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Present Environment and Sustainable Development
Volume 16, number 2, 2022
DOI: https://doi.org/10.47743/pesd2022162015
PESD2022, 16, 2; DOI: https://doi.org/10.47743/pesd2022162015 www.pesd.ro
Minimum air temperatures going to
extremes in Brașov city (Romania)
Raul-Gabriel ILEA1,2, Nicoleta IONAC1
1University of Bucharest, Faculty of Geography, Bucharest, Romania
2National Meteorological Administration, Bucharest, Romania
*Correspondence: raul11_bv@yahoo.com (R.G.I.); ionac.nicoleta@geo.unibuc.ro (N.I.)
Keywords: minimum air temperatures; indices of extremes; Brașov city; Romania
Abstract: Brașov is the largest city in the Romanian Carpathians, being located inside the largest
low-lying area in the country and exposed to western climatic influences, with well-defined local
features, especially in the cold season, when many thermal extremes may occur. The main
objective of this study was to analyze the minimum air temperatures and highlight their extreme
values over the 1980-2015 period. The climatic data were collected from the Brașov-Ghimbav
meteorological station and a set of specific indices, generically called as indices of extremes and
climate change by the WMO’s Expert Team on Climate Change Detection and Indices (ETCCDI),
were subsequently calculated. The resulting quantitative assessments of minimum
air-temperature variations in the above mentioned area has also proved that they can be useful
tools in calculating other derived technical parameters which may be relevant for the design and
onset of heating or cooling installations, in order to maintain a comfortable indoor climate in
residential buildings.
1. Introduction
The municipality of Brașov is known as the largest city in the central part of Romania,
being located in the Brașov Depression, at an average altitude of 625 m (Figure 1)
(Ielenicz, 2005).
Figure 1. The geographical position of Brașov city in the central part of Romania
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The Braşov Depression is a major tectonic low-lying area (3900 km2) on the inner
border of the great Carpathian Curvature. It evolved from the marine and stream erosion
of sedimentary rocks to an extensive floodplain (550-650 m-high), bordered by a wide
belt of pediments (800-1200 m-high). Located in the central part of Romania, the Braşov
Depression is characterized by a mid-latitude climate of transition between the humid
western or north-western and the frequent continental eastern or south-eastern
air-masses (Ciulache, Ionac, 1995).
The surrounding mountains act as important climatic barriers, greatly influencing
local climates in this area (Ciulache, 1997). In the southern part of the depression there
are the Piatra Craiului, Bucegi and Ciucaş Mts., greatly restricting the advancement of
southern air-masses. In winter, the cold air invading from N or NE often settles over the
basin, causing persistent temperature inversions and low air-temperature values. On the
western side, there lie the Perșani Mts., which allow humid western air-masses to extend
over the area, attenuating air-temperature ranges (Ionac, Ciulache, 2005).
In the western compartment of the Brașov Depression, there is the Brașov city
(Figure 2), exposed to western mid-latitude climatic influences, with well-defined local
features. The main climatic aspects highlighted by the existing literature, as well as the
climatic parameters recorded at the Brașov-Ghimbav weather station show that the
western type of general atmosphere circulation is dominant. However, some excessive
features of continentalism may become evident, especially during the cold season, when
strong and persistent air temperature inversions may occur (Ciulache, 2006).
Figure 2. The location of Brașov city in the western compartment of the Brașov
Depression
The minimum air temperatures often go to extremes (Busuioc et al., 2015), having a
great variability in time all over the area of reference, thus reflecting the importance of
their analysis (Bartholy, Pongrácz, 2007). Thermal extremes are also frequently recorded
during the cold season in other parts of the country (Croitoru, Piticar, 2013) or the world
(Founda et al., 2004; Miȩtus, Filipiak, 2004; Marengo, Camargo, 2008; Roushdi et al.,
2016; Sui et al., 2020). Besides, there are some minimum air temperature – related
indices which may be calculated in order to estimate the impact of cumulative extremely
low air-temperature values on the energy demands for heating or cooling devices in
indoor spaces.
All these things being taken into consideration, the practical importance of this study
relates to the fact that specific minimum air-temperature parameters and associated
indices may be used to assess the bioclimatic comfort of the population living in Brașov
city and the energy needs for either heating buildings during the cold season or
PESD2022, 16, 2 197
air-cooling their inner spaces during summertime. It is well-known that the largest
population in the Romanian Carpathians is concentrated in this area and this is important
especially during the winter season, when more heat is needed. The respective
air-temperature related indices can be used as guidelines for sufficient amounts of heat
being provided to the population, but also to ensure that no energy is wasted in the
process of production. It is also important to mention that this study can be applied not
only to Brașov city, but also to other big cities in Romania, which are mainly based on the
public heat energy supply systems.
2. Materials and Methods
The most representative weather station for the Brașov city area is located at
Ghimbav (4.5 km west from Brașov), at 45°41’N latitude and 25°31’E longitude, at an
altitude of 534 m and is included in the national network of meteorological stations,
operated by the National Meteorological Administration (NMA).
In order to relevantly outline the most extreme climatic features of this important
intra-mountainous area, the authors have opted for some of the WMO’s Expert Team on
Climate Change Detection and Indices (ETCCDI) indices which, unlike the
”commonly-used” weather and climatic indices that describe extreme events and
phenomena, they mainly refer only to the most rare cases occurring ”within the reference
statistical distribution of some specific weather elements in a certain area” (Houghton et
al., 2001), that is mostly to those specific climatic elements on which systematic and
accurate daily measurements and observations are being made, namely: air temperature
and rainfall amounts (Vincent et al., 2005; Tang et al., 2013; Piccarreta et al., 2015).
Out of all the 27 existing ETCCDI indices best describing the main characteristics of
some extreme weather and climatic events, such as their frequency, amplitude and
duration (Zhang et al., 2011), the following have been selected and calculated according
to RClimDex or FClimDex methods (CLIMDEX – Datasets for Indices of Climate Extremes
- http://www.climdex.org/index.html), over the 1980-2015 period: mean of daily
minimum temperature (TN), monthly minimum value of daily minimum temperatures
(TNn), monthly maximum value of daily minimum temperatures (TNx), frost days (FD),
maximum number of consecutive frost days (CFD), cold spell duration index (CSDI) and
heating degree-days (HDDs).
As the period of reference (1980-2015) included homogenous data series, the
respective ETCCDI indices were then processed in Excel table and graphical forms,
mainly based on the corresponding daily minimum air temperature values (TN) available
for all months during the previously-mentioned period. A prospective time evolution
trend could also be indicated and the statistical significance of the trend was sustained by
simply calculating the R-squared index in Excel.
3. Results
The most favorable synoptic context which is responsible for the lowest minimum
air temperatures in Brașov city area is the presence of an extensive anticyclone. This
produces the intense radiative cooling of the ground and also strong temperature
inversions, especially during the cold season nights (Sfîcă et al., 2019). Over the
1980-2015 period, the lowest minimum air temperature that has ever been recorded
(absolute minimum temperature) in Brașov was -33.3°C (on 8th January 2015). The
average of the corresponding lowest annual values was -24.9°C (Table 1).
The most extreme values ranged between -33.3°C in 2015 (on 8th January) as the
absolute lowest minimum value, and -17.6°C in 1994 (on 16th February and on 20th
December), as the highest minimum value recorded. An interesting fact is that 13
absolute yearly minimum air temperatures were registered in January, 12 in December
and February respectively and a single one in March (-27.3°C, on 5th March 1987), out of
all the 36 years taken into consideration. Even though it is not scientifically customary, if
calculating the average mean value of all these 36 yearly minimum air temperatures, the
PESD2022, 16, 2 198
result would be: -24.9°C (colored in yellow below). This value shows that there are
extreme cold conditions, which can easily occur in this area, especially due to persistent
thermal inversions during the cold season associated mostly with anticyclonic conditions
(Mihai, Teodoreanu, 1969; Mihai, 1971). It is well-known that there are differences
between the air-temperatures recorded in Brașov city and those in Ghimbav (4.5 km far
west from the city), where the weather station is located (Ciulache, 2006). Due to the
development of Brașov town area (located at a higher altitude than Ghimbav), there is an
increasing role of the urban heat island, which complicates the understanding of
temperature inversions in this area, especially in the cold season.
Table 1. The lowest minimum air temperature (°C) of each year and the date of their
occurrence at the Brașov-Ghimbav meteorological station over the period 1980-2015.
(Source: NMA archive for temperature values)
The highest monthly mean of daily minimum air temperature (TN) is recorded
in July (with a value of 12.8°C), and the lowest in January (with a value of -7.9°C). The
annual average is 2.5°C, as showed in Figure 3-A.
For the 1980-2015 period, the highest monthly mean of daily minimum
air-temperatures reached as high as 3.9°C (in 2015) and the lowest value is 0.7°C (just
above the freezing level, in 1985). The trend of this index is statistically significant
(Figure 3-B), increasing with an average value of 1.5°C all through the 1980-2015
period.
Data source: NMA archive for temperature values
Figure 3. The annual (A) and inter-annual variation (B) of the TN index at the
Brașov-Ghimbav meteorological station for the period 1980-2015.
The monthly minimum value of daily minimum air temperature (TNn) for this
period is recorded on 8th January 2015 (-33.3°C). This index has positive values only in
summer, reaching as high as 3.3°C, in July (Figure 4-A). The annual average is calculated
as -13.6°C, highlighting an extreme value for Romania, on condition that, out of all the
minimum air-temperatures of the 30/31 days of each month, the lowest value of each
month was taken into consideration.
The inter-annual variability of the TNn index is pretty large and the trend is slowly
decreasing over time but not statistically significant, as showed in Figure 4-B. The highest
TNn value is recorded on 16th of February 1994 (-17.6°C) and the lowest value on 8th
January 2015 (-33.3°C). The last value mentioned is not far away from the lowest
B
A
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minimum air temperature ever recorded in Romania (-38.5°C, on 24th January 1942, in
Bod, Brașov County).
Data source: NMA archive for temperature values
Figure 4. The annual (A) and inter-annual variation (B) of the TNn index at the
Brașov-Ghimbav meteorological station for the period 1980-2015.
The monthly maximum value of daily minimum air temperature (TNx) for the
analyzed period took place on 30th August 2003 (23.4°C), as represented in Figure 5-A.
The lowest TNx value is calculated in January (6.1°C) and the annual average for this
index is 14.4°C. In this case, among all the minimum air-temperatures of the 30/31 days
of one month, the highest value of each month was taken into consideration.
The inter-annual variability of the TNx index is moderate and the trend of this index
is slowly increasing (Figure 5-B), but not statistically significant, exactly as in the case of
the TNn index. The highest value has obviously been recorded on 30th August 2003
(23.4°C), turning it into a typical value for a tropical night (when TNx>20°C). The lowest
value has occurred twice, on 10th June 1981 and 29th July 1997 (15.2°C).
Data source: NMA archive for temperature values
Figure 5. The annual (A) and inter-annual variation (B) of the TNx index at the
Brașov-Ghimbav meteorological station for the period 1980-2015.
The highest number of frost days (FD) has been recorded in January (28 days for
the 1980-2015 period). A frost day is a day in which the minimum air-temperature keeps
below 0°C (TN<0°C). An interesting fact is that from May to September, the average FD
value is zero, which means that all minimum air temperatures occurring these months
are positive (Figure 6-A). This case is similar for May too, but there are years with some
negative air-temperature values occurring during this month, especially in cases of late
frosts with potential damaging effects on crop plants.
The highest FD value was calculated for 2011 (158 days) and the lowest for 2014
(104 days), as showed in Figure 6-B. The most relevant aspect is that the FD index
decreases from around 140 days to 125 days, showing a statistically significant decrease
over time, possibly due to latest warming trends.
A
B
A
B
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Data source: NMA archive for temperature values
Figure 6. The annual (A) and inter-annual variation (B) of the FD index at the
Brașov-Ghimbav meteorological station for the period 1980-2015.
The highest number of consecutive frost days (CFD) was reached in 1982, with
82 consecutive daily minimum air-temperature values below 0°C (from 4th of January to
26th of March). The lowest CFD index value was recorded in 1994, with only 16
consecutive days (Figure 7-A). The trend of this index shows a slight decrease over time,
from a value close to 40 consecutive frost days to almost 35 days over the whole
analyzed period; yet this decreasing trend is not very statistically significant.
Data source: NMA archive for temperature values
Figure 7. The inter-annual variation of the CFD index (A) and the maximum values of the
CSDI (B) at the Brașov-Ghimbav meteorological station over the period 1980-2015.
Another interesting fact for this meteorological station is that the maximum values of
the cold-spell duration index (CSDI) were highest in 1986 and 2002, when four cold
waves occurred (Figure 7-B). This index represents the number of intervals per each year
where, for at least six consecutive days, the daily minimum air-temperature is lower than
the calendar 10th percentile calculated for a 5-day window centered on each calendar day
over the 1980-2015 period, as specified in Ionac, Ciulache, 2005. In this case, there is no
observed change in tendency over time.
The following degree-days (DDs) indices (Figure 8) are also very important regarding
the amount of energy required to heat or to cool homes. Generically, they represent the
sum of all daily minimum air-temperature values less than 0°C (HDDn0) and/or than
10°C (HDDn10) from each year analyzed. Obviously, the highest heating degree-days
(HDDs) values were summed up especially during the cold season and the lowest values
are usual for summer (for example, in July, the HDDn10 index had a value of 0°C, which
means that all the minimum air-temperature values were greater than 10°C).
The inter-annual variation of heating degree days (HDDs) shows that 1985 was the
coldest year from the entire analyzed period (1,318.9°C for the HDDn0 index,
respectively 3,504.4°C for the HDDn10 index), and the warmest years were both 2014
and 2015 (with 521°C for HDDn0 index, respectively 2,478.4°C for HDDn10 index), as
represented in Figure 8-A. Thus, in 1985, more energy from power plants was needed
A
B
A
B
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than in the other years taken into consideration, which is demonstrated by the high
values of this index. On the other hand, in the last two analyzed years (2014 and 2015),
a lower energy consumption was needed, given the higher values of minimum
air-temperatures and lower values of HDDn0 index recorded.
In each case, these indices showed a significant decrease over time, especially for
the HDDn10 index, pointing to a gradual increasing of the minimum air-temperatures
over the analyzed period. The trend for HDDn0 index is decreasing from a value around
1,000°C to only an average value of 700°C, as showed in Figure 8-B. The trend for
HDDn10 index is decreasing even more pronouncedly, from a value closer to 3,200°C to
almost 2,700°C (Figure 8-C). The main cause of both these trends is most probably the
general ongoing climate warming process, which attenuates the normal climatic cooling
effects, especially in the cold season of the year.
Data source: NMA archive for temperature values
Figure 8. The annual (A), and the inter-annual variation of the HDDn0 index (B) and
HDDn10 index (C) at the Brașov-Ghimbav meteorological station for the period
1980-2015.
The monthly and annual frequency of negative air-temperature values (when
TN<0°C) is calculated in Table 2, and the frequency of air-temperatures below 10°C is
presented in Table 3.
Table 2. The frequency of negative air-temperature values (TN<0°C) for the
Brașov-Ghimbav meteorological station for the period 1980-2015.
Year/Month
I
II
III
IV
V
VI
VII
VIII
IX
X
XI
XII
Annual
TN<0 (%)
91.8
89.4
65.0
22.7
1.4
0.0
0.0
0.0
0.7
22.9
61.6
84.1
36.6
Data source: NMA archive for temperature values
Table 3. The frequency of air temperature values below 10°C for the Brașov-Ghimbav
meteorological station for the period 1980-2015.
Year/Month
I
II
III
IV
V
VI
VII
VIII
IX
X
XI
XII
Annual
TN<10 (%)
100
100
99.9
98.7
79.2
35.9
17.5
25.9
73.5
95.6
99.4
100
77.1
Data source: NMA archive for temperature values
A
B
C
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In the first case, in January, 91.8% of all registered values were below 0°C, followed
by February with 89.4% and December with 84.1%. Obviously, there are no negative
values in summer (from June to August), but in winter (from December to February), all
the values keep below 10°C. In March, the percentage of negative air-temperatures
reached 99.9%, followed by November (99.4%), April (98.7%) and October (95.6%).
4. Conclusions
The high variations of all the extreme values that were recorded in the Brașov
Depression, from 1980 to 2015 reveal not only that the general climatic conditions may
often depreciate, but that there is also a high thermal variability from one month to
another or from one year to another. Our results are in agreement with the previous
knowledge on this topic and they are relevant not only for Brașov city, but also for other
big cities in Romania.
This variability has been confirmed by all the indices that were calculated. The
monthly mean of daily minimum temperatures (TN index) increased with an average
value of 1.5°C in the 1980-2015 period. The monthly minimum value of daily minimum
temperature (TNn index) reached the lowest value on 8th January 2015 (-33.3°C), while
its corresponding annual average is calculated as -13.6°C, a value that’s pretty low even
for Romania. The monthly maximum value of daily minimum temperature (TNx index)
reached its highest value ever recorded on 30th August 2003 (23.4°C), while its annual
average is 14.4°C. For both TNn and TNx indices, there are no statistically significant
changes over time, which is in agreement with the results obtained for other regions in
Romania.
The frost days (FD index) evolution trend decreases all over the analyzed period (17
day in average), but there are also some recent exceptions (in 2011, there were 158 frost
days). The maximum number of consecutive frost days (CFD index) shows about the
same evolution in time.
The cold spell duration index (CSDI index) ranged from one to four cold waves (at
least one cold wave occurring each year). The coldest year for the analyzed period was,
obviously, 1985, when the sum of all daily minimum air temperatures lower than 0°C
(HDDn0 index) and 10°C (HDDn10 index) reached almost record-high values
(1,318.9°C, respectively 3,504.4°C), resulting in high energy and fuel consumption. In
contrast, the lowest values of these indices were calculated for 2014 and 2015 (HDDn0
value less than 600°C and HDDn10 value less than 2,500°C), requiring low energy and
fuel demands. All these results prove the extreme character of the minimum air
temperatures and their great variability in time, but also reflect their importance.
The practical importance of all these quantitative assessments is given by the fact
that they can be useful tools in calculating some important technical parameters, which
may be relevant for the design of heating or cooling installations, in order to ensure
bioclimatic comfort of the population, without energy loss.
The most important fact is that both the average value of these extreme indices and
their duration in time must be taken into consideration and not only the values produced
on a given day, on a specific month or year. By simply identifying the particularities of
minimum air-temperatures in Brașov city may prove useful in the management of heat
energy being produced and provided for domestic purposes.
All in all, the higher the values obtained for these indices, the more energy must be
provided by the power plants to the population of the city. This is very important because
Brașov city is one of the largest cities in the country and definitely the greatest town in
the Romanian Carpathians.
The present study could be applied to other large cities in Romania too, in order to
find out the relationship between the amount of energy provided by power plants and the
actual need for heating homes and for indoor human comfort, depending on the building
characteristics as well.
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