51
• For an urban environment, there is currently no standard method for extrapolating air temperature data down towards screen level (2 m in height).
Other facts to consider during the placement of a thermometer in urban
environments:
• Careful attention to radiation shielding and ventilation is highly recommended.
• Use of the same sensor assemblies (with/without shields and ventilation) inside
a network is strongly advised in order to avoid inter-site differences.
Surface temperature is not commonly measured at urban stations; its measurement is only possible using infrared remote sensing or a downward-facing pyrgeometer, or by employing one or more radiation thermometers for which the
combined field of view covers a representative sample of the urban district.
2.4.2 Humidity
The instruments normally used for measuring humidity can be used in urban areas.
The siting and exposure of air temperature sensors presented in the previous chapter
apply equally to humidity sensors in the Urban Canopy Layer and above the
Roughness Sublayer (see Figs. 2.6 and 2.7). For humidity measurement, the same
sensor is used as in temperature sampling. Because urban environments are far dirtier than rural sites (in terms of dust, oils, and pollutants), thermometers and hygrometers require increased maintenance and frequent service. Yearly changing and
calibration are strongly advised. The provision of shielding from extraneous sources
of solar and long-wave radiation is also recommended.
2.4.3 Wind Speed and Direction
Wind speed and direction are very sensitive to flow distortion by obstacles
including
• effects of local relief due to hills, valleys, and cliffs,
• sharp changes in roughness or in the effective surface elevation (z d : zero-plane
displacement length),
• perturbation of flow around clumps of trees and buildings,
• obstacles in the form of individual trees and buildings (Fig. 2.8),
• disturbances induced by the physical bulk of the tower or mounting arm to which
the instrument is attached.
In choosing the height for wind measurement, some basic principles should be
considered.
2 Urban Heat Island Gold Standard and Urban Heat Island Atlas
• For an urban environment, there is currently no standard method for extrapolating air temperature data down towards screen level (2 m in height).
Other facts to consider during the placement of a thermometer in urban
environments:
• Careful attention to radiation shielding and ventilation is highly recommended.
• Use of the same sensor assemblies (with/without shields and ventilation) inside
a network is strongly advised in order to avoid inter-site differences.
Surface temperature is not commonly measured at urban stations; its measurement is only possible using infrared remote sensing or a downward-facing pyrgeometer, or by employing one or more radiation thermometers for which the
combined field of view covers a representative sample of the urban district.
2.4.2 Humidity
The instruments normally used for measuring humidity can be used in urban areas.
The siting and exposure of air temperature sensors presented in the previous chapter
apply equally to humidity sensors in the Urban Canopy Layer and above the
Roughness Sublayer (see Figs. 2.6 and 2.7). For humidity measurement, the same
sensor is used as in temperature sampling. Because urban environments are far dirtier than rural sites (in terms of dust, oils, and pollutants), thermometers and hygrometers require increased maintenance and frequent service. Yearly changing and
calibration are strongly advised. The provision of shielding from extraneous sources
of solar and long-wave radiation is also recommended.
2.4.3 Wind Speed and Direction
Wind speed and direction are very sensitive to flow distortion by obstacles
including
• effects of local relief due to hills, valleys, and cliffs,
• sharp changes in roughness or in the effective surface elevation (z d : zero-plane
displacement length),
• perturbation of flow around clumps of trees and buildings,
• obstacles in the form of individual trees and buildings (Fig. 2.8),
• disturbances induced by the physical bulk of the tower or mounting arm to which
the instrument is attached.
In choosing the height for wind measurement, some basic principles should be
considered.
2 Urban Heat Island Gold Standard and Urban Heat Island Atlas