The Homeland Security Advisory System provides warnings in the form of a set of graduated “threat levels” that increase as the risk of the threat increases. The greater the risk of a terrorist attack, the higher the threat level. The risk includes both the probability of an attack occurring and its potential gravity. There are five threat conditions, each identified by a description and corresponding color. From lowest to highest, the levels and colors are:

Green: This condition is declared when there is a low risk of terrorist attacks.

Blue: This condition is declared when there is a general risk of terrorist attacks.

Yellow: An elevated condition is declared when there is a significant risk of terrorist attacks.

Orange: A high condition is declared when there is a high risk of terrorist attacks.

Red: A severe condition reflects a severe risk of terrorist attacks.