Agriculture Reference
In-Depth Information
Box 1.3 (
cont.
)
Fig. B1.3.2
Tree damage resulting from a mistral wind event following wildfire in a
Pinus
halapensis
forest in southern France
.
(Photo by J.E. Keeley.)
Santa Ana winds are the best studied of the foehn winds and they are similar
in some but not all respects to foehn wind events in other MTC regions. These
winds peak in frequency in the autumn and spring. It is the former wind events
that produce extremely dangerous fire weather because they follow on the heels
of a 6-month or more annual drought. When anomalous winter drought condi-
tions are followed by spring Santa Ana wind events this may lead to significant
out-of-season fires (Keeley
et al
.
2009b
). Regardless of season these winds
typically have less than 10% relative humidity and produce gusts that exceed
100 km per hour (Fosberg
et al
.
1966
; Ryan
1969
). Although referred to as
“desert winds,” the high temperatures and low humidity are the result of com-
pression as air descends to form the basin air mass (Mitchell
1969
) and, on a
local scale, as it descends through coastal passes (Krick
1933
).
Santa Ana and other foehn wind events typically last for several days. Multiple
such events occur annually and there are usually 40-50 days per year with these
wind events, although the frequency of southern European mistral winds
(
Fig. B1.3.2
) may be double that number (Weber & Kaufmann
1998
). Although
these wind events are described as lasting for a period of days, they actually wax
and wane during this period. Santa Ana winds develop overnight and peak in the
early morning (Edinger
et al
.
1964
), a pattern also evident with the European
mistral winds (Weber & Kaufmann
1998
). With daytime convectional heating
these winds are often held aloft and onshore flow may predominate during
the afternoon, which can change fire spread direction and greatly complicate
firefighting activities (Keeley & Zedler
2009
). Santa Ana wind events include
Continued
