3. Stability

Determining Stability Exercise

Skew-T showing sounding for stability exercise

In this sounding, the Tv curve is shown as the dashed line from 1000 to 800 hPa. Above 800 hPa, Tv nearly equals T.

Determine the stability for each of the following layers using both temperature and virtual temperature:

1000 to 950 hPa:

The correct answer is: Unstable

The slopes of the T and Tv curves are both greater than the slope of the dry adiabat. This layer is absolutely unstable.

950 to 850 hPa:

The correct answer is: Conditionally Unstable

The slopes of the T and Tv curves are both less than the slope of the dry adiabats, but greater than that of the saturation adiabats. Therefore, this layer is in a conditional state. Since the T and Td curves practically coincide, the layer is essentially saturated. The slopes of both the T and Tv curves, when compared with that of the saturation adiabats, show the layer to be unstable as long as it remains saturated.

850 to 800 hPa:

The correct answer is: Conditionally Unstable

The slope of the Tv curve is greater than that of the saturation adiabats and slightly less than that of the dry adiabats. The bottom layer interface is saturated, so that a saturated parcel lifted from the bottom of the layer would be unstable. Therefore, the layer is conditionally unstable.

Note that the T and Td curves indicate the remainder of the layer is definitely unsaturated. A comparison of the Tv curve and the dry adiabats shows the layer to be stable to unsaturated ascents. The T curve also indicates greater stability than the Tv curve.

800 to 600 hPa:

The correct answer is: Stable

The layer is unsaturated, and the slope of the T curve is less than that of the dry adiabats; the layer is stable.

600 to 550 hPa:

The correct answer is: Neutral

Through this unsaturated layer, the T curve is parallel to the dry adiabats; hence, the layer is in equilibrium for small upward and downward displacements of parcels within it.

550 to 500 hPa:

The correct answer is: Stable

The slope of the T curve is less than that of the dry or saturation adiabats; the layer is absolutely stable.

500 to 400 hPa:

The correct answer is: Neutral

This layer is in equilibrium since the T curve parallels the dry adiabats and the air is unsaturated. The remarks on the layer from 600 to 550 hPa apply here also.

400 to 300 hPa:

The correct answer is: Stable

The air is absolutely stable above 400 hPa.