Critical fire weather patterns

Critical fire weather patterns are defined as the atmospheric conditions that produce extreme fire behaviour, resulting in large and destructive wildland fires.

Paul werth, “synthesis of knowledge of extreme fire behaviour”

Three critical weather elements produce extreme fire behaviour:

– Low atmospheric humidity
– Strong surface wind
– Unstable atmosphere

Atmospheric humidity

The amount of water contained in the atmosphere influences fire behaviour indirectly by determining the moisture content of fine dead fuels.

The moisture content of fine dead fuels, such as dead leaves and small twigs, reaches equilibrium with atmospheric moisture relatively quickly, typically within a few hours.

Since fine dead fuels ignite more readily, their moisture content plays a key role in determining landscape flammability and subsequently rate of spread; lower fine dead fuel moisture content translates to higher flammability and larger rate of spread.

In that regard, wildfires can break out more easily and spread faster early in the afternoon, when the moisture content of fine dead fuels is typically lowest, responding to peaking temperature and lowest humidity.

Surface wind

Wind is inarguably the weather element with the largest influence on fire behaviour. It controls the flux of oxygen required for sustaining the burning of fuels, while it also influences the exchange of moisture between the atmosphere and the fuels, and determines the movement direction of the fire fronts. The stronger the wind, the faster the wildfire will spread.

When the wind is gusty, it promotes spotting activity by effectively carrying burning embers over large distances and consequently setting up new ignitions.

When blowing from varying directions, strong winds can easily turn a surface wildfire into a crown wildfire.

Unstable atmosphere

The stability of the atmosphere is one important weather element that defines if and when a wildfire will blow up.

Simply put, atmospheric stability describes the resistance of the atmosphere to vertical motions; a stable atmosphere suppresses vertical motions, whereas an unstable atmosphere enhances vertical motions.

The heat released by a wildfire warms the air, which is then forced to rise, generating updrafts. The stability of the atmosphere controls the strength and extent of these updrafts.

An unstable atmosphere enhances the updrafts generated by a wildfire, encouraging the growth of the convective column above it. This, in turn, increases combustion rates, as more oxygen is vigorously supplied to the flaming zone. As a result, the strength and depth of the convective column continues to increase, eventually leading to the blow up of the wildfire and the occurrence of transient fire phenomena, such as fire whirls and pyrocumulus or pyrocumulonimbus clouds.

Our contribution

✔︎ FLAME will compile the critical fire weather patterns of Greece, defining their key characteristics (frequency, strength, duration) and verifying relationships with large-scale atmospheric teleconnections and/or sea-surface temperature anomalies.

✔︎ The self-organising map (SOM) approach will be employed for classifying synoptic-scale circulation patterns into different weather types.

✔︎ Extreme fire weather conditions will be identified based on various fire weather indices, and fire occurrence and burnt area data.

✔︎ Relationships between certain SOM-based weather types and the occurrence of extreme fire weather will be verified statistically.