Atmospheric Stability Questions and Answers

Q: Which stability effect does a snow cover produce?

Snow has a high albedo, so it absorbs little solar radiation, and a high emissivity, so it radiates efficiently at night, and its low conductivity insulates it from the ground beneath. The combination produces extremely strong surface inversions and the lowest temperatures recorded on the continents. Reflection cools rather than warms the surface. Naming the cost of a choice alongside its benefit is what turns a fact into a design decision.

Q: Which record low temperatures result from that combination?

The Antarctic plateau combines high altitude, continuous darkness, extreme dryness and a snow surface, and satellite measurements have recorded surface temperatures below minus 90 degrees. Vostok holds the air temperature record at minus 89.2 degrees. The Siberian record of about minus 68 degrees is the northern hemisphere extreme.

Q: Which effect does the extreme dryness contribute?

Water vapour is the principal source of downward long wave radiation, so extremely dry air returns very little energy to the surface and allows it to cool far below what a moist atmosphere would permit. This is why deserts and polar plateaus both show extreme nocturnal cooling. Sublimation is far too slow to contribute measurably.

Q: Which layer does the strongest inversion occupy in such conditions?

Antarctic surface inversions of twenty to thirty kelvin are commonly confined to the lowest few hundred metres, above which the profile returns to a more normal lapse rate. The intensity rather than the depth is what is remarkable. That shallow intense structure drives the katabatic winds that flow off the plateau.

Q: Which wind results from that surface layer over sloping ice?

Radiatively cooled air adjacent to the sloping ice becomes denser than the free air at the same level and drains downhill, reaching hurricane force where the terrain funnels it. The winds at Cape Denison average over 70 kilometres per hour throughout the year. Barrier winds are a genuine phenomenon in Antarctica too but arise from blocking rather than from drainage. Checking the units of each quantity catches a wrong formula before the arithmetic begins.

Q: Which effect does stability have on wind shear near the surface?

Suppressed mixing means momentum is not transferred efficiently, so the wind at the surface falls light while the air a hundred metres up retains its speed, producing very strong shear. This is why nocturnal low level jets form and why aircraft encounter shear on approach in stable conditions. Unstable conditions mix momentum downward and reduce the shear. Checking the units of each quantity catches a wrong formula before the arithmetic begins.

Q: Which hazard does that shear present to aviation?

An aircraft descending through a shear layer experiences a change in headwind that translates directly into a change of airspeed, which must be anticipated and corrected. Low level wind shear alert systems monitor for exactly this. The stable layer is typically smooth rather than turbulent, which is why the hazard is the airspeed change rather than the buffeting. The shared properties explain why these belong to one family, and the differences explain why more than one exists.

Q: Which stability regime produces the smoothest flying conditions?

Stable stratification suppresses vertical motion, so an aircraft flying in a stable layer with little shear encounters very smooth air. Convective boundary layers give a bumpy ride from the thermals. A Richardson number below the critical value indicates that the layer is about to become turbulent, which is the least smooth condition of all. The quantity that varies and the quantity that does not are worth naming before any substitution is attempted.

Q: Which time of day gives the smoothest low level flying?

The nocturnal stable layer persists until surface heating begins to erode it, so the hours around dawn offer the smoothest conditions at low level. Glider pilots wait for the opposite conditions in the afternoon. Late evening can also be smooth once the convection has decayed, which makes that a reasonable secondary answer.

Q: Which quantity does static stability affect in a numerical model?

The buoyancy frequency appears directly in the dispersion relation for internal gravity waves, so a more strongly stratified atmosphere supports faster waves. That does bear on the time step through the Courant condition, which makes the last option partly defensible. The Coriolis parameter depends on latitude alone and resolution is a design choice. Naming the cost of a choice alongside its benefit is what turns a fact into a design decision.

41.

Which stability effect does a snow cover produce?

A.A deeper mixed layer, because the smooth snow surface generates more mechanical turbulence in the flow above
B.Enhanced surface cooling and a stronger inversion
C.Enhanced surface heating during the day, since the bright surface reflects radiation back into the air above it
D.Neutral conditions

Answer: B

Snow has a high albedo, so it absorbs little solar radiation, and a high emissivity, so it radiates efficiently at night, and its low conductivity insulates it from the ground beneath. The combination produces extremely strong surface inversions and the lowest temperatures recorded on the continents. Reflection cools rather than warms the surface. Naming the cost of a choice alongside its benefit is what turns a fact into a design decision.

Discuss in Forum

42.

Which record low temperatures result from that combination?

A.About minus 20 degrees
B.About minus 60 degrees, which is the record for the northern hemisphere and represents the physical limit
C.Below minus 80 degrees Celsius on the Antarctic plateau
D.About minus 40 degrees, which is the lowest temperature that a snow surface is able to reach by radiation

Answer: C

The Antarctic plateau combines high altitude, continuous darkness, extreme dryness and a snow surface, and satellite measurements have recorded surface temperatures below minus 90 degrees. Vostok holds the air temperature record at minus 89.2 degrees. The Siberian record of about minus 68 degrees is the northern hemisphere extreme.

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43.

Which effect does the extreme dryness contribute?

A.Increased evaporation from the snow surface, which cools it further by removing latent heat from the surface
B.Stronger winds
C.Enhanced absorption of solar radiation by the very clear atmosphere overlying the surface of the ice sheet
D.Reduced downward long wave radiation from the atmosphere

Answer: D

Water vapour is the principal source of downward long wave radiation, so extremely dry air returns very little energy to the surface and allows it to cool far below what a moist atmosphere would permit. This is why deserts and polar plateaus both show extreme nocturnal cooling. Sublimation is far too slow to contribute measurably.

Discuss in Forum

44.

Which layer does the strongest inversion occupy in such conditions?

A.The lowest few hundred metres above the surface
B.The whole depth of the troposphere, which becomes stably stratified from the surface to the tropopause above
C.The stratosphere
D.A layer beginning about two kilometres above the surface and extending upward to the top of the boundary layer

Answer: A

Antarctic surface inversions of twenty to thirty kelvin are commonly confined to the lowest few hundred metres, above which the profile returns to a more normal lapse rate. The intensity rather than the depth is what is remarkable. That shallow intense structure drives the katabatic winds that flow off the plateau.

Discuss in Forum

45.

Which wind results from that surface layer over sloping ice?

A.A barrier wind, which flows parallel to a mountain range where the flow is blocked from crossing it directly
B.A katabatic wind
C.An anabatic wind, which flows up the slope during the day as the surface is heated by the incoming sunlight
D.A foehn

Answer: B

Radiatively cooled air adjacent to the sloping ice becomes denser than the free air at the same level and drains downhill, reaching hurricane force where the terrain funnels it. The winds at Cape Denison average over 70 kilometres per hour throughout the year. Barrier winds are a genuine phenomenon in Antarctica too but arise from blocking rather than from drainage. Checking the units of each quantity catches a wrong formula before the arithmetic begins.

Discuss in Forum

46.

Which effect does stability have on wind shear near the surface?

A.Stability has no effect
B.Stable conditions produce a deeper layer of gentle shear extending well above the top of the boundary layer
C.Stable conditions produce strong shear over a shallow depth
D.Stable conditions remove the shear entirely because the flow becomes uniform through the depth of the layer

Answer: C

Suppressed mixing means momentum is not transferred efficiently, so the wind at the surface falls light while the air a hundred metres up retains its speed, producing very strong shear. This is why nocturnal low level jets form and why aircraft encounter shear on approach in stable conditions. Unstable conditions mix momentum downward and reduce the shear. Checking the units of each quantity catches a wrong formula before the arithmetic begins.

Discuss in Forum

47.

Which hazard does that shear present to aviation?

A.Severe icing, produced by the supercooled droplets within the stable layer through which the aircraft descends
B.Reduced visibility
C.Turbulence severe enough to cause structural damage, since the shear layer is both strong and very deep
D.A rapid airspeed change during approach or departure

Answer: D

An aircraft descending through a shear layer experiences a change in headwind that translates directly into a change of airspeed, which must be anticipated and corrected. Low level wind shear alert systems monitor for exactly this. The stable layer is typically smooth rather than turbulent, which is why the hazard is the airspeed change rather than the buffeting. The shared properties explain why these belong to one family, and the differences explain why more than one exists.

Discuss in Forum

48.

Which stability regime produces the smoothest flying conditions?

A.A stable layer with weak shear
B.A strongly unstable boundary layer, within which the thermals produce a comfortably uniform ride at low level
C.Neutral conditions with strong wind
D.A layer in which the Richardson number lies just below the critical value of one quarter throughout its depth

Answer: A

Stable stratification suppresses vertical motion, so an aircraft flying in a stable layer with little shear encounters very smooth air. Convective boundary layers give a bumpy ride from the thermals. A Richardson number below the critical value indicates that the layer is about to become turbulent, which is the least smooth condition of all. The quantity that varies and the quantity that does not are worth naming before any substitution is attempted.

Discuss in Forum

49.

Which time of day gives the smoothest low level flying?

A.Around noon, when the thermals are most vigorous and therefore most regularly spaced across the whole area
B.Early morning before the convective layer develops
C.Mid-afternoon, when the mixing has become deep enough for the turbulence to be evenly distributed through it
D.Late evening

Answer: B

The nocturnal stable layer persists until surface heating begins to erode it, so the hours around dawn offer the smoothest conditions at low level. Glider pilots wait for the opposite conditions in the afternoon. Late evening can also be smooth once the convection has decayed, which makes that a reasonable secondary answer.

Discuss in Forum

50.

Which quantity does static stability affect in a numerical model?

A.The horizontal resolution
B.The time step, which must be shortened whenever the atmosphere becomes more stably stratified than usual
C.The vertical propagation and speed of gravity waves
D.The Coriolis parameter, which increases with stability because the rotation of the flow becomes more coherent

Answer: C

The buoyancy frequency appears directly in the dispersion relation for internal gravity waves, so a more strongly stratified atmosphere supports faster waves. That does bear on the time step through the Courant condition, which makes the last option partly defensible. The Coriolis parameter depends on latitude alone and resolution is a design choice. Naming the cost of a choice alongside its benefit is what turns a fact into a design decision.

Discuss in Forum

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