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PwsFWI: Difference between revisions
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The output is then generated with the calculated list.
=== Interpretation of the output ===
The value which is used for the user to display - the Current value - can be set to be today or yesterday (default) according to the value of the inifile parameter ''CurrentIndexDay''. Yesterday is the default because that is the last complete day and thus (true) day. Today is still to be measured and therefore is a prediction.
For the output the chosen format is used with a warning level 5 or 6 depending on the choice of the user. 5 means a maximum warning level of '''Extreme''' fire danger and 6 means a maximum warning level of '''Catastrophic'''. '''Catastrophic''' has been added as a warning level in Australia some years ago. The warning levels have been calibrated (non-scientifically) during the first year and were correct according to other systems.
So what do the colours mean and how should they be interpreted? In the theoretical explanation is is said that:
#Wet wood does not burn easily (if at all);
#Moisture content of the fuel (wood) is of great importance;
#Wind does not spark fire but assists drying and is dangerous for propagation;
#Rain contradicts drying but not immediately;
#Drying timber in a forest is not a single day event.
This Fire Weather Index is all about the dryness of the timber and the meteorological environment. Other than the Canadian FWI there is no species component involved, nor is there a distinction for slopes and/or herbs. It is clear to everybody involved in wildfire that a dense forest with lots of undergrowth has a lot more wood to burn than a savanna with sparse trees. But that is not the point here. Management of the forests and wild lands is one thing. But the weather and the humidity condition of the vegetation is the direct environment where iginition and propagation takes place.
A lot has been said about the interaction between vegetation, weather and fire and I do not have the illusion to finish that discussion now.
This Fire Weather Index is about the risk of ignition and the ease of propagation. So I the colour/number output of the index says basically: high number (red and purple) : high risk, low number (green) : low risk.
#Green: Wood is wet to very humid and at the end of the scale the evaporation of the water content in timber is beginning. The wood is difficult to ignite.
#Blue: The evaporation continues but no real dry conditions exist. The wood is still difficult to ignite but thin twigs might be used to assemble and start a fire. Propagation is not fast is any.
#Yellow: The evaporation is now speeding up and with wind it catches up. After some days the wood is easier but still requires to be lit. A spark would not be enough to ignite, propagation would still be slow.
#Orange: The humidity of the environment becomes low, typically 40% or lower and the wood now really gets dry. Pools and soil dry out and undergrowth and litter become easy to ignite. With wind the drying process speeds up.
#Red: In the Red phase the relative humidity get to 30% or lower, the wood is dry and ignition becomes easy. Dead materials all have dried out and the stomata of the living plants have closed during the heat of the day. When this phase has a long duration sparks (electricity, lighting, stones hutting each other or human causes) may be a cause of ignition.
#Purple: During the catastrophic phase (Australia, but in general valid in semi-arid climates) the relative humidity can approach zero (it has been observed to be as low as 5 percent or less), there is hardly any moisture left in the vegetation and ignition is easy. Wind will definitely create dangerous spread and the situation may even create it's own [https://en.wikipedia.org/wiki/Flammagenitus_cloud weather systems] if enough fuel is at hand ([https://en.wikipedia.org/wiki/June_2017_Portugal_wildfires Portugal 2017] and [https://en.wikipedia.org/wiki/Camp_Fire_(2018) California Camp fire]).
With warning levels red and purple - and really starting with Orange - everybody has to take real care. And if you are in a forest, especially with species like Pinus and Eucalyptus, which contain flammable resins, you better check your escape routes.
== Example result from the Australian firy summer 2019/2020 ==
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