Geoscience Reference
In-Depth Information
possible, because they are aware of the fact that flames will spread as time goes on
at exponential speed, and as a result, it becomes more difficult to extinguish the fire.
Obviously, the methods used to fight forest fires should be called upon as soon as
possible after the alert of a fire burning has been given. The best way to achieve this
is to establish a pre-alert phase in which the emergency services are informed of
what may happen, and as a result they have time to prepare themselves and prepare
the equipment that will be used to help fight such fires. The use of such a policy is
not too expensive from both an economic and social viewpoint, if it is carried out
effectively as far as risk management is concerned and if the correct methods to
fight against the fire are used. A compromise needs to be reached so that the
resources used are only used in the area where the risk of the fire spreading is
extremely high so that resources are not wasted.
The different meteorological components that influence fires and their effects
have been described earlier in this section. However, the weather that occurs at a
place P and at a time T is the result of a combination of all of these different
meteorological components. Each of the individual components can change at any
given time and as a result be favorable or unfavorable to the development of a fire.
For example, these components could lead to the production of cold and dry
weather, or warm and humid weather, which is an example of two opposite weather
conditions. However, on certain occasions the different components could group
together and add their potential in a same direction. Whenever it rains during cold
and wet periods, and when there is no wind, forests are in no danger of bursting into
flames. On the opposite, periods of hot, dry, and windy weather in which there is
strong solar radiation are the most dangerous weather conditions for forests,
especially when plants have been dehydrated for several weeks without rainfall.
Certain types of atmospheric circulation favor the different combinations of
these dangerous weather factors. An example of such an atmospheric condition is
the Foehn wind, which is a hot, dry wind associated with an extremely clear and
transparent sky, meaning that there is a high level of direct solar radiation. This type
of wind tends to affect mountain slopes that are “down wind” (on the slope opposed
to the incoming wind) [CAR 02]. For example, mountain slopes that are leeward
may have a temperature range of 5-10°C higher than the temperature experienced on
the windward slopes. The relative humidity of the leeward slopes can fall to below
20%, and sometimes to even below 10%, although this is not very common. From
this information we can see that if this wind were to make contact with the ground,
all of the variables necessary to start a fire would be present. These variables would
also help a fire to spread rapidly. The majority of catastrophic fires that occur in the
French Mediterranean region are started in this way.
Considering the costs involved in trying to prevent fires from spreading, the
decision makers in charge of the operation find themselves faced with a daily
dilemma: although such a very dangerous event does not occur all the time, nor are
the risks associated with fires extremely low all the time. This is why, several
decades ago, the idea of allocating a single value to fire risk was adopted. This
