WILD FIRE

We in MetService pass our condolences to the people in Victoria for their recent wild fires. This web article is written as a tribute to remember this, the worst ever natural disaster in Australia. Click on this link to YOU TUBE for images and to make a donation or here for more images

For more information about this Satellite image from NASA click here.

In New Zealand, the wild fire season peaks in February and March, and, for regions such as Canterbury and Hawke’s Bay may extend from October to April.

When experts explain fire, they often talk about the "fire triangle" and its three equal sides, representing heat, fuel and oxygen. All three must interact for a fire to get started and to keep going. Take away any one of these ingredients and the fire is suppressed.

Fuel

Fuel is really just anything that is combustible. That could be vegetation, peat, coal and/or buildings. What's important to know here is that it's the moisture content of a fuel that determines how easily it will burn.

Living trees contain a lot of moisture. Dead trees have very little. In areas where the vegetation is dry, the temperature needed for ignition is lower than normal. If it’s a really hot day, the moisture content of fuel means very little.

Heat

Heat is responsible for igniting a fire and allows it to spread. The trigger may be a stray match, or a spark such as when a motor mower strikes a stone, or a flash of lightning.

Once a fire has started, heat emanates from the flames and warms the surrounding air and preheats fuel in its path, evaporating away the fuel’s moisture content and priming it ready to burn.

Experts in the National Rural Fire Authority NRFA describe the movement of heat using three different methods: conduction, convection and radiation.

Conduction is the transfer of heat through a solid, like the heat we feel when we touch the outside of a hot stove.

Convection is the transfer of heat by moving particles or liquids or gases, like the heat that flows from hot water to peas when we boil them or the heat that flows out of the kettle with steam.

Radiation is the transfer of heat by infrared electromagnetic radiance. That's the kind of heat we can feel without touching the source, like the heat from the sun or a campfire.

Radiation accounts for most of the preheating of fuels surrounding a fire. The temperature of these fuels can sometimes grow so high that they ignite even before they come into contact with flames. This is called spontaneous combustion.

All three of these methods combine in a flashover which is how a fire can suddenly spread through the air without flame. It happens when a fire releases a smoky layer of hot gas that radiates more and more heat to surrounding fuel. The flashover happens when the smoky gas reaches temperatures of about 600 Celsius. It's at this point that the surrounding fuel becomes so hot it appears to spontaneously burst into flame.

Air

For a fuel to burn, it needs to react with oxygen from the surrounding air so that it can release heat, which in turn helps the fire to spread. The air we breathe contains roughly 21 per cent oxygen. Most fires require only 16 per cent oxygen. Wildfires can replenish their supply if the oxygen level dips below 16 per cent. Hot flames heat the air, causing it to rise and thin out. Fresh air then rushes in to replace this partial vacuum, providing the fire with new oxygen. This is the way that heat creates wind that helps to fan the flames. Embers are blown away from the seat of the fire and help to spread the fire through the crown top of the trees.

The hot, dry gusty northwester winds that tumble down New Zealand’s main ranges onto eastern regions such as Canterbury and Hawke’s Bay exacerbate our fire risk and have been associated with our most damaging wild fires.

Wild fires create their own weather conditions that can lead to more fires. Smoke contributes to the clouds for thousands of miles downstream. Often the skies in New Zealand have a rosy or brownish glow a few days after wild fires in Australia.

Coal and Peat fires are raging around the world

In the former town of Centralia, Pennsylvania, USA, coal fires have been burning underground for more than 46 years. The blaze was sparked in 1962 on the outskirts of town when a trash fire in the pit of an abandoned strip mine ignited a coal vein running near the surface. Workers battled for decades to put out the fire without success. The town was abandoned in the mid-1980s after an engineering study suggested the fire could burn for a century or more. Today, it continues to release thick plumes of toxic smoke throughout the town. For a report on this fire on YOU TUBE click here

In the deserts of northern Mali in Africa, there have been areas of hot ground and smoking potholes for centuries. Click here for more info. This was thought to be volcanic activity, but now it is known to be due to a layer of peat that is burning less than a metre below. It may have originally ignited by heat created through bacterial activity in the same way that a compost pile can heat up from decay. The region is dry and the ground porous, which allows oxygen into the ground and keeps the fire smouldering.

During the strong El Nino of 1997-1998 Indonesia was coated with a choking haze caused by sustained peat fires on Sumatra and Borneo. Click here for more info .

Fire Weather in New Zealand

The National Rural Fire Authority (NRFA) use MetService computer models to help track the changing fire danger. When bush builds up or grows rapidly in the spring and then dries out in summer it becomes a fire risk. Hot days with low relative humidity exacerbate the fire risk, and strong gusty winds increase the rate of spread. To monitor the dryness of the bush NRFA daily compute a specialised Drought Code (DC) that gives a numerical rating of the average moisture content of deep, compact, organic layers. This code is also a very useful indicator for anyone interested in seasonal drought effects. It is based on data collected for the past 52 days and typically takes on values between 0 and 800. Numbers over 500 are associated with the bite of drought. To check a map of the latest DC readings click here and then click on the FUEL MOISTURE tab. These maps are loaded around 2pm daily so if none is available for TODAY then click on the YESTERDAY tab.

FEUL MOISTURE

The DC is just one of the factors taken into account when considering the Fire Weather Index, and when the DC is above 500 the Fire Danger Code is usually ranked as high. This NRFA site also shows maps of the other parameters of Fuel Moisture, and parameters of Fire Behaviour such as the Build Up Index and Initial Spread Index, and parameters of Fire Danger suchas the heat level needed to start a fire and the likely rate of spread. These parameters are combined to form the Fire Weather Index (see right hand map of the FIRE BEHAVIOUR tab) and this is then turned into a Fire Danger Class (see middle map of the FIRE DANGER tab), as shown on the coloured hemispheres signs seen around the country.

This web site also contains maps under the WEATHER tab of maximum temperatures, Relative Humidity, Wind and Rain as measured daily by NRFA sites.

Note the change in Relative humidity between Sunday 8 February (our most likely candidate for hottest day of the year) and Tuesday 10 February (our most likely candidate for warmest night of the year):

Above: Sunday 8 February 2009, likely candidate for hottest day of the year, and very dry air with low relative humidity.

Above: Tuesday 10 February 2009: High relative Humidity from the tropics

Here are the MetService tips to beat the heat:

Heat stress is tiring. To keep cool, follow these rules.

1. Drink lots of water - more than you think you need.

• Avoid drinking alcohol, coffee or cola. Avoid heavy meals.

2. Dress Right. Wear light and loose fitting clothing.

3. Fan yourself or get into a breeze.

• Skin temperature is 32 degrees C, so once the air gets over 28 degrees C this has less impact.

4. Slow down and seek the shade. Shun the sun and close the curtains.

5. Take a trip to air conditioned places. Shopping malls, theatres or community centres.

6. Take a dip or a cool shower. Wear something cool and wet.

• Help cool and settle anyone you see with signs of heat stroke such as headache, dizziness, confusion, nausea or rapid or difficult breathing.

MetService Weather Ambassador, February 2009