Discussion
Two identical sources are modelled, one on an overcast windy day and one on a clear calm night.
Which produces the higher ground level concentration near the source?
Answer: B
Correct answer: The calm night, on both stability and wind
The calm night gives the higher concentration, and for two reinforcing reasons. Stability is greater - a radiation inversion suppresses vertical mixing, confining the emission to a shallow layer - and the wind is lighter, so the same emission rate is diluted into less air per second, with concentration going as \( 1/u \).
The overcast windy day is the opposite on both counts: cloud prevents strong surface heating and the wind supplies mechanical turbulence, giving neutral class D and vigorous mixing. This is why worst-case dispersion modelling examines light-wind stable conditions rather than average weather.
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