293
10.2.2.2.2 Trees
PET in 1.5 m height was found to be around 10 K lower under trees compared to
green areas and 25 K lower than over asphalt (Fig. 10.9 ). Therefore, shading by
trees could reduce the frequency of daytime heat stress signifi cantly (Fig. 10.10 ).
The increasing number of trees in the Olga Hospital area has no signifi cant
impact on the averaged air temperature during moderate warm conditions. However,
during hot summer days it could reduce the air temperature in this area by 3.0 K
(spatial average).
10.2.2.2.3 Green Roofs
The effect of green roofs was quantifi ed using ENVI-met and by changing all roofs
of the hospital scenario into green roofs. The effect of green roofs on the local
thermal conditions experienced by humans on street level are on a very low level
(ΔPET < 0.06 K). The local air temperature differences on street level are even
lower. However, green roofs signifi cantly reduce the warming of urban roof surfaces
in daytime. Inside green roofs the accumulation of heat is decreased, resulting in a
lesser heat emission in nighttime. A large-scale revegetate of roofs is an effectively
measure for the mitigation of UHI intensity especially in nightime.
10.2.2.2.4 Urban Morphology
The urban morphology is analyzed using RayMan Pro (Matzarakis et al. 2007 , 2010 ).
The morphology of street canyons infl uences solar access and radiation and therefore
thermal comfort. The importance of solar access for city dwellers depends on the
climate zone. While south of the Alps sun is considered as harmful, solar access is
favored in northern cities such as Stuttgart. East-west oriented street canyons do not
have solar access during winter months due to the low zenith angle of the sun. But
during summer, the street canyon and especially the northern façade is illuminated
during the whole day. Accordingly high is the frequency of heat stress in this E-W
oriented street canyon. A N-S oriented street canyon is accessed by sunshine during
the midday hours throughout the year (Ketterer and Matzarakis 2014b ).
The daily maximum value of PET could be reduced by 10 K due to a changing
H/W ratio from 0.5 to 3.5 and an orientation of 120° on a hot summer day.
Throughout the year, the frequency of heat stress can be reduced by 477 h (4.3 %).
Additionally, the occurrence of thermal comfort conditions could be increased by
10 %. However, a change in H/W ratio from 0.5 to 1 (2.5) could already reduce the
frequency of heat stress by 192 (333) hours per year (Fig. 10.10 ).
10 Pilot Actions in European Cities – Stuttgart
10.2.2.2.2 Trees
PET in 1.5 m height was found to be around 10 K lower under trees compared to
green areas and 25 K lower than over asphalt (Fig. 10.9 ). Therefore, shading by
trees could reduce the frequency of daytime heat stress signifi cantly (Fig. 10.10 ).
The increasing number of trees in the Olga Hospital area has no signifi cant
impact on the averaged air temperature during moderate warm conditions. However,
during hot summer days it could reduce the air temperature in this area by 3.0 K
(spatial average).
10.2.2.2.3 Green Roofs
The effect of green roofs was quantifi ed using ENVI-met and by changing all roofs
of the hospital scenario into green roofs. The effect of green roofs on the local
thermal conditions experienced by humans on street level are on a very low level
(ΔPET < 0.06 K). The local air temperature differences on street level are even
lower. However, green roofs signifi cantly reduce the warming of urban roof surfaces
in daytime. Inside green roofs the accumulation of heat is decreased, resulting in a
lesser heat emission in nighttime. A large-scale revegetate of roofs is an effectively
measure for the mitigation of UHI intensity especially in nightime.
10.2.2.2.4 Urban Morphology
The urban morphology is analyzed using RayMan Pro (Matzarakis et al. 2007 , 2010 ).
The morphology of street canyons infl uences solar access and radiation and therefore
thermal comfort. The importance of solar access for city dwellers depends on the
climate zone. While south of the Alps sun is considered as harmful, solar access is
favored in northern cities such as Stuttgart. East-west oriented street canyons do not
have solar access during winter months due to the low zenith angle of the sun. But
during summer, the street canyon and especially the northern façade is illuminated
during the whole day. Accordingly high is the frequency of heat stress in this E-W
oriented street canyon. A N-S oriented street canyon is accessed by sunshine during
the midday hours throughout the year (Ketterer and Matzarakis 2014b ).
The daily maximum value of PET could be reduced by 10 K due to a changing
H/W ratio from 0.5 to 3.5 and an orientation of 120° on a hot summer day.
Throughout the year, the frequency of heat stress can be reduced by 477 h (4.3 %).
Additionally, the occurrence of thermal comfort conditions could be increased by
10 %. However, a change in H/W ratio from 0.5 to 1 (2.5) could already reduce the
frequency of heat stress by 192 (333) hours per year (Fig. 10.10 ).
10 Pilot Actions in European Cities – Stuttgart