Severe Bushfire Attacks
SEVERE BUSHFIRE ATTACKS are unwanted or unexpected flames or ember fall on your property.
They usually occur on Total Fire Ban Days, but any hot dry day with a strong wind can cause them.
What are TOTAL FIRE BAN days?
In Australia, the Bureau of Meteorology (BOM) issues a Fire Danger Rating – see above table – based on forecast weather.
The BOM forecasts maximum temperature, minimum Relative Humidity and maximum wind speed and they then calculate Fire Danger Index (based on forest and grass fire danger indices), which allows them to calculate Fire Danger Rating.
Fire Agencies use the forecast Fire Danger Rating to issue fire warnings and to declare TOTAL FIRE BAN DAYS. Fire agencies tend to declare a TOTAL FIRE BAN when the Fire Danger Index estimate reaches 50.
The Fire Danger Index is a logarithmic scale, and was originally based on a range from 0 to 100.
Prior to 2009, the 50 – 100 range was called EXTREME. Since then, EXTREME has been subdivided into SEVERE, EXTREME and CATASTROPHIC.
Although the Fire Danger Index was originally designed to peak at 100, some weather conditions have exceeded 100. These forecast days will now be called CATASTROPHIC or CODE RED.
This means TOTAL FIRE BAN DAYS now include Severe, Extreme or Catastrophic levels.
What do these categories mean to the person on the property?
We must not forget that all three category days can generate SEVERE BUSHFIRE ATTACKS. Because the Fire Danger scale is logarithmic, the major weather differences between the three categories are minor.
If we take 40*C and 10% RH as a very hot dry summer day, wind speed fluctuations are the main cause of Fire Danger Index (FDI) changes. A wind speed of 20kph hits FDI 55, a 35 kph wind speed hits FDI 75 and a 50kph wind hits FDI 100. These wind speed fluctuations typically occur throughout a windy summer day.
Or again, if we take 42*C and 7% RH, a 40 kph wind will hit FDI 100.
What is the influence of SEVERE to CODE RED on the head fire?
Slight influence on flame height in a given fuel bed
Double wind speed tends to double fire speed
What is the influence of SEVERE to CODE RED on ember generation?
A long unburnt patch of forest will generate many more embers and tend to throw them a bit further.
A non-ember generating fuel bed? There will be no difference.
Are the fluctuations significant to the self defence architect ?
Even if ember fall rate doubles or quadruples, they fall on managed parts of the property and the outcome is already known.
If they fall on LOW FLAME zones, flame height will be as predicted
If they fall onto non flammable surfaces, they will all extinguish.
Therefore, no significant extra threat.