_A method

The _A method (Tables 6-9a and 6-9b) is a turbulence-based method which uses the standard deviation of the wind direction in combination with the scalar mean wind speed. The criteria in Table 6-9a and Table 6-9b 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.6-19 measurement height should never be less than 1 m. The upper bound should never be less than 10 m. To obtain 1-hour 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 1-hour _A is defined using 15-minute values (see Equation. 6.2.10).

Table 6-9a  Lateral Turbulence a Criteria for Initial Estimate of Pasquill-Gifford (P-G) Stability Category. For use with Table 6-8b
Initial estimate of P-G 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, . Sigma-A, Sigma-Theta, and A are aliases for

Table 6-9b Wind Speed Adjustments for Determining Final Estimate of P-G Stability Category from A. For use with Table 6-9a
	Initial Estate of P-G Category	10-meter wind speed (m/s)	Final Estimate of P-G 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 6-9a 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 6-9a 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 P-G stability category with values as follows: 

P-G 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 mid-afternoon 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 Plume-Rise Estimates  
      6.3.2 Vertical Temperature Gradient 
  6.4 Stability 
      6.4.1 Turner's method  
      6.4.2 Solar radiation/delta-T (SRDT) method 
      6.4.3  _E method 
      6.4.4 _Amethod 
      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