Question:
You often hear speak of a "Five-alarm fire", or a "three-alarm fire". I am looking for a brief description of the severity of fires, and how they would be classified into these "alarm" levels.

Answer: -The Fire Chief (Senior Fire Official) on site determines the need for Mutual Aid. A two alarm fire, is the call for another station to respond and help. Three, four, five, etc. It's the number of responding stations to aid in efforts to put out a fire. It's determining the need for help, not a classification of severity.

-Depends on the standards of the district. One district may define a one alarm fire as a response for an engine. Another would say an engine and a ladder.

In my old department the responses were:

One Alarm: Engine and aide unit. Second Alarm: Another engine and a tanker...all stations and units man and move to standby positions. Third Alarm: all hands for the district and a Medic unit Fourth Alarm: Mutual Aid. Call in resources from all other surrounding districts depending on incident. More tankers,engines, or special equipment.

In each case, it would be the incident commander or first on officer to make the call. The officer can make any, all, or a combination of the calls. Things may have changed.

This was for fire. Aid calls where a different thing. You get a multiple car pile-up...