Wildfire officials are keeping a close eye on this key wildfire indicator

PENTICTON – Wildfire officers trying to predict fire behaviour are paying close attention to an indicator known as "crossover" this week.

Crossover occurs when the relative humidity, expressed as a percentage, is less than the air temperature, measured in degrees Celsius.

It can be an indicator of how dry forest fuels are and what fire behaviour might look like on a given day, says B.C. Wildfire Information Officer Kyla Fraser.

“The concept is relevant as fire behaviour is driven more by humidity levels than by temperature. Low humidity levels can be enough to dominate the fire weather pattern for the day,” she said in an email.

Humidity levels are important to consider even if the temperature is cool, Fraser says, but the crossover point is more easily reached in hot weather.

The lower the humidity, the more “drying power” the atmosphere has on forest fuels. Lower fuel moisture content means more of the energy of combustion is felt as heat since less energy goes to boiling off the water contained in the forest fuels.

Fraser says crossover usually occurs during afternoon heating, and the longer the time period where crossover occurs, the more aggressive fire behaviour can be anticipated.

With this week’s hot temperatures anticipated, it’s likely crossover will be reached earlier in the day and the crossover period will last longer, meaning a wider window for optimum burning conditions.

Fraser says crossover is a good first indication of how dry forest fuels currently are, in addition to providing information about what fire behaviour might look like for the day.

“It is extremely important to note that this is only one indicator of fire behaviour, and it can’t be used in isolation,” she said.

“Winds play an extremely important role when predicting fire behaviour and must also be taken into consideration in combination with temperatures and relative humidity levels,” she says.

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