Lesson 1: What are the causes of wildfires?

Learning objectives:

To be able to define the term 'wildfire'.

To be able to describe the location of areas in which wildfires are common and explain why they happen here.

To be able to give and explain at least 5 causes of wildfires.

Wildfires are unplanned, unwanted wild land fires, including unauthorised human-induced fires. They are sometimes known as bushfires in Australia and brushfires in the USA.

Wildfires are a common occurence in some parts of the world. They occur on every continent except Antarctica. They happen most frequently in hot areas where there are extended periods of drought. Fires need heat, fuel (eg. dried out vegetation) and oxygen to continue to burn.  High temperatures can preheat fuels in the fire's path so that they burn more readily. Strong winds can fan the flames, pushing them towards new fuel sources as well as transferring burning embers and sparks which can start spot-fires. During the day, the sunlight heats the ground and warm air rises. When hot air currents travel up sloped landscapes they can cause fires to start.

Forest fuels consist of ground, surface and aerial materials.

Ground fuels lie below the earth's surface eg. tree roots.

Surface fuels include twigs, grasses, wood and needles.

Aerial fuels include tree crowns, branches and hanging mosses.

The map below shows the global distribution of wildfires in August and February 2008. The maps show different patterns for the two times of year. In AUgust, the fires were concentrated in two areas - just north of the Black Sea in Romania and the Ukraine and in a band across the African continent at about 15-20 degrees south of the equator.  The map for February shows a concentration in Venezuala (in the north of South America) and in a band across the African continent at about 10-15 degrees north of the equator.

Why do wildfires happen?

The causes of wildfires can be divided into 2 categories - natural and human. WIldfires are not like the other hazards that we have studied because, unlike earthquakes/volcanoes/storms, people can cause wildfires. Experts have suggested that 90% of wildfires are actually caused by people.

Natural causes of wildfires

Lightning is the single biggest natural cause of wildfires. Most fires started by lightning are small and burn out quickly but if the conditions are right then fires started by lightning can spread very rapidly. About 8 million lightning strikes hit the earth every day!

Spontaneous heating is where material becomes heated to the point at which it catches fire without a spark. This is common wherelots of leaves and branches have fallen to the ground and not been cleared away - the flow of air is restricted and often leads to fire.

Volcanic eruptions give out red hot lava and ash whch can start wildfires.

Wildfires are more common on south-facing slopes because the sun dries out the vegetation. Steep slopes also help fires to spread as the flames hit the upslope vegetation.

Human causes of wildfires

There are many human causes of wildfires including...

Arson - deliberate fire-starting. This is illegal.

Sparks for train wheels or from machinery

Military training

Household chimneys

BBQs and camp fires which haven't been extinguished properly

Broken bottles acting as a magnifying glass and concentrating the sun's rays

Slash and burn farming techniques (particualrly common in Brazil)

Discarded cigarettes

Children playing with matches

Electricity pylons falling down in high winds

Useful links:

Interactive about the sources of fire

US active fires - updapted in real time

How Stuff Works - Causes of wildfires