be sent using many different forms of communication. In fact, Internet and wireless technology make it possible to receive emergency weather alerts as notifications on your PC, cell phone or other mobile device.

But what form of emergency alert works best in which situation? And how do you sign up to receive emergency alerts? Read on to find out.