3.5 Urban Locations
Urban
areas are characterized by increased heat flux and surface roughness. These
effects, which vary horizontally and vertically within the urban area, alter
the wind pattern relative to the outlying rural areas (e.g., average wind
speeds are decreased). The close proximity of buildings in downtown urban
areas often precludes strict compliance with the previous sensor exposure
guidance. For example, it may be necessary to locate instruments on the roof
of the tallest available building. In such cases, the measurement height
should take into account the proximity of nearby tall buildings and the
difference in height between the building (on which the instruments are
located) and the other nearby tall buildings.
In
general, multiple sites are needed to provide representative measurements in
a large urban area. This is especially true for ground-level sources, where
low-level, local influences, such as street canyon effects, are important,
and for multiple elevated sources scattered over an urban area.
However, due to the limitations of the recommended steady-state guideline
models (i.e. they recognize only a single value for each input variable on
an hourly basis), and resource and practical constraints, the use of a
single site is necessary. At the very least, the single site should be
located as close as possible to the source in question.
3. SITING AND EXPOSURE
3.1 Representativeness
3.1.1 Objectives for Siting
3.1.2 Factors to Consider
3.2 Simple Terrain Locations
3.2.1 Speed and Wind Direction
3.2.2 Temperature, Temperature
Difference, and Humidity
3.2.3 Precipitation
3.2.4 Pressure
3.2.5 Radiation
3.3 Complex Terrain Locations
3.3.1 Wind Speed
3.3.2 Wind Direction
3.3.3 Temperature Difference
3.4 Coastal Locations
3.5 Urban Locations
3.6 Recommendations