The term “slope” in reference to the Skew-T chart is the angle from the horizontal (the isobars) counter clockwise to a section of the T curve.
For stability, the smaller the angle, the greater stability there will be. The larger this angle, the more instability there will be.
The stability of air parcels in an atmospheric layer is indicated by comparing the slope of the virtual temperature to the slope of the dry or saturation adiabats. Virtual temperature is compared to the dry adiabats when the parcel is unsaturated and to the saturation adiabats when the parcel is saturated. The temperature curve is normally used instead of the virtual temperature curve for a quick determination of the stability; however this may cause errors under certain circumstances. For this reason be sure when looking at stability on a Skew-T to use the virtual temperature.
Description of Stability Criteria
Absolutely Stable – If the slope of the T curve is less than the slope of the saturation adiabat and the slope of the dry adiabat then the layer is considered absolutely stable.
Figure 2.The area between 700 and 800mb is an example of an absolutely stable layer.
Absolutely Unstable – If the slope of the T curve is greater than the slope of the dry adiabat and the saturation adiabat, the layer is considered absolutely unstable.
Figure 3.The area between 850 and 950mb is an example of an absolutely unstable layer.
Conditionally Unstable/Stable – If the slope of the T curve is less than the slope of the dry adiabat but greater than the slope of the saturation adiabat, the layer is conditionally unstable/stable. This means that the layer is stable only if it is unsaturated and unstable if the layer is saturated.
Figure 4. The area between 600 and 700mb is an example of a conditionally unstable layer.
Neutrally Stable – If the T curve is parallel to either a saturation or a dry adiabat, the layer is in neutral equilibrium with the surrounding atmosphere. If the curve is parallel to a saturation adiabat, then the upward movement of saturated parcels will not be aided or hindered by the environment. Likewise, if the T curve is parallel to a dry adiabat, the parcel’s upward displacement of unsaturated parcels is not helped or hindered by the environment.
Figure 5. The area between 600 and 800mb is an example of a neutral layer with respect to the dry adiabatic lapse rate.
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“Atmospheric Stability.” Department of Atmospheric Sciences - University of Arizona . 1999. http://www.atmo.arizona.edu/students/courselinks/fall99/atmo171-mcc/atmo171_f99_09.html
Department of Atmospheric Science – University of Wyoming . Use of Skew T-log p diagram . Air Weather Service (MAC), United States Air Force. (1969).