|
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.
# <span style="color:white;background:green;">Green: </span> 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.
# <span style="background:blue;color: white;">Blue: </span> 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.
# <span style="background:yellow;">Yellow:</span> 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.
# <span style="background:orange;">Orange:</span> 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.
# <span style="background:red;">Red: </span> 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.
# <span style="color:white;background:purple;">Purple:</span> 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.
| 