Atmospheric Stability Questions and Answers

Answer: C

The parcel conserves its potential temperature during the adiabatic displacement, so in a layer where the environmental potential temperature increases upward the parcel finds itself colder and denser than its surroundings and sinks back. The reason therefore explains the assertion completely. The same argument run in reverse gives the criterion for instability.

Answer: D

The reason states the assumption correctly, and where the ascending region occupies a large fraction of the area that assumption fails and the slice method gives a more stable criterion. The assertion is therefore too strong. For isolated thermals the parcel method is entirely adequate, which is why it remains the standard approach despite the known limitation.

Answer: A

Shear instability arises from the shear overcoming the stratification, whereas convective instability arises from the stratification alone and occurs equally in the absence of any shear. The two are the classical members of a family that also includes symmetric instability, which requires both shear and rotation. Identifying which ingredients each instability needs is what allows the right one to be diagnosed from a sounding.

Answer: B

The first two statements are correct, but symmetric instability involves a slanted displacement along a path between the isentropes and the absolute momentum surfaces. A purely vertical displacement would find the layer stable. That slanted geometry is exactly what distinguishes it from ordinary convection and is why it was recognised comparatively late. Framing the question as which quantity the change acts upon makes the answer follow without calculation.

Answer: C

Fumigation occurs during the transition as a nocturnal inversion is eroded from below, not during persistently stable conditions. It is a transient event rather than a steady state. The other three pairings are correct, and the four plume shapes together give a visual classification of boundary layer stability that predates any instrumentation. A limit that survives making every external component ideal is an intrinsic one, and worth knowing for that reason.

Answer: D

A dry adiabatic lapse rate corresponds to neutral stratification, in which a displaced parcel experiences no restoring force at all. Inversions and isothermal layers are both stable, and fanning is the plume behaviour characteristic of stable conditions. The distinction between neutral and stable matters because a neutral layer mixes readily while a stable one does not. Reading the definition literally, rather than recalling a formula, is the reliable route through questions of this shape.

Answer: A

Fanning is the classic signature of a stable layer, which permits horizontal spreading by wind direction fluctuations while suppressing vertical motion entirely. It typically occurs at night or in the early morning. The hazard is that the accumulated plume will be brought to the ground as a fumigation event once heating erodes the inversion. Most errors here come from carrying a result across from a neighbouring case where it does not hold.

Answer: B

The Brunt Vaisala or buoyancy frequency has a square proportional to the vertical gradient of potential temperature, and its value near 0.01 per second in the troposphere corresponds to a period of about ten minutes. The inertial frequency involves rotation instead and gives a period of seventeen hours at mid-latitudes. Both appear together in the dispersion relation for inertia-gravity waves. Working one numerical case through both cases side by side fixes the distinction better than any rule of thumb.

Answer: C

A superadiabatic layer is unstable, a dry adiabatic layer neutral, an isothermal layer stable and an inversion the most stable of all, giving 3, 2, 1, 4. Laying the four out along a single axis of lapse rate makes the classification immediate, and each has a characteristic appearance on a sounding that can be recognised at a glance.

Answer: D

Convective instability requires an unstable stratification, Kelvin-Helmholtz instability requires shear sufficient to overcome a stable one, and symmetric instability requires a slanted displacement in a rotating baroclinic flow. Each produces a characteristic structure, respectively cells, billows and bands, which is how they are identified in observations. Working one numerical case through both cases side by side fixes the distinction better than any rule of thumb.