Discussion
Temperature profiles measured on four occasions.
| Occasion | Surface (C) | At 500 m (C) | Lapse rate (C/km) | Stability |
|---|---|---|---|---|
| W | 24.0 | 18.5 | 11.0 | Unstable |
| X | 20.0 | 15.1 | 9.8 | Neutral |
| Y | 16.0 | 12.5 | 7.0 | Stable |
| Z | 8.0 | 10.5 | -5.0 | Inversion |
Which occasion would produce the highest ground level concentrations from a low stack?
Answer: A
Correct answer: Z, where the inversion suppresses mixing
Occasion Z is the inversion: the temperature rises from \( 8.0 \) degrees at the surface to \( 10.5 \) degrees at \( 500 \) m, giving the negative lapse rate of \( -5.0 \) degrees per kilometre shown in the table. Vertical motion is strongly suppressed, so an emission from a low stack is trapped in a shallow layer near the ground and concentrations build.
Occasion W, with a lapse rate of \( 11.0 \) exceeding the adiabatic \( 9.8 \), is superadiabatic and mixes most vigorously, giving the lowest concentrations. X is neutral and Y mildly stable - both disperse far better than an inversion, in which the atmosphere actively opposes the vertical motion that dilution requires.
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