Page 396 - Fundamentals of Air Pollution 3E

350 21. Air Pollution Climatology

The use of a measurement generally dictates the circumstances of data
collection. For example, to provide a best estimate of plume transport
direction, hour by hour, of a release from a 75-m stack, a wind vane at the
100-m level of a tower will probably provide more representative wind
direction measurements than a vane at 10 m above ground. If the release
has buoyancy so that it rises appreciably before leveling off, even the
100-m measurement may not be totally adequate.
If one is studying the transport of material through the tree canopy of
a forest, it is most desirable to disturb the natural environment as little as
possible in making a wind measurement in the canopy. An extremely
sensitive wind system is necessary because one would expect the winds
to be extremely light. Also, it may be necessary to make supporting mea-
surements both above and below the canopy, so that a wind speed profile
is obtained.
Frequently, it is necessary to determine whether a record made at one
location can be used to infer values of a particular parameter at another
location. Many factors help determine whether one location's measure-
ments of a parameter are similar to those made at another location, such as
the parameter in question, distance between sites, intervening topography,
local topography, surface roughness, local vegetative cover, and the height
above ground of the measurement. Generally, no measurements are made
at both sites to check for comparability; if there were, there would be little
need to use measurements at one site as a substitute for those at the other.
The foregoing factors are not independent. Two sites 50 km apart in
Kansas, a state that has quite flat terrain largely planted in field crops, may
be much more representative of one another than two sites 5 km apart in
more rugged mountainous, wooded terrain, e.g., in the western part of
North Carolina.
Data for one full year (1964) for Nashville, Tennessee, and Knoxville,
Tennessee, 265 km (165 mi) apart, were compared to determine the extent
to which the frequencies of various parameters were similar. Knoxville is
located in an area with mountainous ridges oriented southwest-northeast;
Nashville is situated in a comparatively flat area. The data available are the
number of hours during which each of 36 wind directions (every 10° azi-
muth) occurred, the average wind speed for each direction, the number of
hours of each Pasquill stability class for each direction, and the mean annual
wind speed.
Table 21-3 indicates that average wind speeds are lower in Knoxville,
where the average wind is 0.86 of that at Nashville. Considering the differ-
ence in topography, it is surprising that the difference is not greater. The
percentage of calms, slightly greater at Knoxville, is entirely consistent with
the wind speeds.
Because of the slight difference in average wind speed, one might expect
this to cause a greater frequency of both stable and unstable conditions at