Geoscience Reference
In-Depth Information
estimate the amount of water vapor produced by such a system in a period less than
a day, except in a laboratory.
ETP is a concept used to express the efficiency of a particular area for
evaporating water. It is also used to indicate the exact amount of water a surface
would evaporate should the provision of water not be limited. It can be compared to
a type of virtual ETR in some ways. This potential of an area to evaporate water
does not depend on the amount of water available but only on the set of factors that
favor the evaporation process (temperature, radiation, humidity, and wind). The
value of ETP can never be lower than the value of ETR. A low ETR value is, either
due to an absence of water that is to be evaporated, irrespective of what the ETP
value might be even very high like in the Sahara, or due to a low ETP, like in
Spitzberg for instance.
The wind is the final major atmospheric variable that plays an important role in
the combustion process. The wind can be measured using various different scales:
meters per second; knots (kt), where 1,852 m/hour is the equivalent to 1 kt; in
kilometers per hour; or on the Beaufort scale. The Beaufort scale tends to be used to
measure wind speeds at sea. The wind tends to help fires more when the winds are
stronger, at a maximum of 70 km/hour, which is a little less than 40 kt or a little less
than 20 m/second. At speeds greater than this the wind forces the flame to lose a lot
of its vertical height. This theoretical limit, however, should be considered with
caution, because as soon as the relief of the landscape becomes rugged wind speeds
start to differ and start to interfere with the flow of air. Wind is the source of oxygen
for the fire, and also increases the chances of combustion taking place. This is
caused by two different factors: first, by the surface of the radiant band, and second,
by the constraints that the convection column is subjected to:
- the radiant band is the surface of the flame, which, when it is loses its vertical
height because of the wind, extends outwards and burns vegetation that had not
been burnt previously;
- the convection column, which is fueled by burning gases created by the
combustion processes, is also leveled by the wind, which in turn sends gusts of fire
towards the plants.
This information shows that the wind plays a major role in the development and
spreading of fires, as we had expected.
8.2.3.
Meteorological risk factors associated with forest fires
8.2.3.1.
The need for indexes
The risk linked to a forest fire spreading sometimes through densely populated
forest areas in the Mediterranean region of France has led to great efforts being
undertaken to prevent any development of a new fire igniting. As soon as a fire is
located, the aim of the emergency services is to put it out in the quickest time
