Urban impacts on climate - The Global Climate System: Patterns, Processes, and Teleconnections - page 236

Figure 7.9 The maximum
urban heat island intensity
( T u-r (max)) versus urban
sky view factor ( Y s). (After
Oke 1981 , Table III) The
equation is T u-r (max) ¼
13.3 USVFþ14.86
(r 2 ¼0.87, standard error of
the estimate ¼0.96 8 C).
(From Brazel and Quatrocchi
2005 )
maximum air UHI and Ys between European, North American, and Australian
cities, but there are no reported results for cities in developing countries, such as
China.
In G¨ teborg, with an average UHI of 2 to 4.5 8C, which is strongest in the
summer, the role of urban canyons in establishing the strength and spatial
distribution of the air UHI was investigated in depth, using Ys and temperature
measurements on various canyon surfaces (Eliasson 1994 , 1996 ). In the old city,
Ys ranged from 0.25 to 0.45, but in the western section, were higher, 0.5 to 1.0
(in open parkland). Overall the canyon Ys had warmer UHI of about 1 8C in all
seasons, but the relation is very complex.
There was a significant correlation (p < 0.05) between Ys and surface tem-
perature, but the correlation between Ys and air temperature was poor. Detailed
temperature measurements in the canyons established that the surface tempera-
ture variations (partially due to differential rates of cooling) do not translate very
well into the air temperature above 0.1m in height (Figure 7.5 ). Whether within
a canyon, or in a traverse across the city central area, the urban air temperature
distribution in central G¨ teborg remained relative uniform ( < 3 8C variation).
Urban canyons also create major changes in both the wind speed and wind
direction of the prevailing airflow (Johnson and Hunter 1999 ). If the prevailing
wind is parallel to the direction of the canyon, then down-canyon airflow occurs.
The major influence will be friction, variations in canyon orientation, and gaps
in the canyon sides (side streets and alleyways). If winds are light and variable, it
is difficult to establish a relation between the wind flow within the canyon, its