The Four Stages of Fire Growth Explained

Explore the 4 stages of fire growth, from ignition all the way to decay, with this in-depth guide from the experts at the Western Fire Chiefs Association.

Understanding the four stages of fire growth—incipient, Growth, Fully Developed, and Decay—is essential for effective wildfire management. As soon as a fire ignites, it can rapidly escalate from small flames to a raging wildfire, so early detection and assessment are imperative. By managing these four key stages, firefighters work to control wildfires, protect ecosystems, and ensure the safety of lives and property.

What are the Four Stages?

The first step in the process of firefighting begins with spotting a growing wildfire and assessing its growth. Fire spreads rapidly under certain conditions and can progress from a small campfire to a huge wildfire in just a few hours. There are four stages of fire growth: Incipient, Growth, Fully Developed, and Decay. Monitoring fire behavior and determining the risk it poses based on these stages is an important part of firefighting.¹

Incipient (also known as Ignition)

A fire starts when heat, oxygen, and a fuel source create ignition. The beginning stage of a fire is the easiest to extinguish or control with portable firefighting equipment. An example of this kind of fire would be candles, campfire embers, or cigarettes.

A fire is at the incipient stage when it has small flames and low heat. Smoke from an incipient fire does not obstruct visibility. It is key to monitor fires at this stage, before they can grow into devastating wildfires.

Growth

A fire enters the growth stage as it continues to burn. Spreading brushfires are an example of this. The surrounding area and amount of combustible material affect the speed at which the fire grows during this phase. Temperatures rise, and a defined layer of smoke above the flames appears.

There are greater potential dangers when the flames are in a more closed-off area, such as beneath tree cover. The fire’s heat at the growth phase can ignite everything combustible, including smoke from the fire itself. This is called a flashover when temperatures can rise to 1,000 degrees Fahrenheit in seconds.² A flashover is extremely dangerous and can be fatal to anyone in a confined space when a flashover occurs.

Fully Developed

After flashover occurs, the fire is in the fully developed stage when the temperature reaches its highest point, sometimes almost 2,000 degrees Fahrenheit.³ These are dangerous wildfires that spread rapidly and can consume trees and huge areas of forest and grassland. Fires at this stage have dark, dense smoke that limits visibility and can obscure the flames. A fully developed wildfire will continue to burn until it runs out of fuel.

Decay

The final phase of fire growth is the decay stage. This is often the longest stage of a fire, as the flames decrease in size and the heat of the fire begins to drop. Firefighters work to force wildfires to reach this stage by limiting the area it burns with firebreaks or firelines and by dampening and cooling the surrounding area.⁵

A wildfire that is decreasing in size and running out of combustible material is in the decay stage, but the fire can still be dangerous. A fire can restart if there is still fuel to burn and if winds pick up to add more oxygen to the fire.

Controlled Burns and Safely Managing Fire Growth

Firefighting in wild areas has shifted in recent years to address the importance of regular, controlled burns that clear debris and potential fuel from the landscape. All of this work is done to ensure wildfires benefit the ecosystem, and to ensure that the stages of fire growth can be safely managed. Firefighting agencies focus on protecting human lives, structures, and resources when determining wildfire response needs.⁶