The science of fire bans

Local Journalism Initiative
·4 min read

Now that camping season has arrived, many residents have been keeping their eye on the fire advisories to see where they can have a campfire.

At the regular council meeting on May 1, Council received an education from Drayton Valley/Brazeau County Fire Services Chief Tom Thomson on how those advisories are calculated for the area.

“Because it is fire season we do get a lot of questions asking why we have different bans at different times,” said Thomson.

Thomson says the Fire Weather Index is what departments use to calculate their different levels of fire advisories. He says this model is one that is used across the country and consists of several different factors: the fine fuel moisture code, the duff moisture code, the drought code, the initial spread index, the buildup index, and the fire weather index.

“It’s based on data that is collected by Alberta Forestry on a daily basis,” said Thomson.

He said these different factors are all dependent on the readings in a specific area. As the topography of each area can vary, the level of fire advisories can also vary from one county to another.

Thomson said the fine fuel moisture code is based on how dry the grass is. “It only takes about 0.6 mm to change that rating,” said Thomson.

He says if it has a rating of 80 or higher, it will support free burning. At 90 or higher, there is a chance it will ignite and will cause spot fires throughout the area.

The duff moisture code focuses on the topsoil on the ground, such as the dead or decaying wood material. He said this layer can ignite easily if it is dry enough. Anything with a rating over 20 can support a fire ignition and sustained burning.

The average amount of moisture that is deep in the ground forms the basis of the drought code. Thomson says this number will determine how deep into the ground the fire will burn.

“This is a concern for us considering this is where we have issues where the roots will burn on the bottom of the tree,” said Thomson. “You end up with what is called Widow Maker Trees where all the roots are burned out underneath and then the tree, if there is any wind, will fall over onto firefighters or anyone else around.”

He said a rating of 15 is low, and the ground is pretty saturated with water. Anything 300 or higher will sustain fire for a long time.

The initial spread index that involves the wind speed and the fine fuel. He said it doesn’t take into account the different types of fuel, but rather just considers the grass in the area. A rating of 10 or higher will result in a quick spread and will have the potential of candling, where the fire goes into the treetops.

The total amount of fuel that is available to burn is calculated using the buildup index.

“You take the duff moisture code and the drought code together and you create a rating,” said Thomson.

A rating of 60 or higher is a concern because it’s difficult to extinguish that type of fire.

Thomson says the Fire Weather Index is one that most firefighters will generally talk about.

He said it refers to the intensity that the fire will burn at and is based on the initial spread index and the buildup index.

Thomson said all of these different factors combine to make one figure, which is called the head fire intensity. He said last year, with the Buck Creek Fire, the HFI was at a level six, which is the worst it can be.

He said the conditions in the area last year made for the perfect storm. All of the indicators were high, and that was why it was difficult to extinguish the fires.

Thomson said Alberta Forestry creates daily reports for municipalities, and that is what those areas base their fire advisories on. He says if they see that any of those numbers are in the red zone for more than a few days, they will change the advisory in the area.

“Some people think we just pull this out of the air,” said Thomson. “That’s not the case. We look at this every day and we take general trends [into consideration].”

One day of low numbers doesn’t make a difference, but he said if there are several days of lower numbers, the advisory can be lowered.

The Fire Weather Index Map can be found at www.alberta.ca/fire-danger.

Amanda Jeffery, Local Journalism Initiative Reporter, Drayton Valley and District Free Press

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