Which Pasquill stability class represents the most ‘unstable’ atmospheric condition, typically occurring on a sunny day with low wind speed?
- AClass ACorrect
- BClass D
- CClass F
- DClass G
Solution
Concept
Pasquill-Gifford atmospheric stability categories
Formula
Pasquill Scale: A = Extremely unstable, D = Neutral, F = Moderately stable
Step-by-step solution
1. Atmospheric stability is classified from Class A to Class F based on surface wind speed and solar radiation. 2. Class A designates extremely unstable conditions characterized by intense thermal convection on sunny days with light winds. 3. Class D represents neutral stability (overcast or windy conditions). 4. Class F represents stable conditions with nighttime inversions. 5. Therefore, Class A is the most unstable condition. Answer: A
Explanation
The Pasquill-Gifford atmospheric stability classification categorizes atmospheric dispersion into six classes: A (Extremely Unstable), B (Moderately Unstable), C (Slightly Unstable), D (Neutral), E (Slightly Stable), and F (Moderately Stable). Class A represents the most unstable atmosphere, driven by strong solar insolation during clear sunny days with light surface winds (<2 m/s), producing vigorous thermal turbulence.
Why the other options are wrong
B) Class D represents neutral atmospheric stability typical of overcast skies or high wind speeds. C) Class F represents moderately stable nighttime conditions with ground-based inversions. D) Class G is occasionally used for extremely stable nocturnal conditions.
Shortcut
Pasquill Class A = Extremely Unstable (Sunny + Low Wind); Class D = Neutral; Class F = Stable (Night).
Common mistake
Selecting Class F or D, confusing stability (calm/inversion) with instability (turbulent mixing).
Correct answer: A) Class A