3.2.2 Temperature, Temperature Difference, and Humidity
The siting and exposure criteria for temperature, temperature difference and humidity are
similar. Consequently, these variables are discussed as a group in the following; exceptions are
noted as necessary.
220.127.116.11 Probe placement
Ambient temperature and humidity should be measured at 2 m, consistent with the World
Meteorological Organization (WMO) standards for ambient measurements . Probe placement
for temperature difference measurements depend on the application.. For use in estimating
surface layer scaling parameters (Section 6.6.4), the temperature difference should be measured
between 20z0 and 100z0; the same recommendation applies to temperature difference
measurements for use in estimating the P-G stability category using the solar radiation delta-T
method (Section 18.104.22.168). For use in estimating stable plume rise, temperature difference
measurements should be made across the plume rise layer, a minimum separation of 50 m is
recommended. For sites that experience large amounts of snow, adjustments to the temperature
measurement height may be necessary, however, the ambient temperature measurement should
not extend above 10 m. For analysis of cooling tower impacts, measurements of temperature and
humidity should also be obtained at source height and within the range of final plume height.
The measurement of temperature difference for analysis of critical dividing streamline height,
Hcrit, a parameter used in complex terrain modeling, is discussed in Section 3.3.3.
Temperature and humidity sensors should be located over an open, level area at least 9 m
in diameter. The surface should be covered by short grass, or, where grass does not grow, the
natural earth surface [2, 9]. Instruments should be protected from thermal radiation (from the
earth, sun, sky, and any surrounding objects) and adequately ventilated using aspirated shields.
Forced aspiration velocity should exceed 3 m/s, except for lithium chloride dew cells which
operate best in still air . If louvered shelters are used instead for protection (at ground level
only), then they should be oriented with the door facing north (in the Northern Hemisphere).
Temperature and humidity data obtained from naturally-ventilated shelters will be subject to
large errors when wind speeds are light (less than about 3 m/s).
Temperature and humidity sensors on towers should be mounted on booms at a distance
of about one diameter/diagonal of the tower (from the nearest point on the tower) . In this
case, downward facing aspiration shields are necessary.
Temperature and humidity sensors should be located at a distance of at least four times
the height of any nearby obstruction and at least 30 m from large paved areas , . Other
situations to avoid include: large industrial heat sources, rooftops, steep slopes, sheltered
hollows, high vegetation, shaded areas, swamps, areas where frequent snow drifts occur, low
places that hold standing water after rains, and the vicinity of air exhausts (e.g., from a tunnel or
subway) [2, 9].
22.214.171.124 Siting considerations
In siting temperature sensors, care must be taken to preserve the characteristics of the
local environment, especially the surface. Protection from thermal radiation (with aspirated
radiation shields) and significant heat sources and sinks is critical. Siting recommendations are
similar for humidity measurements, which may be used for modeling input in situations
involving moist releases, such as cooling towers. For temperature difference measurements,
sensors should be housed in identical aspirated radiation shields with equal exposure.
3. SITING AND EXPOSURE
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
3.3 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