Meteorology and Dispersion Questions and Answers

Answer: A

Correct answer: A is false but R is true

The assertion is false. The dry adiabatic lapse rate is not a property of the atmosphere at any given moment; it is a fixed theoretical value of about \( 9.8 \) degrees per kilometre, following from thermodynamics and the acceleration due to gravity, and it does not change with the weather.

The reason is true and explains what the rate actually is: the cooling of a parcel raised without exchanging heat with its surroundings. The measured profile is the environmental lapse rate, which does vary from hour to hour; stability is judged by comparing the two.

Answer: B

Correct answer: 1 and 2 only

Statement 1 is correct: when the environmental lapse rate exceeds the adiabatic rate, a rising parcel stays warmer and less dense than its surroundings and keeps rising, which is instability. Statement 2 is correct: an inversion, with temperature increasing upward, is the most stable condition and suppresses vertical motion most strongly.

Statement 3 is false. Neutral conditions arise when the environmental rate equals the dry adiabatic rate of about \( 9.8 \) degrees per kilometre, not when it is zero. A zero lapse rate is an isothermal layer, which is stable, since a rising parcel cools while its surroundings do not.

Answer: C

Correct answer: 1 and 2 only

Statement 1 is correct: \( u \) appears once in the denominator, so concentration is inversely proportional to wind speed. Statement 2 is correct: the dispersion coefficients are read from curves indexed by Pasquill stability class and downwind distance, growing as the plume travels.

Statement 3 is false and reverses the effect. The reflection term treats the ground as a perfect reflector and adds an image source below it, so it increases the calculated ground level concentration. Omitting it would understate the result, which is why the conservative assumption of no surface removal is the usual default.

Answer: D

Correct answer: 1 and 2 only

Statement 1 is correct: a radiation inversion forms on clear calm nights, when the ground radiates heat to a cloudless sky and cools the air immediately above it. Statement 2 is correct: a subsidence inversion forms aloft in a high pressure system, where descending air is compressed and warms adiabatically.

Statement 3 is false and inverts the physics. Inversions suppress vertical mixing: a parcel rising into warmer, lighter air becomes colder and denser than its surroundings and sinks back. That suppression is precisely why inversions are associated with pollution episodes.

Answer: A

Correct answer: 1 and 2 only

Statement 1 is correct: the ventilation coefficient is the product of mixing height and mean wind speed, in metres squared per second. Statement 2 is correct: a low value means little depth and little through-flow for dilution, so it indicates poor dispersion potential.

Statement 3 is false. The coefficient varies enormously through the day, because both of its factors do - the mixing height may be a hundred metres at dawn and well over a thousand by mid-afternoon, and the wind typically strengthens with the daytime mixing. Swings of a factor of forty between dawn and afternoon are entirely ordinary.

Answer: B

Correct answer: A-2, B-1, C-4, D-3

Each plume form follows from the stability profile through the depth the plume occupies. Looping (A) needs strongly unstable air throughout (2), whose large convective eddies throw the plume up and down. Coning (B) is the neutral case (1), in which the plume expands smoothly and symmetrically.

Fanning (C) requires stable air throughout (4), which suppresses vertical growth and leaves a thin ribbon. Lofting (D) needs stable air below the stack with unstable air above (3), so the plume disperses upward while the stable layer beneath keeps it off the ground - the most favourable arrangement of the four.

Answer: C

Correct answer: A-2, B-1, C-4, D-3

Each inversion takes its name from the process that creates it. A radiation inversion (A) comes from nocturnal cooling of the ground under a clear sky (2), which chills the air in contact with it. A subsidence inversion (B) comes from descending air in a high pressure system (1) warming as it is compressed.

A frontal inversion (C) forms where warm air overruns cold air at a front (4), the warm air riding above the denser cold wedge. An advection inversion (D) forms where warm air moves horizontally over a colder surface (3) - sea air crossing cold land, for instance - and is cooled from below.

Answer: D

Correct answer: A-2, B-1, C-4, D-3

Each quantity is matched to its determinants. The dispersion coefficients (A) are read from curves indexed by stability class and downwind distance (2). The ventilation coefficient (B) is mixing height times mean wind speed (1).

The effective stack height (C) is the physical stack height plus the plume rise (4), and the Pasquill class (D) is assigned from wind speed together with insolation or cloud cover (3). Note the ordering the list implies: the Pasquill class must be fixed first, since the dispersion coefficients cannot be read until it is known.

Answer: A

Correct answer: A-2, B-1, C-4, D-3

Each change is matched to its effect through the dependence in the plume equation. Doubling the wind speed (A) halves the concentration (2), since \( C \propto 1/u \). Doubling the effective stack height (B) roughly quarters the maximum (1), since \( C_{max} \propto 1/H^2 \).

Halving the mixing height (C) roughly doubles the concentration in a box model (4), because the same emission is confined to half the depth. Doubling the emission rate (D) doubles the concentration (3), the one strictly linear relation of the four - which is why the inverse-square height term is so much more powerful a control than proportional reductions in output.

Answer: B

Correct answer: Effective stack height

The effective stack height is a property of the source, not of the atmosphere: it is the physical stack height plus the plume rise, and both depend on the design of the stack and the temperature and velocity of the gas leaving it.

The other three describe the atmosphere. The Pasquill class summarises its turbulent state, the mixing height is the depth available for dilution, and the environmental lapse rate is the measured temperature profile. Keeping source and atmospheric terms apart is the basic organising idea of dispersion modelling, since only the source terms can be engineered.