Largely copied from this site
The atmospheric stability is important in determination of lateral and vertical horizontal dispersion parameters. The most commonly used classification is that of Pasquill (1961), later modified by Gifford (1961), and referred to as the Pasquill-Gifford (P-G) stability class. Unstable conditions are represented by the letter ‘A’ (or the number 1), while increasingly more stable conditions are denoted with successive letters of the alphabet, such that extremely stable conditions are represented by ‘F’ and/or ‘G’ (6 and/or 7). Neutral atmospheric conditions are given by the ‘D’ (4) classification. Turner (1964) devised a scheme for determining the stability class of the atmosphere using data customarily collected at NWS sites by considering radiative and wind speed effects. Net radiation calculations require knowledge of total cloud cover, ceiling height and solar altitude (itself a function of the time of day and year, as well as geographic location). The following tables, reproduced from EPA-454/R-99-005 (2000), provide the mechanism for determining the P-G stability classes.
The solar altitude, q, is the angle the sun makes with respect to the earth’s surface. When the sun is directly overhead (i.e. ‘high noon’), q = 90° leading to the strongest insolation, while weak insolation occurs when the sun is near the horizon. This angle is uniquely determined from the latitude and longitude of the location of interest, the time of day, as well as the time of year. Once determined, an insolation class number (IN) is defined, as given in Table A.1.
The Pasquill-Gifford stability categories (A through G) as a function of wind speed and net radiation index (NRI) are shown in Table A.2. Once the solar altitude and insolation class number are known (Table A.1), the procedure shown in Table A.3 requiring the total cloud cover (TC) and ceiling height (CH) is used to determine the NRI. Knowing the NRI and wind speed for a given time yields the P-G stability class (Table A.2).
Table A.1: Insolation class number (IN) as a function of solar altitude, q.
Solar altitude (°)
Insolation
IN
60 <= q
Strong
4
35 <= q < 60
Moderate
3
15 <= q < 35
Slight
2
q <= 15
Weak
1
Table A.2: Turner’s conversion from NRI to stability class.
Wind speed
Net Radiation Index (NRI)
[m/sec]
[knots]
0
-1
-2^{*}
0 to 0.7
0, 1
A
B
C
D
F
G
0.8 to 1.8
2, 3
1.9 to 2.8
4, 5
E
2.9 to 3.3
6
3.4 to 3.8
7
3.9 to 4.8
8, 9
4.9 to 5.4
10
5.5 to 5.9
11
> 6.0
> 12
Table A.3: Procedure to determine Net Radiation Index (NRI). Remark: The conversion from feet to meters seems to be wrong (using a wrong conversion value 0.3281 instead of 0.3048 [a conversion direction error?])