Fire behaviour in the outdoors

Fires behave differently in different situations. Fire behaviour in the outdoors is quite different from the behaviour of fire inside a building or structure. The biggest difference is that indoor or structural fires are contained within a certain space, but outdoor fires can spread a lot more easily. They are burning in an open environment. As long as there is enough fuel (vegetation) that is dry enough and there is some wind to move it along, the fire will just keep burning, spreading and developing.

Recent research is showing that combinations of certain exotic plants with native ones can influence how quickly a fire ignites and how long it burns for. An upside of the research is the potential for plant species with low flammability being used to slow or extinguish fires in order to protect buildings. You can learn more in the articles Invasive weeds and wildfires and how Low flammability plants could help our homes survive bushfires.

Understanding fire behaviour helps scientists to give recommendations to firefighters and fire managers about how to fight or manage fire. This helps to increase fire safety of firefighters and the general public.

Project FuSE

Some of the research that Stuart Anderson and Grant Pearce do involves lighting fires in scrubland vegetation to improve understanding about fire behaviour in this environment. One research study was a 6-year project called Project FuSE (Fire in Scrub Experiments). This project took place at sites in the South Island high country. The benefit of research in the field is that it is done under real conditions with actual weather rather than in a laboratory or with a simulated computer model, which do not always represent real conditions.

Fire on slopes

The main thing the researchers were investigating in this project was the behaviour of fire on slopes.

The researchers already knew that fire could burn very fast up the slope of a hill. This is mostly due to radiation and convection (heat transfer). Hot air rises and radiates to heat the next section of the hill, causing the vegetation to heat up quickly, ready for combustion, before the flames have reached it. The steeper the hill, the more preheating and the more quickly the fire will spread.

Wind

During Project FuSE, the scientists were interested to learn that wind has a major effect on fire on slopes. Even a minor change in wind direction can cause the fire to spread across the slope rather than up it or to not spread at all.

The wind is a big factor in how scrub and other vegetation fires burn. Even on steep slopes, newly lit fires struggled to burn if the wind blew against the fire. Some of the initial fires blew out because the fire was pushed back to where the fuel had already been burned. However, when the wind was blowing up the slope, even if it was quite mild, the fire spread rapidly.

Moisture

The hill wasn’t the only factor to affect fire behaviour. The moisture content in the vegetation had a big effect too. Dead, dry scrub burned a lot more quickly than live plants. Researchers noticed that, even after one dry day, the scrub burned much more fiercely than it had the day before.

The moisture content of fuel is related to relative humidity, which describes the amount of moisture in the air. If there is a high relative humidity, the fire risk should be lower because even the dead fuels will take up moisture from the air. A low relative humidity means the air is getting dry. Fuels will lose moisture and dry out. The lower the relative humidity, the greater the fire risk.

The researchers found that changes to the environment (weather, terrain or fuel) could happen very quickly, resulting in dramatic escalations in fire behaviour.