Meteorology and Dispersion Questions and Answers

Answer: A

A parcel of dry air rising without exchanging heat with its surroundings cools at this rate as it expands. Comparing it with the actual environmental lapse rate is what tells you whether the atmosphere is stable or unstable.

Answer: B

Correct answer: Greater than the dry adiabatic lapse rate

The atmosphere is unstable when the environmental lapse rate exceeds the dry adiabatic rate of about \( 9.8 \) degrees per kilometre - the superadiabatic condition. A parcel displaced upward cools at the adiabatic rate, but the surrounding air cools faster with height, so the parcel finds itself warmer and less dense than its surroundings and keeps rising.

If the environmental rate is less than adiabatic the parcel becomes colder and denser than its surroundings and sinks back, which is stable. Equality gives neutral conditions, in which a displaced parcel neither rises nor sinks. Instability favours vigorous vertical mixing and rapid dilution of a plume.

Answer: C

Correct answer: Increases with height

An inversion is a layer in which temperature increases with height, inverting the normal decrease. Because the air aloft is warmer and therefore lighter, a parcel rising from below arrives colder and denser than its surroundings and sinks back, so vertical motion is strongly suppressed.

That suppression is why inversions matter for air quality: pollutants released beneath one are confined to a shallow layer and accumulate. Temperature constant with height is an isothermal layer, and a decrease with height at the adiabatic rate is the neutral case - neither is an inversion.

Answer: D

The surface cools faster than the air above it, so the lowest layer becomes the coldest. It breaks up soon after sunrise as the ground warms again, which is why morning traffic emissions are trapped and afternoon ones are not.

Answer: A

Correct answer: A stagnant high pressure system

A subsidence inversion forms in a stagnant high pressure system. Air descends slowly over a wide area, and as it sinks it is compressed and warms adiabatically, producing a warm layer that caps the cooler air beneath it. Because the system may persist for days, the cap is persistent - the classic setting for a severe pollution episode.

Nocturnal radiative cooling produces a different and much shallower inversion at the ground, which breaks up after sunrise. A moving cold front and convective thunderstorms both promote vigorous mixing rather than suppressing it.

Answer: B

Correct answer: The depth through which pollutants disperse

The mixing height is the depth of the layer through which pollutants released near the surface are readily dispersed by turbulence. It is bounded above by a stable layer or inversion that acts as a lid, and it varies through the day, being shallow at night and deepening as surface heating drives convection.

It is the volume available for dilution, which is why a low mixing height and light winds together produce high concentrations. It has nothing to do with the anemometer height, the tallest stack on the site, or the tropopause, which lies far above the layer in which surface pollution disperses.

Answer: C

Correct answer: Mixing height times mean wind speed

The ventilation coefficient is the product of the mixing height and the mean wind speed within the mixed layer, with units of metres squared per second. It combines the two things that dilute an emission: the depth available for vertical mixing and the rate at which air is carried through the area.

A large value means the atmosphere can disperse a given emission well; a small value - a shallow mixing height with light winds - signals poor ventilation and the likelihood of an episode. Dividing rather than multiplying, or bringing in stack height or stability class, gives no such physical meaning.

Answer: D

Correct answer: A for very unstable to F for stable

The Pasquill classes run from A, very unstable, through B and C (moderately and slightly unstable), D (neutral), to E (slightly stable) and F (stable). The class is assigned from surface wind speed together with incoming solar radiation by day or cloud cover by night, and it selects the dispersion coefficients used in the plume calculation.

The scale is therefore ordered from most to least turbulent, not the reverse, and it is not numerical. Nor does it separate day from night as such: neutral class D occurs in overcast or windy conditions at any hour.

Answer: A

Correct answer: Normal, with standard deviations growing

The Gaussian plume model assumes the concentration is normally distributed about the plume centreline in both the crosswind and the vertical directions. The standard deviations \( \sigma_y \) and \( \sigma_z \) grow with downwind distance as turbulence spreads the plume, and their values are read from curves selected by the Pasquill stability class.

The assumption follows from treating turbulent spreading as the sum of many independent random displacements. It is what gives the model its familiar form, with the peak on the centreline and the concentration falling off symmetrically to either side, rather than being uniform, triangular, or decaying only with distance from the source.

Answer: B

They are read from curves or correlations indexed by stability and distance, and they are what distinguish an unstable day from a stable night. The emission rate scales the whole concentration field but does not change its shape.