Atmospheric Stability Questions and Answers

Q: Which profile of humidity produces potential instability?

A moist layer beneath a dry one has equivalent potential temperature falling with height, and lifting saturates the base first, releasing latent heat there while the top continues to cool at the dry rate. The lapse rate steepens until the layer becomes unstable. Uniform humidity or complete saturation give no such differential effect. Naming the cost of a choice alongside its benefit is what turns a fact into a design decision.

Q: Which classification describes atmospheric stability for dispersion modelling?

Pasquill defined six classes from A for very unstable to F for moderately stable, based on the wind speed together with the insolation by day or the cloud cover by night. They determine the spread of a plume in dispersion calculations. The Beaufort, Fujita and Saffir-Simpson scales classify wind, tornado damage and hurricane intensity respectively. A limit that survives making every external component ideal is an intrinsic one, and worth knowing for that reason.

Q: Which conditions correspond to Pasquill class A?

Very unstable conditions require strong surface heating with little wind to disrupt the thermals, so light winds and strong sunshine give class A. Clear calm nights give the stable class F at the other extreme. Strong winds with overcast skies give neutral class D, in which mechanical turbulence dominates and the stability plays little part. Working one numerical case through both cases side by side fixes the distinction better than any rule of thumb.

Q: Which plume behaviour corresponds to very unstable conditions?

In very unstable conditions large thermals carry the plume up and down in a looping pattern, bringing high concentrations to the ground intermittently close to the stack. Fanning occurs in stable conditions, coning in neutral ones and lofting where a stable layer lies beneath an unstable one. The plume shape is a direct visual indicator of the stability class. Naming the cost of a choice alongside its benefit is what turns a fact into a design decision.

Q: Which plume behaviour occurs in a strong surface inversion?

A stable layer suppresses vertical mixing entirely, so the plume spreads horizontally into a thin fan while remaining at its release height. Ground level concentrations near the source are very low. The hazard comes later, when morning heating erodes the inversion from below and brings the whole plume down in a fumigation event.

Q: Which situation produces the highest ground level concentrations?

When morning heating erodes a nocturnal inversion up to the level of a plume that has been fanning through the night, the accumulated pollutant is mixed rapidly to the ground over a wide area. Fumigation episodes produce the highest short term concentrations in classical dispersion theory. Lofting is the safest configuration, since the stable layer beneath keeps the plume aloft. Framing the question as which quantity the change acts upon makes the answer follow without calculation.

Q: Which design measure reduces ground level concentrations?

Releasing above the mixing height allows the plume to disperse in the free atmosphere and to loft rather than fumigate, which was the rationale for the tall stacks built in the mid twentieth century. Momentum and buoyancy of the exhaust add to the effective height, so a narrow hot fast release rises further. The strategy transferred the problem to longer range transport, which is why it was later criticised. Working one numerical case through both cases side by side fixes the distinction better than any rule of thumb.

Q: Which quantity determines the effective height of a plume?

Buoyant and momentum driven rise adds substantially to the physical stack height, often doubling it, and formulae by Briggs are used to estimate the addition. The rise depends on the exhaust temperature and velocity and on the ambient stability and wind. Neglecting it would badly overestimate ground level concentrations.

Q: Which condition limits the plume rise most severely?

Strong wind bends the plume over rapidly and dilutes its buoyancy, limiting the rise, which is why the highest ground level concentrations in neutral conditions occur at moderate to strong winds. Calm conditions allow maximum rise. High exhaust temperature increases the buoyancy and therefore the rise, and instability enhances the rise rather than suppressing it. Framing the question as which quantity the change acts upon makes the answer follow without calculation.

21.

Which profile of humidity produces potential instability?

A.Saturation through the whole depth of the layer, which allows the latent heat to be released everywhere at once
B.Moist air beneath dry air
C.Dry air beneath moist air, so that the top of the layer saturates first as the whole layer is lifted upward
D.Uniform humidity

Answer: B

A moist layer beneath a dry one has equivalent potential temperature falling with height, and lifting saturates the base first, releasing latent heat there while the top continues to cool at the dry rate. The lapse rate steepens until the layer becomes unstable. Uniform humidity or complete saturation give no such differential effect. Naming the cost of a choice alongside its benefit is what turns a fact into a design decision.

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22.

Which classification describes atmospheric stability for dispersion modelling?

A.The Fujita scale
B.The Saffir-Simpson scale, which classifies tropical cyclones by the maximum sustained wind speed they attain
C.The Pasquill stability classes
D.The Beaufort scale, which classifies the wind speed according to its observable effects upon the sea and land

Answer: C

Pasquill defined six classes from A for very unstable to F for moderately stable, based on the wind speed together with the insolation by day or the cloud cover by night. They determine the spread of a plume in dispersion calculations. The Beaufort, Fujita and Saffir-Simpson scales classify wind, tornado damage and hurricane intensity respectively. A limit that survives making every external component ideal is an intrinsic one, and worth knowing for that reason.

Discuss in Forum

23.

Which conditions correspond to Pasquill class A?

A.Strong winds with overcast skies, which produce the most thorough mixing of the pollutant through the layer
B.Clear calm nights
C.Moderate winds with slight cloud cover during the hours immediately following sunrise on a spring morning
D.Light winds with strong daytime insolation

Answer: D

Very unstable conditions require strong surface heating with little wind to disrupt the thermals, so light winds and strong sunshine give class A. Clear calm nights give the stable class F at the other extreme. Strong winds with overcast skies give neutral class D, in which mechanical turbulence dominates and the stability plays little part. Working one numerical case through both cases side by side fixes the distinction better than any rule of thumb.

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24.

Which plume behaviour corresponds to very unstable conditions?

A.Looping
B.Fanning, in which the plume spreads horizontally but not vertically because the stable layer suppresses mixing
C.Coning
D.Lofting, in which the plume disperses upward but is prevented from reaching the ground by a stable layer below

Answer: A

In very unstable conditions large thermals carry the plume up and down in a looping pattern, bringing high concentrations to the ground intermittently close to the stack. Fanning occurs in stable conditions, coning in neutral ones and lofting where a stable layer lies beneath an unstable one. The plume shape is a direct visual indicator of the stability class. Naming the cost of a choice alongside its benefit is what turns a fact into a design decision.

Discuss in Forum

25.

Which plume behaviour occurs in a strong surface inversion?

A.Coning, in which the plume spreads at a steady angle in both the vertical and the horizontal as it travels
B.Fanning
C.Looping, in which large thermals carry the plume alternately up and down as it travels downwind of the stack
D.Fumigation

Answer: B

A stable layer suppresses vertical mixing entirely, so the plume spreads horizontally into a thin fan while remaining at its release height. Ground level concentrations near the source are very low. The hazard comes later, when morning heating erodes the inversion from below and brings the whole plume down in a fumigation event.

Discuss in Forum

26.

Which situation produces the highest ground level concentrations?

A.Lofting
B.Coning under neutral conditions, in which the plume spreads steadily and reaches the surface a long way downwind
C.Fumigation as an inversion is eroded from below
D.Fanning, in which the plume remains concentrated in a narrow layer and therefore reaches the ground undiluted

Answer: C

When morning heating erodes a nocturnal inversion up to the level of a plume that has been fanning through the night, the accumulated pollutant is mixed rapidly to the ground over a wide area. Fumigation episodes produce the highest short term concentrations in classical dispersion theory. Lofting is the safest configuration, since the stable layer beneath keeps the plume aloft. Framing the question as which quantity the change acts upon makes the answer follow without calculation.

Discuss in Forum

27.

Which design measure reduces ground level concentrations?

A.A wider stack, which reduces the exit velocity of the gases and therefore lowers the effective release height
B.A cooler exhaust
C.Releasing the gases horizontally rather than vertically so that they spread over a wider area near the source
D.A taller stack releasing above the inversion

Answer: D

Releasing above the mixing height allows the plume to disperse in the free atmosphere and to loft rather than fumigate, which was the rationale for the tall stacks built in the mid twentieth century. Momentum and buoyancy of the exhaust add to the effective height, so a narrow hot fast release rises further. The strategy transferred the problem to longer range transport, which is why it was later criticised. Working one numerical case through both cases side by side fixes the distinction better than any rule of thumb.

Discuss in Forum

28.

Which consequence followed from the tall stack policy?

A.Long range transport of acidifying pollutants
B.A complete elimination of the problem, since the pollutants were dispersed to concentrations below any concern
C.Increased local concentrations
D.A reduction in the total quantity of sulphur emitted, since the taller stacks allowed more efficient combustion

Answer: A

Raising the release height reduced local concentrations but allowed sulphur and nitrogen compounds to travel hundreds of kilometres before deposition, producing acid rain in Scandinavia from British and German sources. Emissions were unchanged in quantity. The episode is a standard illustration that dispersion is not the same as removal. Framing the question as which quantity the change acts upon makes the answer follow without calculation.

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29.

Which quantity determines the effective height of a plume?

A.The height at which the ambient wind speed equals the exit velocity of the gases leaving the top of the stack
B.The stack height plus the plume rise
C.The stack height alone, since the exhaust gases mix with the ambient air immediately upon leaving the stack
D.The mixing height

Answer: B

Buoyant and momentum driven rise adds substantially to the physical stack height, often doubling it, and formulae by Briggs are used to estimate the addition. The rise depends on the exhaust temperature and velocity and on the ambient stability and wind. Neglecting it would badly overestimate ground level concentrations.

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30.

Which condition limits the plume rise most severely?

A.A high exhaust temperature
B.A strongly unstable atmosphere, within which the thermals suppress the upward motion of the buoyant plume
C.Strong ambient wind
D.Calm conditions, in which there is no ventilation to carry the plume away from the vicinity of the source

Answer: C

Strong wind bends the plume over rapidly and dilutes its buoyancy, limiting the rise, which is why the highest ground level concentrations in neutral conditions occur at moderate to strong winds. Calm conditions allow maximum rise. High exhaust temperature increases the buoyancy and therefore the rise, and instability enhances the rise rather than suppressing it. Framing the question as which quantity the change acts upon makes the answer follow without calculation.

Discuss in Forum

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