Terror alert system

Terror alert systems are standardised emergency population warning systems for describing and disseminating information about terrorism-related threats. They became more popular after the September 11 attacks on the United States in 2001.

Australia

National Counter-Terrorism Alert Level

* Low - terrorist attack is not expected
* Medium - terrorist attack could occur
* High - terrorist attack is likely
* Extreme - terrorist attack is imminent or has occurred

Administered by the Federal Attorney-General's Department.

France

Plan Vigipirate

* Yellow - to stress vigilance
* Orange - to warn of terrorist action
* Red - to warn of serious attempts
* Crimson - to warn of major attempts

United Kingdom

UK Threat Level

* Low - an attack is unlikely
* Moderate - an attack is possible, but not likely
* Substantial - an attack is a strong possibility
* Severe - an attack is highly likely
* Critical - an attack is expected imminently

Administered by the Joint Terrorist Analysis Centre

United States

Homeland Security Advisory System

* Low - Low risk of terrorist attacks
* Guarded - General risk of terrorist attacks
* Elevated - Significant risk of terrorist attacks
* High - High risk of terrorist attacks
* Severe - Severe risk of terrorist attacks

Administered by the Department of Homeland Security