Weather & Contributing Factors

Environmental conditions at the time, coupled with the severe topography and the geographical location, combined to make this fire virtually uncontrollable.

Several factors contributed to the severity of this fire.

• Extremely low precipitation through the late summer and fall.

• Abnormally warm conditions with considerable amounts of wind in the weeks preceding the fire, resulting in the desiccation of the grasses and the stubble, and the absence of soil moisture or even frost.

• Isolation from fire fighting units was another logistical problem. Pincher Creek was 15 miles from the origin of ignition; Fort Macleod was 25 miles, and Granum was 30 miles.

• This fire spread into steep coulees where there was no road access.

• The topography makes communications unreliable, and in certain locations non-existent. Cell phone and radios in some areas will not work at all.

The relative humidity combined with high seasonal ambient temperatures over a long period of time make the fuels extremely volatile. Relative humidity below ambient temperatures creates a high intensity fire that leads to spontaneous combustion of the grasses and stubble in advance of the flames as the heat wave passes over them. Relative humidity below 30% is a concern to fire control officials.

As was the case with this firestorm, there was evidence of trees and grasses igniting 60 to 180 meters in advance of the existing flames due to the above described conditions.

The relative humidity dropped below 30% at 9:00 a.m. on December 12, and continued to decline to 15% at the time of ignition. On the flats near Granum at 3:00 p.m. on December 14, the relative humidity dropped as low as 10% and the temperature soared to 16 degrees Celsius.

Winds had been light December 13, but on December 14 became strong and gusty shortly after ignition. Winds recorded at the Pincher Creek weather station were 28 kilometers at 10:00 a.m. on December 14. and increased to 90 kilometers by 1:00 p.m. The spotter plane reported wind speeds in excess of 160 kilometers at 1000 feet above ground level at 3:00 p.m.

These high winds led to rapid fire spread of high intensity. Alberta Forestry concludes that anything greater than 10,000 kilowatts per meter is uncontrollable. This fire was measured that day at 21,717 kilowatts per meter.

The fire was traveling at a 161 meters per minute average. It had tremendously long fire lines which involved large areas over a short period of time.

High winds, high temperatures, low humidity, and dry conditions resulted in several waves of flame spreading rapidly over fire control barriers.

Continue reading about the Losses.