Fire Weather Terminology

Haines Index combines the effects of instability and dry air to determine conditions in which an existing fire has the potential to spread rapidly. It should not be used to determine if a fire will start. Since wind is not a parameter of the Haines Index, it is best used for plume-dominated fires which can develop extreme surface winds through their own internal heat. The Haines Index values are as follows: 2 or 3 - very low; 4 - low; 5 - moderate; 6 - high. When the value is 5 or 6, the probability of extreme fire behavior, including torching, crowning and spotting is significantly increased.

Lightning Activity Level (LAL): A number, on a scale of 1 to 6, that reflects frequency and character of cloud-to-ground lightning. The scale is exponential, based on powers of 2 (i.e., LAL 3 indicates twice the lightning of LAL 2).

A red flag warning is issued only when all three of the following conditions are met; a) sustained one-minute winds at 20 feet are at or above 25 mph; b) minimum relative humidity is at or less than 25 percent; c) temperatures at or greater than 75 degrees F. In addition, the fuels must be dry enough to burn.

National Interagency Fire Center