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RISCURI ŞI CATASTROFE, NR. XV, VOL. 18, NR. 1/2016
31
COLD AND HEAT WAVES IN THE BARLAD PLATEAU BETWEEN
1961-2013
M.C. HUSTIU1
ABSTRACT. – Cold and heat waves in the Barlad Plateau.
Among the meteorological phenomena with severe impact upon individual humans,
upon society and the environment one distinguishes cold and heat waves. The
geographic location of the Bârlad Plateau – east of the mountainous barrier of the
Eastern Carpathians and in a region where main pressure centers travel along the
year – facilitates the occurrence of cold and heat waves. Such events were analysed,
for the interval 1961-2013, both as the frequency of massive warming and cooling
situations, and their duration – taking as a reference intervals of at least 5
consecutive days when in any month thermal anomalies are recorded, either positive
or negative.
Key words: cold, heat wave, freezing nights
1. INTRODUCTION
As with canicular days, there are no universally-accepted definitions neither
for a cold wave, nor for a heat wave. As such, the temperature values that the
inhabitants of a region consider to be normal, may be extreme in another region if
in the latter those values deviate largely from the multiannual daily means.
Recently, with relation to a WHO methodology, the WMO makes a
recommendation to which we subscribe: Heat or hot weather that lasts for several
days, often referred to as a “heatwave”, is a pervasive natural hazard that can exact
a heavy toll on human systems, affecting health, livelihoods and infrastructure.
Natural systems can also be severely affected by the impacts sustained beyond the
duration of a heatwave. Although there is no universally acceptable definition of
heatwaves (Perkins and Alexander, 2013;Robinson, 2001), they are understood to
be periods of unusually hot and dry or hot and humid weather that have a subtle
onset and cessation, a duration of at least two to three days and a discernible impact
on human activities (WMO-WHO, Heatwaves and Health, 2015). As a result, this
new take on heat waves is fully adequate for thermal hazards impacting upon
1 National Meteorological Admistration, Bucharest, Romania, e-mail: [email protected],
M. C. HUSTIU
32
people and at the same time it does not require that strict criteria – of timespan and
threshold – have to be met.
The warming trend of climate is obvious in the region we analysed herein
and in the last years it is manifesting itself by frequent weather extremes – mostly
of thermal nature. The last winters were generally warm, with short intervals of
excessive drops in temperature and heavy snowfall.
The geographic location of the Bârlad Plateau – the south-east of Europe
(fig.1), in a region where main pressure centers travel along the year and whose
activity drives the entire mechanism of atmospheric circulation – is a cause for
strong deviations in the temporal and spatial evolutions of climatic factors, from
the multiannual means which are being taken as the norm.
They constitute aperiodic variations of the climate, whose intensity amplifies
or diminishes in direct relation to the conditions of the underlying surface, and
those of the low landscape especially.
Fig. 1. Hypsometric map and relief units of
Plateau of Barlad
COLD AND HEAT WAVES IN THE BARLAD PLATEAU BETWEEN 1961-2013
33
2. DATA AND METHODS.
HWDI/CWDI (Heat/Cold Wave Duration Index) – deviations of
maximum/minimum temperature of at least 5 degrees from the multiannual means,
lasting at least 5 days.
To highlight the cold/heat waves having a climatc hazard character,
(Bălteanu, Şerban, 2005), we have considered the local climate – to wit, the mean
temperature derived from the minimum values in January (fig. 2 - a) (minimum
values of -6...-5ºC) and the mean temperature derived from the maximum values in
July (maximum values of 28...29ºC) (fig. 2 - b)
a) b)
Fig. 2. Mean temperature derived from the minimum value (a – in January)
and the mean temperature (b – in July)
M. C. HUSTIU
34
3. RESULTS AND DISCUSSION
The absolute, extreme temperatures
To get the limits of the temperature variations, data has been extracted from
the database of the National Meteorological Administration in Romania (NMA):
the absolute maximum temperatures in the cold season (October to March) and the
absolute minimum temperatures in the warm season (April to September).
The lowest absolute minimum temperatures occur when arctic air masses
pass over Romania, and during radiative cooling in an anticyclonic context.
During the analysed interval (1961-2010), the absolute minimum temperatures
occurred in months with the largest monthly temperature deviations – specifically, in
January 1963 and January 1985, when the minimum values of air temperature got as
low as -25.1 ºC at Bârlad (on January 18, 1963); -27.5 ºC at Tecuci (on January 13,
1985) and -30.8 ºC at Bacău (on January 18, 1963) (table 1).
Table 1. Minimum monthly temperatures (extremes) at stations within
Bârlad Plateau, between 1961 and 2010.
Staţia X XI XII I II III
27/79 28/93 26/02 13/85 13/69 01/85
Tecuci -9,5 -20 -22,9 -27,5 -23 -20,2
27/88 26/93 28/96 18/63 09/76 01/86
Bârlad -7,4 -18,4 -21,2 -25,1 -22 -17,4
27/79 26/93 26/02 18/63 09/76 01/86
Bacău -7,8 -21,4 -23,4 -30,8 -25 -21,5 Source: The archive database of NMA, in Bucharest
Since the beginning of measurements, the absolute minimum temperatures
were: -32.0 ºC at Vaslui (on January 15, 1911) and -30.5ºC at Bârlad (on January
25, 1942).
In the warmest years of the 21st Century, the absolute maximum
temperatures measured in the same period have surpassed the previous top values
of 39.7 ºC at Bârlad (on July 30, 1936) and 39.1 ºC at Vaslui (on July 5, 2000).
In 2007, these maximum temperatures were recorded: 39.9 ºC at Bârlad on
July 23; 39.1 ºC at Tecuci on July, 20 (tabel 2); 39.7 ºC at Negreşti on July 24; 40.3
ºC at Bacău on July 24, and the absolute top value for Bârlad Plateau was 40.8 ºC,
recorded at Vaslui on July 19, 2007.
In August of 2012 these outstanding values of absolute temperature were
surpasses anew, so that the new ones were: 40.4 ºC at Tecuci; 39.8ºC at Bârlad
(a tenth of degree lower than the previous top value in 2007), on August 8, and
42.5ºC were recorded at Bacău the same day.
COLD AND HEAT WAVES IN THE BARLAD PLATEAU BETWEEN 1961-2013
35
Table 2. Maximum monthly temperatures (extremes) at stations
within the Bârlad Plateau, between 1961 and 2010
Staţia IV V VI VII VIII IX
06/98 17/69 26/07 20/07 25/07 27/87
Tecuci 31.2 33.6 37.4 39.1 37.9 35.1
06/98 17/69 26/07 23/07 25/07 27/87
Bârlad 30.5 35 38 39.9 38.2 34.5
06/98 16/69 26/07 24/07 24/07 18/86
Bacău 29.8 34.5 38.4 40.3 37.4 35.1 Source: The archive database of NMA, in Bucharest
Aperiodic variations of temperature in January and July, and the trend in
its evolution.
From the graphs representing the variations - and the trend of evolution – of
the mean temperature in January, one can see that the coldest month in the
analysed interval was January, 1963 (fig.3) – when the negative deviations in
Bârlad Plateau varied between 8 and 10 degrees. During that month, the mean
temperature values were -12.3 0C at Vaslui, -11
0C at Huşi, and -10. 4
0C at Bârlad.
The analysis of graphs depicting the variation and trend of the deviations
from the July norm – a month which is taken as representative for the warm season
(in fact, the warmest month) – shows a trend of acclerated increase in the last two
decades. During those years the deviation was positive, with the notable
maximums of July, 2007 when it was 3.74 0C (fig.3) and of July, 2012 when the
mean deviation was 4.5 0C.
-8,09
6,48
-2,59
3,74
-8,50
-6,50
-4,50
-2,50
-0,50
1,50
3,50
5,50
19
61
19
64
19
67
19
70
19
73
19
76
19
79
19
82
19
85
19
88
19
91
19
94
19
97
20
00
20
03
20
06
20
09
ianuarie iulie Liniară (ianuarie) Liniară (iulie)
Fig. 3. Average monthly mean deviations and the linerar trend (January and July) in
Barlad Plateau (1961-2010)
M. C. HUSTIU
36
Regarding the comparison between the evolution of mean deviations of
temperature in January and July, one can see a larger amplitude of deviations in
January.
The linear trend of evolution in analyzed period (1961-2010) of the mean
temperature in July and January shows significant increase, of about 2.5 0C
Similarly, from the analysis of the variation and trend of evolution of the mean
temperatures during July and January – representative months for the warm and
cold season, respectively – we can conclude that the warming trend spotted for the
mean annual temperature is mostly caused in a higher degree by the stronger
warming trend during winter and summer, than during transition seasons.
Cold waves
By its position – in front of the mountainous obstacle of the Romanian
Carpathians, the Bârlad Plateau is frequently affected by cold waves during the
cold season of the year.
Looking at the entire country, its eastern, south-eastern and southern regions
have distinguished themselves in the 20th Century by the highest frequency of
massive temperature drops (minimum temperatures ≤ -30 ºC), that is 53 cases
which amount to 47.3% of the total (Bogdan, 1998, Bogdan, Niculescu, 1999).
Knowing the minimum temperatures during the analysed interval, it was
possible to compute the frequency of cooling episodes/temperature drops ≤ -10 ºC
in the Barlad Plateau, judging by value thresholds.
As for the intensity of cooling episodes (deviation from normal), the leading
months are January, 1963 and February, 1985.
During the analysed period there were identified 41 intervals with remarkable
low temperature(s). The most intense and frequent cold waves have occurred in the
coldest months as well – the winers of 1963, 1969, 1985, 1987 and 2012.
The cold waves with the highest frequency were those ranging in 7 to 14
days (53.7% of total cases), followed by those ranging in 5 to 7 days, and lastly by
the cold waves longer than 14 days (Table 3)
Table 3. The frequency of cold waves within Bârlad Plateau, by their duration (1961-2013)
Duration < 5 days 5... 7 days 7...14 days > 14 days
Num. cases 3 8 22 8
% 7.3 19.5 53.7 19.5 Source: Processed data from the archive database of NMA, in Bucharest.
The longest cold wave – 33 days – has occurred during the winter of 1987,
when the nights were extremely cold (≤ -10 ºC) and the days were especially
COLD AND HEAT WAVES IN THE BARLAD PLATEAU BETWEEN 1961-2013
37
wintry (≤ 0 ºC) over the entire analysed geographic area. This cold wave was not very intense, since the minimum temperature has dropped below -25 ºC in a single instance, and below -15 ºC it has dropped 58 times.
The next cold wave, by duration was that of January-February 2012, when for almost a month (27 days) the temperatures were below climatological means of that time of the year.
The most intense cold waves have occurred during the winters of 1963 and 1985. In the winter of 1963 there were 3 episodes of excessive cooling, when in two instances the minimum temperatures have dropped below -30 ºC, in 11 instances they have dropped below -25 ºC, and below -15 ºC in 90 instances.
During the winter of 1985, although there were two episodes only of excessive cooling, their duration was much longer (January 1… 22, and February 8 – March 1). That winter there were 5 instances only of temperature below -25 ºC, none with values below -30 ºC; however, in 118 instances (Fig.4) the minimum temperatures were between -15 ºC and -25 ºC.
0
15
30
45
60
75
90
105
120
1961
1963
1965
1967
1969
1971
1973
1975
1977
1979
1981
1983
1985
1987
1989
1991
1993
1995
1997
1999
2001
2003
2005
2007
2009
2011
2013
>-25 -15...-24,9°C -10…-14,9°C
Linear (-15...-24,9°C) Linear (-10…-14,9°C)
Fig. 4. The evolution of minimum temperatures by value thresholds
(total number of cases at 5 stations in and around the Bârlad Plateau) (1961-2013)
As for the time of year that these cold waves have occurred, the highest
frequency was in January with almost 50% of the total cases (Table 4). February
comes in second, with 24,4% of the total, yet in the last two decades this frequency
has increased during December too, amounting to 22% over the entire interval.
Table 4. The frequency of cold waves in Bârlad Plateau,
by month of occurrence (1961-2013)
M. C. HUSTIU
38
Month November December January February March
Num. cases 1 9 20 10 1
% 2,4 22,0 48,8 24,4 2,4 Source: Processed data from the archive database of NMA, in Bucharest.
During the analysed interval there were cold waves at the beginning and the
end of the cold season as well. As such, the earliest cold wave has occurred in the
last two thirds of November 1993, when the temperature dropped to -20 ºC at
Tecuci, -18.7 ºC grade at Bârlad, -21.4 ºC at Bacău and -21.1 ºC at Iaşi.
The latest cold wave has occurred between March 3 and 16, 1987 when the
lowest minimum temperatures were -14.4 ºC at Tecuci, -15.6 ºC at Bârlad, -17.8 ºC
grade at Bacău and -17.4 ºC at Iaşi.
Heat waves
The Moldova Plateau is located in the temperate zone as well as in a
continental area subject to multiple climatic influences, over which air masses travel
that originate in the tropical zone, a situation that derives into positive deviations
from the climatologic means which at times get an unique status or establish
themselves as climatic records (Bogdan, 1992, 1994; Bogdan, Niculescu, 1999).
Such deviations are the result of the large, aperiodic variations of the climate
and as such they appear to be isolated or singular, therefore the name of positive
thermal singularities.
The positive thermal singularities are the manifestation of heat waves,
namely of warm air of a tropical and continental nature (or of warm, maritime and
tropical air which gets above Moldova Plateau already continentalised and devoid
o precipitation/precipitable water), generated by continental highs that develop
either in the south-east of Europe, or in the south-west of Asia, or in the Black Sea
basin, or in the Balkan Peninsula, or iin the north-west of Africa etc.
At times, such positive thermal singularities are caused by warm air masses
of a tropical nature that reside at the margin of oceanic cyclones that extend over
the central and eastern areas of Europe; or, southern air masses tag along retrograde
cyclones that cover the southern parts of Moldova (Clima RPR, I, 1962).
When the high pressure systems persist, the local insolation processes
intensify that further contribute – together with tropical air advections – to the
increase in warming and drying, thus increasing the values of subsequent positive
thermal singularities. The literature on this subject contains frequent links to the strongest
warming instances, that are often (yet, not exclusively) presented as absolute maximum temperatures, and also links to their consequences.
Judging by the mean temperatures of the warmest months (July and August), the most intense warming instances are those ≥ 25°C; by the absolute maximum temperatures (be them monthly or annual), they are those surpassing 30°C (tropical
COLD AND HEAT WAVES IN THE BARLAD PLATEAU BETWEEN 1961-2013
39
days) and higher (≥ 35 °C, canicular days), and judging by the nocturnal minima, they are those ≥20 °C (tropical nights).
In the last century, in the Barlad Plateau there were frequent situations when the absolute maximum temperatures surpassed 30°C in the warmest months and even from May to September. However, not every tropical day may be the manifestation of a strong warming. This depends on the persistence of anticyclones that drive the frequency of tropical days and that of warming processes.
For the warm season, the most dangerous event that southern circulations cause quite often in the geographic area of Romania is represented by the heat
waves, and canicular weather respectively – which in Romania occur under the effect of North-African High's ridge, yet such bouts may be also governed by the hot pulses – less frequent – of the Arabic Low.
As such, it appears of high concern to note the fact that, due to the climate specifics of each geographical region, one cannot formulate an universally-valid definition of heat wave. In Romania, the weather is said to be canicular when during the temperature reaches or rises above 35
oC by day and when it does not
get below 20 oC at night. On the other hand, heat and heat waves are also
frequently defined by parameters that relate to thermal stress or feels like temperature, whose formula include the value of relative humidity along with that of the measured temperature.
As with massive cooling in meteorological terminology, a new term has been established – heat wave or canicular heat wave. Since The World Meteorological Organization recommends a 5 degrees deviation, for at least 5 days, to label an interval as a heat wave, for the Bârlad Plateau maximum temperatures
were considered those ≥ 33°C. As a result, for the analysed region canicular heat waves were considered
those intervals when the maximum temperatures have surpassed 33 0C for 5 days
in a row. During the period of interest, 32 intervals with heat wave were identified
(temperature ≥ 33°C more than 5 days) Until 1987, this meteo-climatic situation has had a insignificant frequency, and its extent was in the south of the region only.
During the last two decades, the canicular heat waves have become more and more common (Fig. 5), noting that between 2000 and 2012 especially the warmest summers were recorded, since the beginning of meteorological observations. The warmest summers in Romania and Bârlad Plateau were those of 2007 and 2012, when new absolute temperature peaks were reached and surpassed, for this part of the country.
During the summer of 2007 there were two canicular heat waves of medium duration (8…10 days) yet maximum intensity, when at the above mentioned stations 32 instances of maximum temperatures above 38 °C were recorded, and 40 cases of values above 35 °C.
M. C. HUSTIU
40
The summer of 2012 remains the warmest summer in the history of meteorological observations, with 4 canicular heat waves; the one in July having a long duration (17 days) yet medium intensity, and those in August having a shorter duration (5…7 days) yet large intensity, when new absolute records were reached. During this summer, in the Bârlad Plateau the largest number of canicular days was recorded (228 cases in 2012, to 153 cases in 2007), which shows the persistence of canicular heat waves for a long time (Fig. 5), but the number of days with the maximum temperature over 38°C was a bit lower than that of 2007 (29 days in 2012, to 32 days in 2007).
228
0
20
40
60
80
100
120
140
160
19
61
19
63
19
65
19
67
19
69
19
71
19
73
19
75
19
77
19
79
19
81
19
83
19
85
19
87
19
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19
91
19
93
19
95
19
97
19
99
20
01
20
03
20
05
20
07
20
09
20
11
T max >38°C T max. 3 5...37,9°C
Tmax 33…34,9°C T max >33 (canicu lă)
Linear (T ma x >33 (caniculă))
Fig. 5. The evolution of maximum temperatures by value thresholds
(total number of cases at 5 stations in and around the Bârlad Plateau) (1961-2013)
The aperiodic variability of freezing nights (≤ -10 ºC)
The mean annual number of such nights, when the minimum air
temperature gets as low as -10 ºC at most – varies over the Bârlad Plateau between 15 and 20, while it can surpass 20 nights both on the upper and medium Bârlad River and in the valleys of the rivers Berheci and Tutova, where the landscape is markedly low. At Tecuci the mean of this number is 14.7, at Bârlad 15.2, at Vaslui it increases at 19.2, at Huşi it is 18.7, while at Negreşti 22.
The aperiodic evolution of such nights shows a considerable decline over the
analysed period, of approximately 23 % (Fig.6), a process caused by the warming
of climate over the past two decades.
COLD AND HEAT WAVES IN THE BARLAD PLATEAU BETWEEN 1961-2013
41
0
5
10
15
20
25
anul 1964 1968 1972 1976 1980 1984 1988 1992 1996 2000 2004 2008
Tecuci Bârlad Vaslui Negreşti huşi Linear (Bârlad)
Fig. 6. The aperiodic variability of freezing nights (≤ -10 ºC) in January
in the Barlad Plateau (1961 – 2013)
The aperiodic variability of the number of tropical days
The mean annual number of tropical days over the analysed interval
surpasses 25 days in the southern areas and is 10-12 days only at altitudes over 300
m(etres) in the Central Moldavian Plateau and Tutova Hills. At Tecuci there were
28 days, at Bârlad 23, at Vaslui 25, at Negreşti 21, while at Huşi 26 days.
The frequency of tropical days varies significantly from a year to the next.
As such, over the analysed interval the maximum monthly number of tropical days
was recorded in August, 1992 and July, 2012 – months when at Tecuci there were
27 such days.
0
5
10
15
20
25
anul
1963
1966
1969
1972
1975
1978
1981
1984
1987
1990
1993
1996
1999
2002
2005
2008
Nr.
zile
Tec uc i Bârlad Vaslu i
Negreşti Huşi L inear (Bârlad)
Fig. 7. The aperiodic variability of the number of tropical days ≥3 0°C in July
in the Barlad Plateau (1961-2013)
M. C. HUSTIU
42
By a multiannual analysis, the trend of increase in the number of tropical
days is obvious in all summer months. Specifically, in July and August when the
largest number of such days is recorded, the increase over the last 50 years s-a
produs was the double of the initial number (an increase of almost 100%). At
Bârlad, both in July (Fig. 7) and August the mean number of such days has
increased from 5...6 days in the 1960s, to 11...13 days at the present
CONCLUSIONS The trends that we have identified (as increase) in the annual frequency of
the number of days with heat waves and especially of days with canicular heat
waves (suggested in this study by the increase in the number of days with
maximum temperature ≥33 ºC) reflects the climatic change noticed in the analysed
region.
The canicular heat waves have extended over the entire region and have
increased exponentially after 1987. In these conditions, a doubling of the number
of tropical days was noted; a special weight carries the fact that, while at the
middle of the last century there were almost no tropical nights, at the present their
number is 3...4 both in July and August
The rate of decrease in the annual mean number of days with cold waves and
that of instances with extreme minimum temperatures (≤-10 ºC) during the cold
waves in the cold season are lower. However, an obvious decrease is noted in the
number of total freeezing nights.
REFERENCES 1. Bălteanu D. Mihaela, Şerban (2005), Modificările globale ale mediului. O evaluare
interdisciplinară a incertitudinilor, Edit. CNI Coresi, Bucureşti.
2. Bogdan Octavia, Niculescu Elena, (1999), Riscurile climatice din România, Institutul
de Geografie al Academiei Române, Bucureşti.
3. ***WMO-No.1142, World Meteorological Organization and World Health
Organization, 2015, Heatwaves and Health:Guidance on Warning-System
Development, 107 pp.
