Trends in adverse weather patterns and large wildland fires in Aragón (NE Spain) from 1978 to 2010

Abstract

This work analyzes the effects of high temperature days on large
wildland fires during 1978-2010 in Aragón (NE Spain). A high
temperature day was established when air temperature was higher than 20
°C at 850 hPa. Temperature at 850 hPa was chosen because it properly
characterizes the low troposphere state, and some of the problems that
affect surface reanalysis do not occur. High temperature days were
analyzed from April to October in the study period, and the number of
these extreme days increased significantly. This temporal trend implied
more frequent adverse weather conditions in later years that could
facilitate extreme fire behavior. The effects of those high temperatures
days in large wildland fire patterns have been increasingly important in
the last years of the series.

Full text

Nat. Hazards Earth Syst. Sci., 13, 1393–1399, 2013
www.nat-hazards-earth-syst-sci.net/13/1393/2013/
doi:10.5194/nhess-13-1393-2013
© Author(s) 2013. CC Attribution 3.0 License.
EGU Journal Logos (RGB)
Advances in
Geosciences
Open Access
Natural Hazards
and Earth System
Sciences
Open Access
Annales
Geophysicae
Open Access
Nonlinear Processes
in Geophysics
Open Access
Atmospheric
Chemistry
and Physics
Open Access
Atmospheric
Chemistry
and Physics
Open Access
Discussions
Atmospheric
Measurement
Techniques
Open Access
Atmospheric
Measurement
Techniques
Open Access
Discussions
Biogeosciences
Open Access
Open Access
Biogeosciences
Discussions
Climate
of the Past
Open Access
Open Access
Climate
of the Past
Discussions
Earth System
Dynamics
Open Access
Open Access
Earth System
Dynamics
Discussions
Geoscientic
Instrumentation
Methods and
Data Systems
Open Access
Geoscientic
Instrumentation
Methods and
Data Systems
Open Access
Discussions
Geoscientic
Model Development
Open Access
Open Access
Geoscientic
Model Development
Discussions
Hydrology and
Earth System
Sciences
Open Access
Hydrology and
Earth System
Sciences
Open Access
Discussions
Ocean Science
Open Access
Open Access
Ocean Science
Discussions
Solid Earth
Open Access
Open Access
Solid Earth
Discussions
The Cryosphere
Open Access
Open Access
The Cryosphere
Discussions
Natural Hazards
and Earth System
Sciences
Open Access
Discussions
Trends in adverse weather patterns and large wildland fires in
Arag´
on (NE Spain) from 1978 to 2010
A. Cardil1, D. M. Molina1, J. Ramirez2, and C. Vega-Garc´
ıa1
1School of Agrifood and Forestry Science and Engineering, University of Lleida, Avenida Rovira Roure 191,
25198 Lleida, Spain
2Department of Agricultural Sciences and Technology, University of Leon, Avenida de Portugal 41, 24071 Le´
on, Spain
Correspondence to: A. Cardil (adriancardil@gmail.com)
Received: 1 December 2012 – Published in Nat. Hazards Earth Syst. Sci. Discuss.: –
Revised: 23 April 2013 – Accepted: 27 April 2013 – Published: 31 May 2013
Abstract. This work analyzes the effects of high temperature
days on large wildland fires during 1978–2010 in Arag´
on
(NE Spain). A high temperature day was established when
air temperature was higher than 20◦C at 850hPa. Tempera-
ture at 850hPa was chosen because it properly characterizes
the low troposphere state, and some of the problems that af-
fect surface reanalysis do not occur. High temperature days
were analyzed from April to October in the study period,
and the number of these extreme days increased significantly.
This temporal trend implied more frequent adverse weather
conditions in later years that could facilitate extreme fire be-
havior. The effects of those high temperatures days in large
wildland fire patterns have been increasingly important in the
last years of the series.
1 Introduction
Mediterranean countries like Spain have numerous wildland
fires each year (Pereira et al., 2011). Fire has always been
part of the traditional Mediterranean agrarian land manage-
ment, occasionally developing into unwanted fires (Mill´
an
et al., 1998). However, over the past 30 yr wildland fires
have became more extreme, with fire behavior more and
more often exceeding firefighting capabilities (Miralles et al.,
2010; Molina et al., 2010), and fire agencies experience dif-
ficulties in suppressing extreme-behavior fires while provid-
ing safety for both firefighters and citizens, as reviewed in
Werth et al. (2011). The social and physical/biological envi-
ronment has changed dramatically, and wildfires constitute
nowadays one of the problems that consistently obtain more
attention from the media in summer. Agricultural abandon-
ment is the main cause of an increased fuel load (Mill´
an et
al., 1998), but wildland fuel homogeneity and continuity are
also major facilitators of both a fast fire propagation and a
higher fire line intensity (Molina et al., 2010; Vega-Garc´
ıa
and Chuvieco, 2006). In addition, climate and weather are
two of the main factors influencing fire regime (Trouet et al.,
2009), and climate change could have an important impact
on ecosystems due to increases in area burned and fire inten-
sity/severity (Flannigan et al., 2000). Regato (2008) showed
that climate change could provide an increase in the inten-
sity and frequency of summer heat waves (short periods with
very hot days, very low air humidity and frequently with
strong winds) that increase the probability of large wildland
fire (LWF).
Typically, just a few LWFs cause the majority of the dam-
age (Alvarado et al., 1998; Ganteaume and Jappiot, 2012)
because they account for a very high percentage of the total
burned area (Stocks et al., 2003), and their severity is nor-
mally higher. In these LWF events fire behavior is often ex-
treme, making suppression difficult. Therefore, LWFs affect
our ecosystems, human safety and properties to the utmost
(Alvarado et al., 1998) and also demand vast resources to
suppress them.
It is essential to know what factors influence LWFs. We
have focused on days with high temperatures (HTDs) to as-
sess their potential impact on the development of LWFs.
Previous works indicate that HTDs might provide more ex-
treme weather conditions (Montserrat, 1998), which have
an important role in forest fire behavior (Crimmins, 2006).
Hot days decrease fuel moisture and increase the ignition
Published by Copernicus Publications on behalf of the European Geosciences Union.

1394 A. Cardil et al.: Trends in adverse weather patterns and large wildland fires
Fig. 1. Geographic location of Arag´
on and air temperature at 850 hPa (legend in ◦C) for 2 July 1994. Source: www.wetterzentrale.de.
probability and, as a result of that influence, other aspects
such as longer flame length, most likely involving crown fire
activity and spotting activity (long distance ignition by con-
vection processes). Therefore, HTD has the potential to in-
crease the probability of having a LWF. In a similar way,
Mills (2005) indicates that unusually strong temperature gra-
dients at 850hPa (which usually stands for a level around
1500 m up in the atmosphere) may have the potential to iden-
tify unusually severe fire weather events. It would be ex-
tremely profitable to be able to discriminate between the sim-
ply “bad” and the “disastrous” fire days with some reason-
able lead time (i.e., 24 or 48h).
The five largest LWFs on record in Arag´
on did develop
under HTDs. In addition, those HTDs were extreme. The
largest fire in Arag´
on affected 16832ha in Villarluengo
(Teruel) on 2 July 1994, and air temperature at 850hPa and
at 00:00UTC in the Arag´
on region was higher than 22.5◦C
(Fig. 1), during the day of the fire and also on the two pre-
vious days. In this case, the synoptic weather pattern that
caused the HTD was a hot air mass inlet (south advection
from the Sahara desert).
The exploration for underlying causes and visible patterns
of LWFs is instrumental to plan best strategies for our sup-
pression resources and to foresee extreme fire behavior. In
this study, we have analyzed HTDs in the Spanish region
of Arag´
on and their relationship to the LWF official records
both in terms of amount and cumulative area burned and
number of LWFs in HTDs versus non-HTDs.
2 Methods
2.1 Study area
Arag´
on is the fourth largest region in Spain (47719 km2)and
is located in the northeastern part of the country (Fig. 1). The
region has 1.34 million inhabitants, comprises the provinces
of Huesca, Teruel and Zaragoza, and it is politically divided
in 33 counties. Arag´
on has a high altitudinal gradient that
generates several ecosystems in the region. There is a ma-
jor river (Ebro) bordered by two mountain chains: the Pyre-
nees (maximum altitude 3404 m, Aneto) and the Iberian Sys-
tem (maximum altitude 2314m, Moncayo). The climate in
Arag´
on can be generally regarded as a Mediterranean cli-
mate with continental nuances, but the irregular topography
influences it and generates local climate variability. The envi-
ronment varies from the high mountains of the north-central
Pyrenees, with perpetual ice (glaciers) to the steppe or semi-
desert areas, such as Monegros, and intense continental cli-
mate in other areas. The mean annual temperature ranges
from 22.5◦C in the Ebro valley to 5◦C in the highest ar-
eas of Pyrenees, and the average annual rainfalls also ranges
from 1800 mm in the highest mountains to 300 mm in the val-
ley (AEMET, 2012). The vegetation is conditioned by relief
and climate. In upland forests there are several tree species
(pine, fir, beech, oak), shrubs and meadows. In the Ebro val-
ley, oak and juniper trees are the most common and there
are degraded areas covered by shrubs and grasslands. Arag´
on
has high ecological value with several protected wilderness
areas.
Nat. Hazards Earth Syst. Sci., 13, 1393–1399, 2013 www.nat-hazards-earth-syst-sci.net/13/1393/2013/

A. Cardil et al.: Trends in adverse weather patterns and large wildland fires 1395
2.2 High temperature days in Arag´
on
In order to characterize the high temperature days, reanaly-
sis data from the National Centers for Environmental Predic-
tion (NCEP) were used (Kalnay et al., 1996). We analyzed
daily air temperature maps (850hPa at 00:00UTC) to assess
whether there was a HTD condition in the territory. Air tem-
perature at 850hPa is the air temperature at an altitude in
the atmosphere where pressure is 850 hPa (around 1500m up
in the atmosphere). The 850hPa air temperature daily maps
were available at Wetterzentrale (2013). We established that
there was a HTD when air temperature at 850hPa was equal
to or higher than 20 ◦C in at least two-thirds of the Arag´
on re-
gion. We chose the temperature at 850 hPa because it is gen-
erally used to analyze past fire weather and fire weather fore-
casts (Mill´
an et al., 1998; Garcia-Ortega et al., 2011; Trigo
et al., 2006). It provides a regional coverage as well because
it is sufficiently close to the surface to be representative of
the low troposphere state, and it avoids some of the prob-
lems that affect near-surface reanalysis variables (Trigo et
al., 2005; Ogi et al., 2005). An air temperature at 850hPa
equal to or higher than 20◦C is associated with heat waves,
and this condition provides high temperatures in surface and
low relative humidity in the territory (Montserrat, 1998).
Weather conditions were characterized every day from
1978 to 2010 in the fire season from April to October (in-
cluded). We analyzed the number of HTDs and the duration
and frequency of the high temperature (HT) phenomena as
a proxy for potential fire behavior. We defined “HT periods”
as the number of uninterrupted times that a HTD occurred.
2.3 Large wildland fires
Large wildland fires (LWFs) are defined in this work as
those over 100ha threshold (Moreno et al., 2011; De Zea
Bermudez et al., 2009). In order to understand the interac-
tions between HTD and LWF in the study period (1978–
2010) in Arag´
on, we processed the historical fire data records
from Spain’s EGIF database (General Statistics on Wild-
land Fires; see www.magrama.gob.es, accessed last time on
30 October 2012), which includes the wildland fire reports
sent to the Ministry of the Environment by the firefighting
and forest management services of all the Spanish regions.
This database has an entry from each fire, regardless of size,
and contains the same fields of information for each fire.
The first years of the database (1968–1977) were not used
in this study because the area burned on private properties
were usually underreported in those years because the For-
est Service mandate was to suppress only on state-owned or
state-controlled forest but not privately owned lands (Anto-
nio Mu˜
noz, Forest Service, personal communication). Many
fires smaller than 100ha in the database most likely burned
a larger area (maybe more than 100ha) because foresters did
not account for the area of burnt private land. Therefore, there
are missing 100ha+fires prior to 1977. We have analyzed
trends in the number of LWFs, LWF area burned and average
LWF size under both HTDs and non-HTDs.
In the study period, there were 193 wildland fires in
Arag´
on larger than 100ha that burned 132000ha approx-
imately. All of them affected forest and agricultural areas,
roads and people. For instance, the four forest fires that
occurred on 22 July 2009 in Teruel (the Aliaga, Alloza,
Cedrillas and Corbal´
an fires) burned about 10000ha in to-
tal.
2.4 Statistical analysis
The relationship between HTDs and LWFs was assessed in
the period up to two days immediately before LWF occur-
rence date, and analyzed according to the following four
HTD classes:
–Class A: LWFs that start on a HTD (day 0), HTD (day 1)
and HTD (day 2), therefore, LWFs under a very strong
HT period.
–Class B: LWFs that start on a HTD (day 0), HTD (day 1)
and non-HTD (day 2), therefore, LWFs under a strong
HT period.
–Class C: LWFs that start on a HTD (day 0), non-HTD
(day 1) and HTD or non-HTD (day 2), therefore, LWFs
under a weak HT period.
–Class D: LWFs that start on a non-HTD (day 0), HTD or
non-HTD (day 1) and HTD or non-HTD (day 2). There-
fore, they were fires with minor influence of HT condi-
tions.
Only two days before all LWFs have been used in this
analysis. Several days before each LWF were analyzed
(5 days), but they did not influence the results, and previous
days (day 3, day 4 and day 5) did not supply more informa-
tion than HT classes used (above). We evaluated the influ-
ence of HTD on LWF on three consecutive days by using
an ANOVA analysis and group comparison with the Fisher
method with a 95% confidence interval.
We also quantified how many LWFs were conditioned by
HTDs (only on the day that the fires started), and we sum-
marized statistics in the studied period. We established the
number of LWFs, burned area swept by them, the average
size and percentage of LWFs under HTDs and non-HTDs.
We determined if there were significant changes (decrease,
increase, no difference) in the studied variables (number of
LWFs, area burned by LWFs and number of HTDs) from
1978 to 2010 with a linear regression analysis on annual
raw data. The annual variability in fire occurrences is high,
both in terms of large fire frequencies and their burned ar-
eas (Stocks et al., 2003). This variability is caused by diverse
environmental factors, such as human influence (Mollicone
et al., 2006) and climate (Gillett et al., 2004). For this rea-
son, we added the evolution in time of the variables with the
www.nat-hazards-earth-syst-sci.net/13/1393/2013/ Nat. Hazards Earth Syst. Sci., 13, 1393–1399, 2013

1396 A. Cardil et al.: Trends in adverse weather patterns and large wildland fires
18
1
Figure 2. Annual number of high temperature days (HTD) (light grey line) in Aragón from2
1978 to 2010 and moving seven-year average (black line) from 1981 to 2007.3
4
0
5
10
15
20
25
30
1978
1983
1988
1993
1998
2003
2008
Number of HTD
Annual number of HTD
Moving seven-year average
Fig. 2. Annual number of high temperature days (HTDs) (light grey
line) in Arag´
on from 1978 to 2010 and moving seven-year average
(black line) from 1981 to 2007.
moving average method in order to obtain a better display
in the figures. This smoothing technique was applied to mit-
igate the effect due to year to year random variation. This
practice, when properly applied, reveals more clearly the un-
derlying trend (Legendre and Legendre, 1998). “The method
calculates successive arithmetic averages over 2m+1 con-
tiguous data as one moves along the data series” (Legendre
and Legendre, 1998). In this study, we used simple moving
average with seven-year periods(m=3).
3 Results
3.1 HTD trends
The annual number of HTDs increased in the study period
significantly (pvalue=0.020). It rose from 8 HTDs in 1981
to 15 in 2006 in terms of seven-year average values, as
shown in Fig. 2. The number of HT periods also increased
(pvalue=0.022). Therefore, in recent years, we have more
periods influenced by HTD phenomena. However, the aver-
age duration of HT periods did not change in the study period
with an average duration of 2.2 days.
The majority of HTD events took place in mid-summer
(July and August) with more than 80% of the total. June has
11.5 % of days and September 5.8%. In April, there were no
HTDs; in October, there was only one HTD in the studied pe-
riod. June had a significant increase in the number of HTDs
from 0.7 days in 1981 to 2.7 in 2006 in terms of seven-year
average. In July, August and September, no significant trends
were observed.
3.2 Large wildland fires
A decrease in the total annual number of LWFs was ob-
served in Arag´
on during the study period (pvalue=0.003).
It diminished from 12 LWFs in 1981 to 3 LWFs in 2007
in terms of seven-year average values. The annual num-
ber of LWFs under non-HTDs also decreased significantly
Table 1. Trends in annual number of large wildland fires (LWFs),
annual area burned, and annual number of high temperature days
(HTDs) in Arag´
on from 1978 to 2010.
Variable Total HTD Non-HTD
Number of LWFs – (0.003) n.s (0.411) – (<0.001)
Area burned n.s (0.968) n.s (0.590) – (0.014)
HTD +(<0.020)
+significantly increased; – significantly decreased at P <0.05; n.s. not significant.
Values in parentheses are the Pstatistic.
(pvalue<0.001). It diminished from 8 LWFs in 1981 to
2 LWFs in 2007 in terms of seven-year average values. By
contrast, the annual number of LWFs under HTDs did not
decrease in the study period. Neither total annual area burned
nor annual area burned under HTDs changed in the study pe-
riod. Nevertheless, a significant decrease was found in the an-
nual area burned under non-HTDs. It decreased from 2204 ha
in 1981 to 780 ha in 2007 in terms of seven-year average val-
ues.
HTDs also influence the average LWF size, and HTD
classes explain the variable average size of the LWFs (p=
0.003). Table 1 lists the number of LWFs, area burned and
average size of LWFs in each HTD class (1978–2010). The
HTD class comparison analysis shows that there was a sig-
nificant difference between both A and B classes and D class.
Average LWF size in D class was a third of those of both A
and B classes (Table 2). No significant difference between
class C and other classes can be established.
We split the study period in two intervals (1978–1993
and 1994–2010) because in 1994 there were LWFs with an
extreme behavior under very strong HTD conditions. The
average LWF size increased significantly between 1978–
1993 and 1994–2010 periods (424 ha vs. 1275ha) (pvalue=
0.001). Additionally, the average LWF size under HTDs is
significantly larger in the 1994–2010 period (1923 ha) than in
the 1978–1993 period (590ha) (pvalue=0.024). However,
the average LWF size under non-HTDs did not change be-
tween two periods (389 ha). In the first interval (1978–1993),
the majority of LWFs were under D class with 5.59 LWFs
and 1987ha burned per year. In the second interval (1994–
2010), the results changed significantly and the annual num-
ber of LWFs was 1.69 and the annual area burned was 858 ha
(Table 2). By contrast, in HTD classes (A, B and C) neither
annual number of LWFs nor annual area burned decreased
between the two time intervals, and the annual area burned
was higher in 1994–2010 interval in both A and B classes
while the percentage of LWF number under HTD versus to-
tal LWF number was 54.2 % and the area burned was 81.7 %.
Additionally, in the 1994–2010 period, most of the surface
(76 %) was burned by LWFs under A and B classes (in which
HTD conditions were strong or very strong). The HTD in-
fluence in LWF increased in Arag´
on in the study period as
shown in Fig. 4 with two ratios that indicate that most LWFs
Nat. Hazards Earth Syst. Sci., 13, 1393–1399, 2013 www.nat-hazards-earth-syst-sci.net/13/1393/2013/

A. Cardil et al.: Trends in adverse weather patterns and large wildland fires 1397
Table 2. Number of large wildland fires (LWFs), area burned and average size in high temperature (HT) classes in Arag´
on from 1978 to
2010.
Classes Number Area Average Annual number Annual number Annual area Annual area
of firesaburnedasizebof firescof firescburnedc(ha) burnedc(ha)
(ha) (ha) 1978–1993 1994–2010 1978–1993 1994–2010
A 25 35311 1412±699 0.82±0.29 0.69±0.25 462±122 1716±1087
B 26 34388 1323±419 0.53±0.22 1.06±0.24 291±182 1840±919
C 20 14 913 746 ±423 0.94 ±0.47 0.25 ±0.06 604 ±390 291 ±66
D 122 47502 389±49 5.59±1.21 1.69±0.39 1987±437 858±251
Total 193 132114 685±115 7.88±1.67 3.69 ±1.04 3344 ±756 4704 ±2156
aAbsolute values in the study period. bMean and standard error (σ/√n) values over the period. cAnnual mean and standard error (σ/√n) values.
19
1
Figure 3. Number of large wildland fires in Aragón under high temperature days (HTD), non2
high temperature days (non-HTD) and total number of large wildland fires (LWF) in Aragón3
from 1978 to 2010. Moving seven-year average from 1981 to 2007. Vertical lines are the4
annual standard error values.5
6
0
2
4
6
8
10
12
14
16
18
1981
1986
1991
1996
2001
2006
Number fo fires
Moving seven year average
HTD
non-HTD
Total
Fig. 3. Number of large wildland fires in Arag´
on under high tem-
perature days (HTDs), non-high-temperature days (non-HTDs) and
total number of large wildland fires (LWFs) in Arag´
on from 1978 to
2010. Moving seven-year average from 1981 to 2007. Vertical lines
are the annual standard error values.
took place nowadays under HTDs and that was not the case
in the 1980s and 1990s. The ratio of LWF number under
HTDs versus total number of LWFs increased in the study
period (Fig. 3) from 0.28 in 1981 to 0.65 in 2007. The ra-
tio of LWF area burned under HTDs versus total area burned
also increased from 0.45 in 1981 to 0.82 in 2007 (Fig. 4). The
values of these ratios are in seven-year average.
In terms of area burned by LWFs, the worst years of the
series were 1994 and 2009 with 32600 ha and 21 925 ha re-
spectively. More than 90 % of the total burned area (in these
two years) was burned under HTD conditions. These years
also have a very high annual number of HTDs: 27 HTDs in
1994 and 23 in 2009. Moreover, the largest fires in Arag´
on
did spread in this HTD conditions.
4 Discussion
While it is recognized that the major elements for fire
weather forecasts are low humidity, high temperatures,
and strong winds near the ground, meteorological indexes
planned to evaluate temporal and spatial dissimilarities in
20
1
Figure 4. Ratio of LWF area burned under HTD versus total area burned and ratio of LWF2
number under HTD versus total number of LWF in Aragón from 1978 to 2010. Moving3
seven-year average from 1981 to 2007.4
5
0.0
0.2
0.4
0.6
0.8
1.0
1981
1986
1991
1996
2001
2006
Ratio
Moving seven year average
Ratio of LWF number
Ratio of LWF area burned
Fig. 4. Ratio of LWF area burned under HTDs versus total area
burned and ratio of LWF number under HTDs versus total number
of LWFs in Arag´
on from 1978 to 2010. Moving seven-year average
from 1981 to 2007.
those elements are not frequently used or available by all
fire weather forecast agencies (Charney and Keyser, 2010;
Crimmins, 2006). For that very reason, we highlight the im-
portance of discerning between HTD and non-HTD defined
by 850hPa synoptic conditions in planning pre-suppression
efforts to stand up to large fires.
An increase was found in the number of HTDs in the study
period, and this agrees with Rodriguez-Puebla et al. (2010).
The main source of this increase might be related to the
weather regime that brings hot dry air masses from the
north of Africa (Rodriguez-Puebla et al., 2010). Different au-
thors suggested that this increase might be linked to an in-
crease of temperature in northeastern Spain due to climate
change (Moreno, 2005; Giannakopoulos et al., 2009; Ket-
tunen et al., 2007). Giannakopoulos et al. (2009) suggested
that the number of hot days (Tmax>30 ◦C) and heat wave
days (Tmax>35 ◦C) will increase in Spain. Giannakopoulos
et al. (2009) estimate that there will be 1 to 3 additional hot
weeks per year. The mean annual temperature will increase,
with greatest warm-up rate in southern Europe (Moreno,
2005; Castro et al., 2005; Kettunen et al., 2007). Van Wagner
and Pickett (1985) remarked that the fire weather index will
increase in summer (i.e., increasing fire risk). Therefore, if
www.nat-hazards-earth-syst-sci.net/13/1393/2013/ Nat. Hazards Earth Syst. Sci., 13, 1393–1399, 2013

1398 A. Cardil et al.: Trends in adverse weather patterns and large wildland fires
HTDs become more frequent and these conditions are able
to decrease air humidity and fuel moisture and increase the
fire behavior potential, we may be facing larger wildland fires
in the future, and very likely extreme-behavior fires beyond
suppression capacity (Molina et al., 2010). We also found
that in the last years of the series there were more days under
HT conditions in June. This may be translated as an increase
in fire season length.
Both the total annual number of LWFs and the annual
number of LWFs under non-HTDsdecreased in Arag´
on from
1978 to 2010. Nevertheless, the annual number of LWFs un-
der HTDs did not decrease in the same period. The total an-
nual area burned did not decrease due to the area burned by
LWFs under HTDs. However, a decrease in the annual area
burned under non-HTD conditions was observed. Addition-
ally the percentage of both LWF occurrence and area swept
by LWFs under HTDs also increased. Three main reasons
could explain this. First, the number of HTDs was greater
at the end of the time series, and it is more likely to have a
LWF under HTDs. Second, in the last years, fire suppression
resources have improved in technology and training, and,
therefore, LWFs under non-HTDs are suppressed more ef-
ficiently because the fuel moisture content is higher. Fires
under HTDs have lower fuel moisture content and can prop-
agate faster and with higher fire line intensity. Third, HTDs
are more prone to have simultaneous fire events (LWFs or
smaller fires) that split suppression resources.
The average LWF size in all fires increased in the study pe-
riod (from 424ha in 1978–1993 period to 1275 ha in 1994–
2010 period), and this could be related to the major percent-
age of LWFs under HTDs in the last years of the series be-
cause the average LWF size in both A and B classes was
larger than LWFs in non-HT conditions or weak HT condi-
tions (class C and D). The fact that average LWF size in-
creased under HTDs, when resources were better organized
and trained than ever, reinforces the importance of these HT
conditions and their influence on both total and average size
per LWF in the period 1994–2010. The largest historical fires
in Arag´
on happened under extreme HTDs in both 1994 and
2009. This supports the statement that HTDs provide more
extreme conditions for fire propagation and more difficulties
to suppress those fires. This has also occurred in other coun-
tries (Trigo et al., 2006; Mills, 2005), such as Russia (2010),
Portugal (2003), Australia (different years), Greece (2007)
and USA (2011, 2012).
5 Conclusions
There are significant effects of HTD conditions in the num-
ber of LWFs, total LWF area burned, and average LWF size
in Arag´
on. As a result, if in the future the number of HTD
conditions increases, fire suppression will be compromised.
This is likely to happen because our study shows that the
incidence of both the number of HTD and HT periods has
increased significantly in the study period.
It would be extremely profitable to be able to discriminate
between the simply “bad” and the “very bad” or “terrible”
fire days with some reasonable lead time (i.e., 24 or 48h).
We suggest that this classification regarding HTDs and non-
HTDs (at 850hPa) be used for that discrimination.
In terms of burned area, a decrease was observed only in
annual area burned under non-HTDs. Total area burn is sta-
ble. This may indicate greater fire damage as more area is
burned under HTDs.
Most HTDs are in July and August (82% of total). How-
ever, June is becoming more active in HTDs lately. This in-
dicates an earlier, longer fire season.
Acknowledgements. We are thankful to the University of Lleida
and Pau Costa Foundation for supporting this study through a
partial grant to fund Cardil’s PhD studies. We also thank Joaquim
Garc´
ıa-Codina, Miguel Lazaro and Luis Besold for help in the
classification of HTDs and other detailed checking in the official
wildland fire data base, and finally to Marta Fajo-Pascual and
Antonio Mu˜
noz for help in the statistical analysis and the EGIF
Database.
Edited by: B. D. Malamud
Reviewed by: M. G. Pereira and two anonymous referees
References
AEMET: Iberian climate atlas. Air temperature and precipitation
(1971–2000), State meteorological Agency of Spain and Depart-
ment of Meteorology and Climatology of the Institute of Meteo-
rology, Portugal, 80 pp., 2012.
Alvarado, E., Sandberg, D. V., and Pickford, S. G.: Modeling large
forest fires as extreme events, Northwest Sci., 72, 66–75, 1998.
Castro, M., Mart´
ın-Vide, J., and Alonso, S.: El clima de Espa˜
na:
pasado presente y escenarios de clima para el siglo XXI, Im-
pactos del cambio clim´
atico en Espa˜
na, Spanish Ministry of the
Environment ed., CSIC, 1–65, 2005.
Charney, J. J. and Keyser, D.: Mesoscale model simulation of the
meteorological conditions during the 2 June 2002 Double Trou-
ble State Park wildfire, Int. J. Wildland Fire, 19, 427–448, 2010.
Crimmins, M. A.: Synoptic climatology of extreme fire-weather
conditions across the southwest United States, Int. J. Climatol.,
26, 1001–1016, 2006.
De Zea Bermudez, P., Mendes, J., Pereira, J. M. C., Turkman, K.
F., and Vasconcelos, M. J. P.: Spatial and temporal extremes of
wildfire sizes in Portugal (1984–2004), Int. J. Wildland Fire, 18,
983–991, 2009.
Flannigan, M. D., Stocks, B. J., and Wotton, B. M.: Climate change
and forest fires. Sci. Total Environ., 262, 221–229, 2000.
Ganteaume, A. and Jappiot, M.: What causes large fires
in Southern France, Forest. Ecol. Manag., 294, 76–85,
doi:10.1016/j.foreco.2012.06.055, 2012.
Garc´
ıa-Herrera, R., D´
ıaz, J., Trigo, R. M., and Hern´
andez, E.:
Extreme summer temperatures in Iberia: health impacts and
Nat. Hazards Earth Syst. Sci., 13, 1393–1399, 2013 www.nat-hazards-earth-syst-sci.net/13/1393/2013/

A. Cardil et al.: Trends in adverse weather patterns and large wildland fires 1399
associated synoptic conditions, Ann. Geophys., 23, 239–251,
doi:10.5194/angeo-23-239-2005, 2005.
Garc´
ıa-Ortega, E., Trobajo, M. T., L´
opez, L., and S´
anchez, J. L.:
Synoptic patterns associated with wildfires caused by lightning
in Castile and Leon, Spain, Nat. Hazards Earth Syst. Sci., 11,
851–863, doi:10.5194/nhess-11-851-2011, 2011.
Giannakopoulos, C., Le Sager, P., Bindi, M., Moriondo, M.,
Kostopoulou, E., and Goodess, C. M.: Climatic changes and as-
sociated impacts in the Mediterranean resulting from a 2 ◦C
global warming, Global Planet Change, 68, 209–224, 2009.
Gillett, N. P., Weaver, A. J., Zwiers, F. W., and Flannigan, M. D.:
Detecting the effect of climate change on Canadian forest fires,
Geophys. Res. Lett., 31, L18211, doi:10.1029/2004GL020876,
2004.
Kalnay, E., Kanamitsu, M., Kistler, R., Collins W., Deaven, D.,
Gandin, L., Iredell, M., Saha, S., White, G., Woollen, J., Zhu, Y.,
Chelliah, M., Ebisuzaki, W., Higgins, W., Janowiak, J., Mo, K.
J., Ropelewski, C., Wang, J., Leetmaa, A., Reynolds, R., Jenne,
R., and Joseph, D.: The NCEP/NCAR 40-year reanalysis project,
Bull. Am. Meteor. Soc., 77, 437–471, 1996.
Kettunen, M., Terry, A., Tucker, G., and Jones, A.: Guidance on the
maintenance of landscape features of major importance for wild
flora and fauna – Guidance on the implementation of Article 3
of the Birds Directive (79/409/EEC) and Article 10 of the Habi-
tats Directive (92/43/EEC), Institute for European Environmental
Policy (IEEP) ed., 114 & anexxes pp., 2007.
Legendre, P. and Legendre, L.: Numerical ecology, 2nd Edn., Else-
vier, 854 pp., 1998.
Mill´
an, M. M., Estrela, M. J., and Badenas, C.: Meteorological pro-
cesses relevant to forest fire dynamics on the Spanish mediter-
ranean coast, J. Appl. Meteorol., 37, 83–100, 1998.
Mills, G. A.: A re-examination of the synoptic and mesoscale me-
teorology of Ash Wednesday 1983, Austr. Meteorol. Mag., 54,
35–55, 2005.
Miralles, M., Kraus, D., Molina, D., Loureiro, C., Delogu, G., Ri-
bet, N., and Vilalta, O.: Improving suppression fire capacity. Re-
search Report – European Forest Institute (EFI), 203–215, 2010.
Molina, D. M., Castellnou, M., Garcia-Marco, D., and Salgueiro,
A.: Improving fire management success through fire behaviour
specialists, Research Report – European Forest Institute (EFI),
105–119, 2010.
Mollicone, D., Eva, H. D., and Achard, F.: Ecology: Human role in
Russian wild fires, Nature, 440, 436–437, 2006.
Montserrat, D.: Situaciones sin´
opticas relacionadas con el inicio de
grandes incendios forestales en Catalu˜
na, NIMBUS, 1–2, 93–
112, 1998.
Montserrat, D., Martin-Vide, J., and Llasat-Botija, C.: Climatolog´
ıa
sin´
optica aplicada a la prevenci´
on de incendios forestales en
Catalu˜
na, Montes, 109, 9–15, 2012.
Moreno, J. M.: Impactos sobre los riesgos naturales, Riesgo de
incendios forestales, Evaluaci´
on preliminar de los impactos en
Espa˜
na por Efecto del Cambio clim´
atico, Spanish Ministry of
the Environment, 581–615, 2005.
Moreno, J. M., Viedma, O., Zavala, G., and Luna, B.: Landscape
variables influencing forest fires in central Spain, Int. J. Wildland
Fire, 20, 678–689, 2011.
Ogi, M., Yamazaki, K., and Tachibana, Y.: The summer northern
annular mode and abnormal summer weather in 2003, Geophys.
Res. Lett., 32, L04706, doi:10.1029/2004GL021528, 2005.
Pereira, M. G., Malamud, B. D., Trigo, R. M., and Alves, P. I.:
The history and characteristics of the 1980–2005 Portuguese ru-
ral fire database, Nat. Hazards Earth Syst. Sci., 11, 3343–3358,
doi:10.5194/nhess-11-3343-2011, 2011.
Regato, P.: Adapting to global change: Mediterranean forests,
Malaga, Spain: IUCN Centre for Mediterranean Cooperation,
254 pp., 2008.
Rodriguez-Puebla C., Encinas, A. H., Garc´
ıa-Casado, L. A., and
Nieto, S.: Trends in warm days and cold nights over the
Iberian Peninsula: relationships to large-scale variables, Climatic
Change, 100, 667–684, 2010.
Stocks, B. J., Mason, J. A., Todd, J. B., Bosch, E. M., Wotton, B. M.,
Amiro, B. D., Flanningan, M. D., Hirsch, K. G., Logan, K. A.,
Martell, D. L., and Skinner, W. R.: Large forest fires in Canada,
1959–1997, J. Geophys. Res. Atmos., 108, 5–12, 2003.
Trigo, R. M., Garc´
ıa-Herrera, R., D´
ıaz, J., Trigo, I. F., and
Valente, M. A.: How exceptional was the early August
2003 heatwave in France?, Geophys. Res. Lett., 32, L10701,
doi:10.1029/2005GL022410, 2005.
Trigo, R. M., Pereira, J. M., Mota, B., Calado, T., Dacamara, C.,
and Santo, F.: Atmospheric conditions associated with the ex-
ceptional fire season of 2003 in Portugal, Int. J. Climatol., 26,
1741–1757, 2006.
Trouet, V., Taylor, A. H., Carleton, A. M., and Skinner, C. N.: Inter-
annual variations in fire weather, fire extent, and synoptic-scale
circulation patterns in northern California and Oregon, Theor.
Appl. Climatol., 95, 349–360, 2009.
Van Wagner, C. E. and Pickett, T. L.: Equations and FORTRAN pro-
gram for the Canadian Forest Fire Weather Index System, Cana-
dian Forestry Service, Forestry Technical Report 33, 1985.
Vega-Garc´
ıa, C. and Chuvieco, E.: Applying local measures of spa-
tial heterogeneity to Landsat-TM images for predicting wildfire
occurrence in Mediterranean landscapes, Landscape Ecol., 21,
595–605, 2006.
Werth, P. A., Potter, B. E., Clements, C. B., Finney, M., Goodrick, S.
L., Alexander, M. E., Cruz, M. G., Forthofer, J. A., and McAllis-
ter, S. S.: Synthesis of knowledge of extreme fire behavior: vol-
ume I for fire managers. Gen. Tech. Rep. PNW-GTR-854, Port-
land, OR: USDA, Forest Service, Pacific Northwest Research
Station, 2011.
Wetterzentrale: Archiv der NOAA-CR20 und NCEP Reanalysis,
available at: http://www.wetterzentrale.de, last access: 5 March
2013.
www.nat-hazards-earth-syst-sci.net/13/1393/2013/ Nat. Hazards Earth Syst. Sci., 13, 1393–1399, 2013