N A T URE’S What Is a Forest Fire What Is a Forest Fire The sight of a fire sweeping furiously through a forest is both frightening and spectacular Forest fires also called wildfires are devastating events Their intense heat burns leaves and branches to a crisp before making them explode roaring and crackling into flames Giant flames leap hundreds of feet into the air along with columns of thick black smoke In the biggest fires spinning tornadoes are created by the rapidly rising heat Flames raging through a coniferous forest in Yosemite National Park in California A fiery path Even the biggest forest fire starts as a tiny spark If the vegetation is dry enough the spark quickly grows into an inferno Flames spread in all directions burning everything in 4
N A T URE’S their path They damage and kill trees incinerate plants on the forest floor and even kill roots underground Buildings in the way are also burned Any unfortunate people or animals who become trapped between the fast-moving flames may be killed by smoke When the flames are gone a charred smoking wasteland is left behind Firefighters do not attempt to put out intense forest fires Instead they try to stop the fires from spreading and allow the flames to burn themselves out Where and why Forest fires happen in places where hot summer weather dries out dense vegetation They happen more often during droughts The dry vegetation catches fire very easily allowing the fire to spread rapidly Fire affects both coniferous forests and deciduous forests It can also burn across grasslands scrub and brush Most forest fires are started accidentally by people but some are started naturally by lightning A NATURAL CYCLE Lightning has been starting forest fires throughout history Layers of charcoal in rocks tell us that there were devastating fires millions of years ago Blackened tree rings in very old trees show that natural forest fires are regular events in some places Although forest fires can be very damaging they are not always bad news Fire clears out old trees and allows new ones to grow There is a natural cycle of fire and regrowth In fact some species of trees could not survive without regular fires Scientists cutting a slice from a giant sequoia tree The tree rings will tell them when forest fires happened in the past W h a t i s a F o r e s t F i r e 5
N A T URE’S What Is a Fire What Is a Fire Before we can investigate how forest fires burn and how firefighters combat them we need to understand what exactly fire is A fire is made up of glowing red-hot material and often flames These give off heat and normally produce smoke Fire happens when a material burns Burning is a chemical reaction between a material and oxygen which is one of the gases in the air Burning is also called combustion The fire triangle A fire must have three things to keep burning They are fuel heat and oxygen These three things make up the fire triangle A fuel is any material that burns not just the fuels we use for heating in cars and so on You can find out about the fuels in forest fires on pages 12 13 Fuel must be heated before it will burn and this heat must be provided for a fire to start Once a fire is burning however it produces its own heat and so it can keep burning Oxygen comes from the air so a fire needs a supply of air This diagram is known as the fire triangle The elements on all three sides must be there for the fire to continue burning OXYGE H E A T FUEL 6 Flames and smoke Plant matter such as wood and leaves is made up of several substances including cellulose and lignin When plant matter is heated these substances begin to break up Some flammable gases are given off These burn in the air giving off heat and light which we see as flames The material that remains is mostly carbon Carbon glows red as it burns Some tiny bits of unburned carbon are carried into the air These mix with the air to form smoke Water from the plant material boils in the heat making steam that rises with the smoke
N A T URE’S A close-up of the intense heat in a fire The tree bark is glowing as the carbon in it burns STOPPING FIRE If you remove one of the three sides of the fire triangle fuel heat or oxygen the fire cannot keep burning So if the fuel runs out the heat is taken away or the oxygen supply is cut off the fire goes out These are the basic ways of fighting fire Firefighters can stop a fire by taking away fuel cooling it with water or smothering it with foam W h a t i s a F i r e 7
