311
winter were made with the use of ENVI-Met software in the Vienna University of
Technology, Department of Building Physics and Building Ecology, Institute of
Architectural Sciences.
11.4 The Role of Urban Vegetation in Reduction
of UHI – The Results of Microclimatic Research
The variation of air temperature (Ta) and heat stress (UTCI) inside residential districts was examined on two sunny days, 21–22 May 2013, in two housing estates,
Koło and Włodarzewska, which differ in the provision of green areas as well as in
the arrangement of buildings (Fig. 11.3a, b ). On the measuring days, the wind was
weak (<2 m · s
−1 ), the mornings had clear sky, Cumulus clouds were created during
the day and a short 10-min shower occurred on 22 May. Global solar radiation in
Koło was more differentiated inside the estate than in Włodarzewska, which signifi -
cantly infl uenced the calculated UTCI values. The air temperatures in the housing
estates and in the reference station (Powsin) fl uctuated between 8 and 15 °C in the
mornings and 20 and 23 °C during the middle of the day.
The differences in air temperature in various micro structures inside both housing estates ranged from 2.5 to 2.8 °C in the early morning to 3.3 to 3.5 °C at noon,
reaching higher values in Włodarzewska. At midday, the spatial differences in air
temperature increased due to an increase in solar radiation (up to about 600–800
W⋅m
−2
). The warmest air was above artifi cial surfaces (asphalt, concrete), at well
insulated sites and the coldest air was found over natural, shaded surfaces (lawns
under trees). The wind tunnel effect was clearly seen at post 2 (close to the tunnel
under the block of fl ats) and 5 (between two buildings) in Koło, and at post 5 (on a
street along a long block of fl ats) in Włodarzewska.
The spread of UTCI values inside the analysed estates was 3 times greater than
that of air temperature, though they mostly fall within one heat stress category,
named “no thermal stress”. In the Koło estate, the differences in UTCI values
between posts at individual points in time reached 10.5 °C in the morning. The coldest was recorded on a vast lawn in the centre of the estate, surrounded by high trees
and buildings (post 3), and the UTCI values indicated “slight cold stress” there. The
warmest was a calm site under a canopy of high trees (post 1), where, at midday, the
differences in UTCI reached 10.4 °C. The coldest was a shaded site under a tree
canopy (post 1) and the warmest – an asphalt surfaced parking area (post 6), where
moderate heat stress was noted.
In the Włodarzewska estate, the differences in UTCI values were much smaller
and they reached only 6.7 °C in the morning. The coldest were pavements next to a
block of fl ats, the windiest in the estate (post 5), and the warmest – a calm and sunny
square between buildings (post 6). At midday, the spread of UTCI values was up to
6.8 °C. The warmest was a small lawn with young trees, squeezed between blocks
of fl ats (post 4).
11 Urban Heat Island and Bioclimatic Comfort in Warsaw
winter were made with the use of ENVI-Met software in the Vienna University of
Technology, Department of Building Physics and Building Ecology, Institute of
Architectural Sciences.
11.4 The Role of Urban Vegetation in Reduction
of UHI – The Results of Microclimatic Research
The variation of air temperature (Ta) and heat stress (UTCI) inside residential districts was examined on two sunny days, 21–22 May 2013, in two housing estates,
Koło and Włodarzewska, which differ in the provision of green areas as well as in
the arrangement of buildings (Fig. 11.3a, b ). On the measuring days, the wind was
weak (<2 m · s
−1 ), the mornings had clear sky, Cumulus clouds were created during
the day and a short 10-min shower occurred on 22 May. Global solar radiation in
Koło was more differentiated inside the estate than in Włodarzewska, which signifi -
cantly infl uenced the calculated UTCI values. The air temperatures in the housing
estates and in the reference station (Powsin) fl uctuated between 8 and 15 °C in the
mornings and 20 and 23 °C during the middle of the day.
The differences in air temperature in various micro structures inside both housing estates ranged from 2.5 to 2.8 °C in the early morning to 3.3 to 3.5 °C at noon,
reaching higher values in Włodarzewska. At midday, the spatial differences in air
temperature increased due to an increase in solar radiation (up to about 600–800
W⋅m
−2
). The warmest air was above artifi cial surfaces (asphalt, concrete), at well
insulated sites and the coldest air was found over natural, shaded surfaces (lawns
under trees). The wind tunnel effect was clearly seen at post 2 (close to the tunnel
under the block of fl ats) and 5 (between two buildings) in Koło, and at post 5 (on a
street along a long block of fl ats) in Włodarzewska.
The spread of UTCI values inside the analysed estates was 3 times greater than
that of air temperature, though they mostly fall within one heat stress category,
named “no thermal stress”. In the Koło estate, the differences in UTCI values
between posts at individual points in time reached 10.5 °C in the morning. The coldest was recorded on a vast lawn in the centre of the estate, surrounded by high trees
and buildings (post 3), and the UTCI values indicated “slight cold stress” there. The
warmest was a calm site under a canopy of high trees (post 1), where, at midday, the
differences in UTCI reached 10.4 °C. The coldest was a shaded site under a tree
canopy (post 1) and the warmest – an asphalt surfaced parking area (post 6), where
moderate heat stress was noted.
In the Włodarzewska estate, the differences in UTCI values were much smaller
and they reached only 6.7 °C in the morning. The coldest were pavements next to a
block of fl ats, the windiest in the estate (post 5), and the warmest – a calm and sunny
square between buildings (post 6). At midday, the spread of UTCI values was up to
6.8 °C. The warmest was a small lawn with young trees, squeezed between blocks
of fl ats (post 4).
11 Urban Heat Island and Bioclimatic Comfort in Warsaw