
_{A} method The
_{A}
method (Tables 69a and 69b) is a turbulencebased method which uses the
standard deviation of the wind direction in combination with the scalar mean
wind speed. The criteria in Table 69a and Table 69b are for data collected
at 10 m and a roughness length of 15 cm. Wind speed and direction data
collected within the height range from 20z_{o} to 100z_{o} should be used. For
sites with very low roughness, these criteria are slightly modified. The
lower bound.619 measurement height should never be less than 1 m.
The upper bound should never be less than 10 m. To obtain 1hour averages,
the recommended sampling duration is 15 minutes, but it should be at least 3
minutes and may be as long as 60 minutes. The relationships employed in the
estimation methods assume conditions are steady state. This is more easily
achieved if the sampling duration is less than 30 minutes. To minimize the
effects of wind meander, the 1hour _{A}
is defined using 15minute values (see Equation. 6.2.10).
Table 69a
Lateral
Turbulence a Criteria for Initial Estimate of PasquillGifford (PG)
Stability Category. For use with Table 68b 
Initial
estimate of PG stability category  Standard
Deviation of wind azimuth angle _{A} 
A  22.5
_{A} 
B  17.5
_{A
}< 22.5 
C  12.5
_{A
}< 17.5 
D  7.5
_{A
}< 12.5 
E  3.8
_{A
}< 7.5 
F  _{A
}< 3.8 
a As indicated
by the standard deviation of the azimuth angle of the wind vector, .
SigmaA, SigmaTheta, and _{A}
are aliases for 
Table 69b
Wind Speed Adjustments for
Determining Final Estimate of PG Stability
Category from A. For use with Table 69a 
 Initial
Estate of PG Category  10meter
wind speed (m/s)  Final
Estimate of PG Category 
Daytime  A  u
< 3  A 
A  3
u
< 4  B 
A  4
u <6  C 
A  6
u  D 
B  u
< 4  B 
B  4
u < 6  C 
B  6
u  D 
C  u
< 6  C 
C  6
u  D 
D,
E, or F  ANY  D 
   
Nighttime  A  u
< 2.9  F 
A  2.9
u
< 3.6  E 
A  3.6
u  D 
B  u
< 2.4  F 
B  2.4
u
< 3.0  E 
B  3.0
u  D 
C  u
< 2.4  E 
C  2.4
u  D 
D  ANY  D 
E  u
< 5  E 
E  5
u  D 
E   
F  u
< 3  F 
F  3
u
< 5  E 
F  5
u  D 
If
the site roughness length is other than 15 cm, the category boundaries
listed in Table 69a may need adjustment. As an initial adjustment, multiply
the values listed by: (z_{o}
/15) ^{0·2} where
z_{o} is the site roughness in centimeters. This factor, while theoretically
sound, has not had widespread testing. It is likely to be a useful
adjustment for cases when z_{o} is greater than 15 cm. It is yet problematical
whether the adjustment is as useful for cases when z_{o} is less than 15 cm. If
the measurement height is other than 10 m, the category boundaries listed in
Table 69a will need adjustment. As an initial adjustment, multiply the
lower bound values listed by: (Z/10)^{P} where
Z is the measurement height in meters. The
exponent P is a function
of the PG stability category with values as follows:
PG
Stability  P_{
} 
A  
0.06 
B  
0.15 
C  
0.17 
D  
0.23 
E  
0.38 
The
above suggestions summarize the results of several studies conducted in
fairly ideal circumstances. It is anticipated that readers of this document
are often faced with conducting analyses in less than ideal circumstances.
Therefore, before trusting the Pasquill category estimates, the results
should be spot checked. This can easily be accomplished. Choose cloudless
days. In midafternoon during a sunny day, categories A and B should occur.
During the few hours just before sunrise, categories E and F should occur.
The bias, if any, in the turbulence criteria will quickly be revealed
through such comparisons. Minor adjustments to the category boundaries will
likely be needed to tailor the turbulence criteria to the particular site
characteristics, and should be made in consultation with the reviewing
agency. 6. METEOROLOGICAL DATA PROCESSING
6.1 Averaging and Sampling Strategies
6.2 Wind Direction and Wind Speed
6.2.1 Scalar Computations
6.2.2 Vector Computations
6.2.3 Treatment of Calms
6.2.4 Turbulence
6.2.5 Wind Speed Profiles
6.3 Temperature
6.3.1 Use in PlumeRise Estimates
6.3.2 Vertical Temperature Gradient
6.4 Stability
6.4.1 Turner's method
6.4.2 Solar radiation/deltaT (SRDT) method
6.4.3
_{E} method
6.4.4 _{A}method
6.4.5 Accuracy of stability category estimates
6.5 Mixing Height
6.5.1 The Holzworth Method
6.6 Boundary Layer Parameters
6.6.1 The Profile Method
6.6.2 The Energy Budget Method
6.6.3 Surface Roughness Length
6.6.4 Guidance for Measurements in the Surface Layer
6.7 Use of Airport Data
6.8 Treatment of Missing Data
6.8.1 Substitution Procedures
6.9 Recommendations
 
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