Page 397 - Fundamentals of Air Pollution 3E

II. Representativeness 351

TABLE 21-3
Average Wind Speed and Frequency of Calms
Nashville Knoxville
Average wind speed 3.79ms" 1 3.26 m s" 1
Percentage of calms 9.21 9.77

Knoxville compared to Nashville. The stability comparisons are given in
Table 21-4. The frequencies are very nearly the same, with only A and B
stabilities being slightly greater at Knoxville at the expense of the D stability.
Stability G is more stable than Pasquill's F.
The maximum number of hours of each stability class in a single wind
direction is given in Table 21-5. The total hours for A, C, and D stabilities
are nearly the same. The maximum number of hours of B stability, with
winds from a single direction, is about 50% higher at Knoxville. For all
three stable cases, E, F, and (G), the maximum number of hours at Knoxville
is about two- thirds that at Nashville.
The frequency of occurrence of winds from each of the 36 directions for
the two sites is given in the second and third columns of Table 21-6.
Knoxville has its maximum from 240°, with frequent winds also from adja-
cent points. There is a secondary maximum across the compass (in the
vicinity of 60°). Nashville, on the other hand, seems to have only one
principal maximum (from 180°). To explore the magnitude of the differ-
ences, the fourth column is the difference for each direction. Summing the
absolute values of the differences and expressing them as a percentage of
the total number of hours of observations shows that the differences consti-
tute 62% of the total.

TABLE 21-4
Occurrence of Pasquill Stability Classes

Nashville Knoxville
Stability
class Percent Cumulative Percent Cumulative
(G) 7.64 7.64 7.35 7.35
F 14.15 21.79 15.57 22.92
E 14.18 35.97 14.67 37.59
D 45.14 81.11 41.42 79.01
C 12.37 93.48 12.32 91.33
B 5.75 99.23 7.21 98.54
A 0.77 100.00 1.46 100.00