Two/Three-Alarm Fire---what is it??

[sharpie]

Newsreporters use these terms on the news whenever there's a fire. Couldn't find the meanings of thess terms in Encarta dictionary or using google. Can anyone explain these terms?

[Wildfire]

When firefighters arrive on the scene of a fire and decide that they will need additional help, they will request a second alarm. The dispatch center will send an alarm to another fire company, thus creating a “2 alarm fire�. The dispatch center may also send multiple alarms at once. The more severe a fire is the more alarms will be sent out so the number of alarms can give one a relative idea of how big a fire is.

[freebie]

Google turned this up:


At 6:39 p.m., it became a three-alarm fire, meaning 33 engine and ladder trucks and 138 firefighters responded. The fire then moved into 40 Avenue B, the adjoining building. At 7:51 p.m., two injured firefighters were taken to Cornell Medical Center. At 8:01 p.m., the blaze was upgraded to four alarms, meaning 39 units and 168 firefighters were at the scene.
http://www.thevillager.com/villager_39/fouralarmfire.html