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TEMPERATURE LAPSE RATES IN THE AIR NEAR THE GROUND IN URBAN
AND RURAL AREAS
Anita Bokwa1
Jagiellonian University, Cracow, Poland
Abstract
Temperature lapse rates in the air layer near the ground were studied for two sites: the meteorological station in
Cracow (city center) and the one in Gaik-Brzezowa, in the Wieliczka Foothills, about 30 km south-east from
Cracow, in rural area, near the Dobczyce Water Reservoir. The study covered the period 1993-1997. The
temperature lapse rates for the air layer 50-200 cm above the ground were calculated, for every day of the
studied period, for 6, 12 and 18 UTC. Apart from descriptive statistics, cluster analysis was used to find the
patterns of daily temperature lapse rates course. Both places show significant differences in the thermal
conditions of the air layer near the ground and their daily changes.
Key words: microclimate, lapse rate, temperature
1. INTRODUCTION
The air layer near the ground (up to 2 m above the ground) is the most differentiated part of the atmosphere as
far as temperature’s spatial and temporal variability is concerned. Most often the changes in temperature
distribution result from the differences in the active surface and that phenomenon has been explained by
numerous authors, e.g. Oke (1987) or Geiger et al. (1995). The influence of urban areas on air temperature has
also been widely presented in many works, but it was considered mainly in mesoscale, e.g. the urban heat island
or heat balance of a urban canyon. The present paper attempts to show how mesoscale factors influence
microclimatic thermal conditions over the same active surfaces, in diurnal and seasonal course.
2. DATA
The presented analysis is based on the data from the period 1993-1997, from the meteorological stations located
in two sites: Cracow and Gaik-Brzezowa. Cracow is a large city, with about 900,000 habitants. The meteorological
station is placed in the Botanical Garden (f =50o04’N, ?=19o58’E, h=206 m a.s.l.), in the city center. GaikBrzezowa is a small village in the Wieliczka Foothills, about 30 km south-east of Cracow, in the vicinity of the
Dobczyce Water Reservoir. The meteorological station (f =49o52’N, ?=20o04’E, h=302 m a.s.l.) is placed at the
top of a flattened ridge where the influence of general atmospheric circulation is much larger than the one of the
water reservoir (Obrebska-Starklowa, 1995). Both stations belong to the Institute of Geography and Spatial
Management, Jagiellonian University, Cracow, Poland. And at both of them, unique continuous
microclimatological measurements has been carried for many years. Air temperature was measured at both
stations daily at 6, 12 and 18 UTC, at 50 and 200 cm above the ground in Stevenson’s screens, with mercury bulb
thermometers. The values of temperature were used to calculate the temperature differences for the mentioned
air layer of 150 cm thickness, which then were turned into the values of temperature lapse rates (in ºC per 1 m).
Temperature lapse rate gives rather synthetic information about thermal conditions of the air layer near the
ground. Both its value and sign are significant; the value characterizes the intensity of energy flow, while the sign
– its direction, as temperature characterizes the thermal state of air. Negative values denote decrease of
temperature with height, while positive values mark the temperature inversion.
3. METHODS
The daily data were grouped and analyzed in standard seasons. The analysis for each month separately would
produce results of little significance, as only 5-year period was taken under consideration. On the other hand, an
analysis for the whole year would give too generalized results. The sunshine duration and intensity vary
significantly throughout the year and influence to a large extent daily temperature changes, therefore at least
seasonal approach is needed to obtain meaningful results. In the case of daily microclimatological data, a 5-year
period is sufficient for statistical analysis. Even though at both stations air temperature is measured 6 times per
day, only three measurements daily were taken to the analysis, as they define the character of temperature lapse
1
Anita Bokwa, Institute of Geography and Spatial Management, Jagiellonian University, Grodzka 64, 31-044
Krakow, Poland; email: a.bokwa@iphils.uj.edu.pl
rates changes during a day well enough (Bokwa, 2000). The data were analyzed with descriptive statistics and
cluster analysis (k-means method).
4. RESULTS
4.1. Descriptive statistics
Table 1 and Figure 1 present some descriptive statistics of the analyzed data. The results for both sites differ
significantly. The mean values (Fig. 1) show that in Cracow in every season a slight temperature inversion occurs
in the morning and in the evening. In Gaik-Brzezowa, the temperature inversion develops always in the evening,
but in the morning it prevails only in winter, while in other seasons normal stratification is already formed at that
time of day. At noon, at both sites normal stratification prevails, with the highest values in summer and lowest in
winter. However, the seasonal values at noon in Cracow are more differentiated than in Gaik-Brzezowa. In
Cracow, in all seasons, the diurnal course of temperature lapse rate is rather similar, the values for morning and
evening are almost the same, only the noon values change significantly. In Gaik-Brzezowa, in winter, the diurnal
course of temperature lapse rate is almost the same as in Cracow, but in other seasons not only the noon values
but also the morning values change a lot. The differences for morning are even greater than for noon and
comparable with those for noon in Cracow.
Tab.1. Descriptive statistics of the seasonal air temperature lapse rate values, in the air layer 50-200 cm, in
Cracow (Cr) and Gaik-Brzezowa (G-B), 1993-1997, at 6, 12 and 18 UTC; Au – autumn, Wi – winter, Sp – spring,
Su - summer.
AuG-B
AuCr
WiG-B
WiCr
SpG-B
SpCr
SuG-B
SuCr
Mean
12
-0.2
-0.4
-0.1
-0.1
-0.4
-0.5
-0.4
-0.7
6
-0.1
0.1
0.1
0.1
-0.3
0.1
-0.5
0.1
18
0.2
0.2
0.1
0.2
0.3
0.2
0.3
0.1
6
0.05
0.10
0.03
0.06
0.13
0.10
0.10
0.07
Variance
12
0.04
0.08
0.04
0.10
0.07
0.24
0.08
0.19
18
0.08
0.08
0.06
0.11
0.12
0.09
0.09
0.05
6
-0.2
2.6
1.5
1.4
-1.1
4.5
-0.5
0.9
Skewness
12
-0.2
-1.0
1.3
10.2
-0.4
4.8
-0.1
0.5
18
1.0
0.6
1.4
1.4
1.4
1.0
0.8
0.5
6
5.3
53.5
2.9
1.8
0.5
47.6
-0.5
0.3
Kurtosis
12
18
1.3
0.8
1.1
-0.3
7.5
1.8
172.7
1.7
0.2
2.2
68.7
0.5
-0.3
0.6
5.5
-0.1
Fig. 1. Mean seasonal values of air temperature lapse rate (ºC/1 m) for Cracow (A) and Gaik-Brzezowa (B), in the
years 1993-1997, at 6, 12 and 18 UTC
B. Gaik-Brzezowa
A. Cracow
0,5
Sp
0,0
Su
Au
-0,5
lapse rate
lapse rate
0,5
Sp
0,0
Su
-0,5
Au
Wi
Wi
-1,0
-1,0
6
12
UTC
18
6
12
18
UTC
The values of variance (Tab. 1) prove that the data sets are not characterized by a high data dispersion. The
values of skewness and kurtosis allow to state that especially for Cracow, some of the data sets do not follow
normal distribution, e.g. the data for winter are characterized by high positive skeweness and leptokurtic
distribution. That is partially due to some extreme values of lapse rate which occurred in Cracow in the studied
period. For example, on 25th Nov., 1994 at 6 UTC the lapse rate was as high as 3.9ºC/1 m, on 30th Mar., 1994 it
amounted to 4.0ºC/1 m at 6 UTC, and on 20th Jan., 1993 to 5.2ºC/1 m also at 6 UTC.
4.2. Cluster analysis
For Gaik-Brzezowa, daily air temperature lapse rate values in the period 1988-97 at 6, 8, 12, 14, 18 and 20 UTC
were studied in detail (Bokwa, 2000). Cluster analysis was applied to distinguish types of diurnal air temperature
lapse rates’ course. The k-means method was used, as the Ward method turned out to be not useful for the
purpose of constructing the typology. Finally, for winter and spring four types were chosen, while for summer and
autumn – five (Bokwa, 2001, 2002). The data sets from the period 1993-1997 from Gaik-Brzezowa and Cracow
were also analyzed using k-means method, in order to find out what is the differentiation of lapse rate diurnal
course in particular seasons. First the extreme cases, including the mentioned above ones, were eliminated from
the data sets. The assumption of three clusters turned out to be sufficient. The results for summer and autumn
are presented in Figure 2 as examples.
Fig. 2. Mean values of the air temperature lapse rate at 6, 12 and 18 UTC in summer and autumn, in Cracow and
Gaik-Brzezowa (1993-1997), in the clusters obtained with the k-means method.
Gaik-Brzezowa - Summer
1
0,5
0
cluster 1
cluster 2
-0,5
-1
-1,5
cluster 3
6
12
lapse rate
lapse rate
Cracow - Summer
1
0,5
0
-0,5
-1
-1,5
18
cluster 1
cluster 2
cluster 3
6
UTC
Gaik-Brzezowa - Autumn
cluster 1
cluster 2
cluster 3
UTC
18
lapse rate
lapse rate
1
0,5
0
-0,5
-1
-1,5
12
18
UTC
Cracow - Autumn
6
12
1
0,5
0
-0,5
-1
-1,5
cluster 1
cluster 2
cluster 3
6
12
18
UTC
The number of cases in clusters is as follows: Cracow: summer: cluster No. 1 – 195 cases, 2 – 139, 3 – 122;
autumn: 1 – 111, 2 – 89, 3 – 253. For Gaik-Brzezowa: summer: 1-93, 2 – 140, 3 – 227; autumn: 1 – 107, 2 – 86, 3
– 262.
The cluster analysis proved that the changes of the temperature lapse rates during the day are differentiated
depending on season and the local environment’s conditions. In autumn, which is one of the transitional seasons,
the differences between Cracow and Gaik-Brzezowa are not as well seen as for summer. From 6 to 12 UTC the
tendencies of lapse rate changes are completely different. Also the mean values are very different, in the morning
larger in Gaik-Brzezowa than in Cracow, while at noon larger in Cracow than in Gaik-Brzezowa. Cluster analysis
proves additionally that the diurnal changes of lapse rate in a certain season are much more complex than can be
seen from the mean seasonal values (cf. Fig. 1). The results obtained for Gaik-Brzezowa are in accordance with
previous results where six values of lapse rate per day were used (Bokwa, 2000).
5. CONCLUSIONS
The analysis of the air temperature lapse rates in the air layer near the ground presented above is a contribution
to the research of the influence of urban areas on microclimate. The station in Cracow is located in the center of a
large city, in the Botanical Garden, surrounded by buildings of different height. It represents the conditions of a
city center, including the fact that in case of a sunny day, most of the time the area of the station is in the shadow.
That is not the case in Gaik-Brzezowa, and that may be the reason of the significant changes in the lapse rate
values between the two sites in the morning, in every season. Additionally, the microclimate at the station in GaikBrzezowa is controlled much more by the general atmospheric circulation than in Cracow, where the urban
climate is well developed. As proved by numerous studies (summarized e.g. in Lewinska, 2000), the wind speed
in Cracow is usually much lower than outside the city and the urban heat island has two local centers. Cracow is
surrounded by hills from north (the slope of the Krakow-Czestochowa Upland), south (the Carpathian Foreland)
and west (isolated limestone horsts), and located in a bottom of a large valley of the Vistula river, which makes
the area much more humid than the flattened ridge top in Gaik-Brzezowa. Larger values of the lapse rate in
summer at noon in Cracow than in Gaik-Brzezowa express larger thermal differentiation of the air layer near the
ground in the city, which results from weaker ventilation and is experienced by people as overheating. Worth
noting is the fact that at both stations the measurement are taken over the same active surface, which is the
grass. Therefore the observed microclimatic differences are effects of other factors controlling the local climate in
urban and non-urban areas.
References
Bokwa, A., 2000, Struktura termiczna przygruntowej warstwy powietrza na Pogorzu Wielickim (na przykladzie
Gaika-Brzezowej), Ph.D. thesis, Jagiellonian University, Krakow, Poland.
Bokwa, A., 2001, Cluster analysis as a methodof typology of thermal conditions in the air layer near the ground,
Annales Univ. M. Curie-Sklodowska, Sec. B, 50/51, 75-80.
Bokwa, A., 2002, Typologia gradientow termicznych w przygruntowej warstwie powietrza, Roczn. AR w Poznaniu,
338, 15-20.
Geiger, R., Aron, R.H., Todhunter, P., 1995, The Climate Near the Ground, Vieweg: Braunschweig/Wiesbaden.
Lewinska, J., 2000, Klimat miasta. Zasoby, zagrozenia, ksztaltowanie, Instytut Gospodarki Przestrzennej i
Komunalnej Oddzial Krakow: Krakow.
Obrebska-Starklowa, B., 1995, Differentiation of topoclimatic conditions in a Carpathian Foreland valley based on
multiannual observations, Zesz. Nauk. UJ Pr. Geogr., 101, 1-110.
Oke, T.R., 1987, Boundary Layer Climates, Methuen: London and New York.