Tornado warning sounds vary by location, but are generally a single high-pitched tone sounded for a specific length of time that resembles the civil defense sirens used for air raid warnings during World War II. Many locations use the same original sirens as tornado warnings.
Most jurisdictions use mechanical sirens that force air through rotors to produce the pitch and intensity of the siren. Modern electronic sirens use sound generators and amplification systems capable of producing various sounds for different types of emergencies, as well as vocal instructions.
The tornado siren sound in Aurora, Colorado is typical of most sirens and sounds for three minutes.to indicate that the National Weather Service has issued a tornado warning, and that a possible tornado has been sighted within city limits. This siren is used for a variety of other emergency situations and alerts residents to immediately head indoors and seek additional information.
Illinois distinguishes tornado warning sirens from other alerts, such as those indicating enemy attacks, by using a single tone for tornado warnings sirens and a tone that rises and falls in pitch for attack warnings.