Nature and Causes of Wildfires (AQA A Level Geography)

Distributions of Wildfires

• Wildfires are uncontrolled fires that occur in natural areas such as forests and grasslands

• There are three types of wildfire:

  • Ground fires burn beneath the ground surface with tree roots and peat layers burning

    • They are smouldering, slow-moving fires that can burn for days and weeks with no visible flame and little smoke

  • Surface fires burn through low-lying vegetation such as grass, bushes and small trees

    • They are the most common and can move quickly

  • Crown fires burn through the canopy layer which is the top layer of vegetation

    • They are usually the most dangerous and destructive type of wildfire because they move quickly and often produce intense heat and flames

• Wildfires occur all over the world (except for Antarctica)

• Wildfires are most common in the tropics and subtropics

• Countries that experience the highest number of wildfires include Brazil, Indonesia and Australia

• They are also common in large, forested areas of dense vegetation

  • e.g. Russia, Canada and USA all experience frequent wildfires

Conditions for Wildfires

Vegetation type

• Denser vegetation tends to lead to more intense fires that can spread quickly and burn for longer

  • e.g. forest fires tend to be more dangerous and intense than grassland fires

• Some types of vegetation are more flammable than others

  • e.g. eucalyptus contains highly flammable oils that burn very easily

Fuel characteristics

• The amount of available fuel, such as trees, grass and dead plant materials affects the likelihood and severity of fires

• Fine, dry material such as grasses and twigs can catch fire and burn very easily

Climate and recent weather

• Dry weather and droughts dry out vegetation and increase its flammability

• High temperatures and heatwaves increase rates of evaporation – this dries out vegetation even more and makes it more susceptible to ignition

• Strong winds can fan the flames of a fire and cause it to spread more quickly

  • Wind can also carry embers from a fire and start new fires elsewhere

• Lightning storms are a common cause of wildfire ignition

• Climate change is increasing the frequency and severity of wildfires in many regions around the world

Fire behaviour

• Fire behaviour refers to the physical characteristics of a fire such as its intensity, rate of spread and flame length

• More intense fires with higher flames can ignite trees and other vegetation causing them to spread more quickly and burn more intensely

• Fires tend to spread more quickly uphill due to increased heat and wind

  • The Carr Fire in California 2018 spread rapidly uphill due to strong winds and dry vegetation

• Fires that have multiple fronts can change direction quickly and move unpredictably

Causes of Wildfires

• Wildfires are caused by both natural and human factors, though most are caused by humans

• Natural causes include lightning strikes, volcanic eruptions and even the heat from the sun

• Human causes include campfires and barbecues getting out of control, cigarettes being discarded when they are still lit, and sparks from machinery or power lines

  • Sometimes people start fires on purpose - this is known as arson