Phases of a Wildfire

Understanding fire progression and behaviour helps predict how a fire will develop and informs suppression strategy. This page covers the five phases of a wildfire, the fire environment framework used to reason about behaviour, and the extreme fire behaviour mechanisms that represent the most dangerous escalations.

The fire environment triangle

Before examining fire phases, it is useful to distinguish two related frameworks:

• The fire triangle — fuel, oxygen, and heat — describes the conditions required for combustion. Remove any one element and the fire cannot sustain.
• The fire environment triangle — fuel, weather, and topography — describes the conditions that drive fire behaviour. This is the framework field personnel use to reason about how a fire will spread, intensify, or change character.

The documentation covers weather variables and fire indices extensively. This page connects those variables to fire behaviour outcomes in a way that can be applied operationally.

The five phases

1. Ignition

An initial heat source ignites fuel. Fire begins at a single point and requires heat, fuel, and oxygen to sustain. This is the critical window for early suppression — a fire extinguished at ignition costs a fraction of the resources required later.

Key characteristics:

• Single ignition point, small and localised
• Fuel moisture and weather determine whether the fire will grow or self-extinguish

Weather factors most relevant at this phase: Temperature, Relative Humidity, Wind & Gusts at ground level.

2. Growth (active phase)

Fire spreads rapidly, consuming available fuel. Heat increases, flame height grows, and fire behaviour intensifies. This is the most critical phase for resource deployment.

Key characteristics:

• Exponential spread rate
• Fire behaviour driven by wind, topography, and fuel type
• Pre-heating of adjacent fuels ahead of the fire front
• Ember spotting may begin

What drives growth:

• Wind — pushes flames forward and pre-heats fuel ahead of the fire front
• Slope — fire moves faster uphill, approximately doubling spread rate every 10° of slope
• Fuel continuity — connected vegetation allows rapid spread

Suppression priority: Establish control lines, protect structures, prevent spot fires.

3. Fully developed

Peak fire intensity. Maximum heat release and flame height. All available fuel in the area is burning simultaneously.

Key characteristics:

• Highest flame lengths and maximum rate of spread
• Crown fires possible in forested areas
• Fire can create its own weather (convective columns, fire whirls)
• Most dangerous phase for suppression crews

Indicative values at the flame front: temperatures of 800–1200°C, radiant heat extending 50+ metres, ember transport 1–5 km ahead of the fire front.

Suppression strategy: Often indirect attack only. Wait for fire intensity to decrease before engaging.

4. Decay (controlled phase)

Fuel consumption decreases and fire intensity drops. Flames lower as available fuel is exhausted. Control lines may be holding.

Key characteristics:

• Reduced flame height and slower spread rate
• Transition from flaming to smouldering combustion
• Hot spots remain active

Fires reach this phase when available fuel is exhausted, weather conditions improve (humidity rises, wind drops, precipitation arrives), natural barriers are reached, or suppression efforts are effective.

Operations focus: Strengthen control lines, mop up hot spots, patrol perimeter.

5. Extinguished

No active flames or heat. All fuel consumed or cooled below ignition temperature. Mop-up operations complete.

Verification methods: hand checks for residual heat, thermal imaging for hidden hot spots, moisture content checks, multiple patrol cycles.

Critical reminder — “Controlled” ≠ “Extinguished”

A fire in the decay phase is controlled but not out. Weather changes can reignite growth, hidden smouldering areas can flare up, wind can carry embers beyond control lines, and fuel moisture can decrease overnight. Always maintain vigilance until confirmed extinguishment — particularly in duff layers, roots, stumps, and logs, where smouldering can persist for days.

Factors that influence phase transitions

Direction	Conditions
Accelerating (Ignition → Fully Developed)	Fuel moisture <10%, temperature >30°C, RH <30%, wind >25 km/h, slope >30°, dense and continuous fuels
Slowing (Growth → Decay)	Fuel moisture >20%, temperature <15°C, RH >70%, wind <10 km/h, natural barriers, effective suppression

Extreme fire behaviour mechanisms

The phase model describes typical progression. Extreme fire behaviour represents a departure from this model — conditions in which fire behaviour exceeds what standard indices predict and where suppression operations become extremely hazardous or impossible.

Fire whirls

Fire whirls form when rotating air — caused by wind shear, terrain effects, or the fire’s own convective column — concentrates into a tight vortex. They can carry burning debris over long distances and produce sudden, extreme local spread. They are most likely to develop at the junction between a head fire and a flank fire, or in terrain that produces rotational airflow. A fire whirl is distinct from a fire tornado; both represent extreme, unpredictable local intensifications.

Pyroconvective fires

Pyroconvective behaviour occurs when a fire generates enough heat to build its own convective column — a self-reinforcing atmospheric process. This transition is the most dangerous escalation in fire behaviour.

When a fire “takes over the column”:

• The updraft supplies its own oxygen, making the fire largely independent of synoptic wind conditions
• Downdrafts around the column produce erratic surface winds in all directions, disorienting any prediction based on forecast wind
• Spotting distances extend from kilometres to tens of kilometres
• Fire behaviour becomes largely unpredictable from ground observation

The Haines Index and CAPE both measure atmospheric conditions that support pyroconvective development. The Pedrógão Grande fire (Portugal, 2017) and the Black Saturday fires (Australia, 2009) both involved major pyroconvective episodes that drove the most destructive phases of each event.

Crown fires

Crown fires represent a step change in fire intensity that fundamentally changes the operational picture. Two types are relevant:

• Passive crown fire (individual tree torching) — single trees or small groups ignite into the canopy, often acting as ember sources
• Active crown fire — continuous fire running through the canopy independent of the surface fire below

The key enabling conditions are low live fuel moisture in the tree canopy, extended drought reducing deep soil moisture, and sufficient wind speed to carry fire through the canopy. The transition to crowning is not predictable from standard surface fire indices alone.

Wind shifts

A shift in wind direction turns the fire’s flank into a new head fire instantly. If a fire has been spreading north for several hours, its eastern and western flanks may extend for kilometres. A 90° wind shift turns the eastern flank into a new, very long head fire advancing rapidly east — and personnel on what was a quiet flank are suddenly in front of an active head fire.

This mechanism is responsible for a significant proportion of firefighter fatalities in Europe, Australia, and California. The 2017 Pedrógão Grande fire became catastrophic after a near-90° wind shift; similar dynamics have caused entrapments across Mediterranean and Australian fire environments.

Note

Wind shifts are visible in forecast data as changes in wind direction through the day. The Wind & Gusts page covers the significance of direction changes. When operating on fire flanks, always know the forecast wind evolution — a flank is only safe relative to a given wind direction.

Sources

• European Commission Joint Research Centre (2023). Current wildfire situation in Europe. EFFIS Annual Reports.
• Castellnou, M., Miralles, M., & Molina, D. (2009). Wildfire management in Mediterranean-type regions: paradigm change in Southern Europe. Wildfire, 18(3): 18–23.
• Viegas, D.X. et al. (2017). O complexo de incêndios de Pedrógão Grande e concelhos limítrofes. University of Coimbra.
• Cheney, N.P. & Sullivan, A. (2008). Grassfires: Fuel, Weather and Fire Behaviour. CSIRO Publishing.