Eyewitness accounts in journals and diaries have documented the relationship between weather and large wildland fire for over a hundred years. Even a hundred years ago, observers recognized short periods of up to several days in every fire season when wildland fuels were unusually susceptible to large fires, depending mainly on the weather. Show (1931) referred to these as 'dangerous periods.' However, not until the 1960s were critical fire weather patterns that produced high fire danger and large wildland fires identified for the United States and Canada. Syverson (1962) recorded the first definition of critical fire weather patterns as the 'critical day, week or month during which blow-up fires are experienced.' Current fire behav­ior training courses define critical fire weather patterns as the atmospheric conditions that encourage extreme fire behavior, resulting in large and destructive wildland fires.