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Appendix A: Pasquill Turbulence Types

Table A1: Meteorological conditions defining Pasquill turbulence types*

Surface wind speed
m/s
Daytime insolation Night-time conditions**
Strong Moderate Slight Thin overcast or > 4/8 low cloud ≤ 3/8 cloudiness

< 2

A

A-B

B

   

2-3

A-B

B

C

E

F

3-4

B

B-C

C

D

E

4-6

C

C-D

D

D

D

> 6

C

C

D

D

D

Key: A: Extremely unstable; B: Moderately unstable; C: Slightly unstable; D: Neutral#;E: Slightly stable; F: Moderately stable.

Notes :

* From FA Gifford, 1976, Turbulent diffusion-typing schemes: A review, Nuc. Saf 17(1): 71.

# Applicable to heavy overcast day or night.

** The degree of cloudiness is defined as that fraction of the sky above the local apparent horizon that is covered by clouds.