Meteorology and Dispersion Questions and Answers

Answer: A

Correct answer: A severe episode under a subsidence inversion

A stagnant high pressure system over a valley is the classic setting for a severe episode. Air descends within the high and warms adiabatically as it is compressed, forming a subsidence inversion that caps the mixed layer; the weak pressure gradients leave winds light; and the valley walls restrict horizontal ventilation further.

Mixing height and wind speed both collapse, so the ventilation coefficient falls to a small value while emissions continue, and concentrations build day after day. The descending air does not disperse pollution - it is precisely what creates the lid, since the sinking motion warms the air rather than mixing it downward.

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.

Answer: C

Correct answer: As poor dispersion potential

A ventilation coefficient of \( 2800 \) square metres per second is low. The coefficient is the mixing height times the mean wind speed, and values below about \( 6000 \) are commonly taken to indicate limited capacity to disperse emissions, so this figure is a warning about the site.

It says nothing directly about stability class, though poor ventilation usually accompanies stable conditions, and it is certainly not irrelevant to siting - it is one of the standard screening measures for whether an area can absorb a new industrial load. A high value, not a low one, would indicate good dispersion.

Answer: D

A rising parcel immediately finds itself colder and denser than its surroundings and sinks back, so vertical motion is strongly suppressed. Such layers trap pollution near the ground and produce the very worst episodes.

Answer: A

It measures the volume of air available per unit time to carry pollutants away, and values below about six thousand square metres per second are regarded as poor. It combines both the depth and the throughput of the mixed layer.

Answer: B

Correct answer: effective

The physical stack height plus the plume rise gives the effective stack height, written \( H = h_s + \Delta h \). This is the height at which the plume levels off and begins to travel horizontally, and it is the height that enters the Gaussian plume equation.

The distinction from the geometric or physical height is important in practice: a hot, fast exhaust may rise a hundred metres or more above the stack top, and because the maximum ground level concentration varies roughly as \( 1/H^2 \), that rise can matter as much as the stack itself. "Critical" and "nominal" are not terms used here.

Answer: C

Correct answer: Pasquill

The A to F classification of atmospheric stability is the Pasquill scheme, later refined by Gifford - hence the Pasquill-Gifford curves from which the dispersion coefficients are read. Class A is very unstable, D neutral and F stable, and the class is assigned from surface wind speed with insolation by day or cloud cover by night.

The Beaufort scale describes wind force, not stability. Briggs is associated with the standard plume rise equations, and Gaussian refers to the assumed shape of the concentration distribution rather than to a stability classification.

Answer: D

Correct answer: fumigation

Fumigation is the rapid downward mixing of an elevated plume when a nocturnal inversion breaks up. Solar heating erodes the inversion from below, and when the growing unstable layer reaches the plume that has been travelling in the stable air above, the accumulated material is brought to the surface within minutes.

The episode is short - typically half an hour to an hour - but produces some of the highest short-term ground level concentrations recorded. Looping, fanning and coning are the plume forms of unstable, stable and neutral conditions respectively, and describe steady behaviour rather than this transition.

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.

Answer: B

Correct answer: 06:00, poorer by a factor of about 42

Dispersion potential is worst where the ventilation coefficient is smallest, which is \( 180 \) square metres per second at 06:00 - a shallow \( 120 \) m mixing height left over from the night combined with a wind of only \( 1.5 \) m/s. The best is 14:00 at \( 7500 \), when surface heating has deepened the mixed layer to \( 1500 \) m and the wind has risen to \( 5 \) m/s.

\[ \frac{7500}{180} \approx 42 \]

So the atmosphere disperses a given emission some forty times less effectively at dawn than in mid-afternoon. Both factors work the same way through the day, which is why the diurnal swing in the coefficient is so large.