Urban impacts on climate - The Global Climate System: Patterns, Processes, and Teleconnections - page 224

AIR

Figure 7.6 A classification
of the most commonly
encountered urban heat
island types with examples
of how they may be
measured. (After Oke 1995 :
permission from Kluwer
Publishers)

UBL UCL

Fixed
e.g. tower,
sodar

Traverse
e.g. aircraft,
tetroon

Fixed
e.g. weather
station

Traverse
e.g. car mounted

Standard exposure
e.g. grass

Non-standard
e.g. roof, roadside

SURFACE

“True” 3-d
surface T
distribution

2-d
bird's-eye T
e.g. satellite

Zero-plane T
e.g. 1-d
model output

Ground or single
surface T
e.g. road

SUBSURFACE

Thus, satellite-based observations of surface temperatures must be interpreted
with care, if conducted in the absence of consideration of geometric condi-
tions of the surface and the viewing conditions.

7.3.3 Identifying urban effects
Documenting urban effects is often difficult. Only rarely are extensive pre-
urban measurements of the climate of a region available against which urban
observations can be compared. More commonly, urban data are collected at a
point and compared with those at a nearby rural ''reference site'' and urban
effects are defined in terms of urban-rural differences.
Returning to the urban effect on air temperature (T ), in this framework, the
UHI can be defined as:

T u r ¼ T u T r

Where, the subscripts u and r refer to the urban area and the non-urban
reference site, respectively. Close attention must be paid to reference sites
used in such comparisons. The reference station, although outside the urban
area, may lie downwind and be subjected to urban influences. Moreover,
many rural areas are significantly influenced by human activities, for exam-
ple, agricultural development, irrigation, etc. Such sites do not necessarily
provide a surrogate for conditions in the city pre-urban development.