Wildfires pose the greatest danger within the boreal ecosystem for fire managers in the Northwest Territories. Although they are beneficial to the control of diseases and insects as well as maintaining biological diversity, fires also threaten human life, property and valuable commercial resources. Lightning typically starts approximately 80% of the forest fires in the NWT. Over the past 10 years, an annual average of about 340 fires have consumed approximately 830,000 ha.

Weather is a critical element to forest fires. Not only does it affect the ignition of fires through lightning, but it also influences fire behaviour. Strong winds, high temperatures and low humidity influence the rate of fire growth. Atmospheric instability, another important factor in fire growth, can influence the spread and intensity of wildfires. Research studies carried out in the United States have shown a relationship between the vertical structure of the atmosphere and fire activity.

In this study, several atmospheric severity indices, including the Haines index (Lower Atmosphere Severity Index [LASI]), and their relationship to fire behaviour are examined and compared for two fire seasons in the Northwest Territories. The summer of 1995 represents an example of an extreme fire year, where fires burned 2.8 Mha. The summer of 1998 represents a more typical season where fires burned about 1.4 Mha.

Data from three radiosonde stations in the Northwest Territories were used to determine lapse rates and moisture values between the 850 and 700 mb layer. Components from the Canadian FBP System were used to determine fire behaviour characteristics.

Results showing the frequency of fires and large fires(>200 ha) to the LASI, other stability indices, and the continental-scale atmospheric environment will be presented. Also, the relationship between head fire intensity and the atmospheric stability and severity indexes will be discussed.