3. Stability

Determining Stability » Stability Types Conditionally Unstable - 1

Skew-T showing conditionally unstable lapse rate

On this plot, the red line (line T-T’) depicts an observed temperature profile from the surface (near 960 hPa) to 400 hPa. If a parcel of air at the surface with temperature T and dewpoint Td is lifted, it will cool following the dry adiabat until it reaches saturation at Point T1. If the parcel is lifted further, it will then cool following the saturation adiabat to Point T3. During its ascent and prior to reaching Point T2 (at about 770 hPa), the parcel is colder and denser than its surroundings, and, thus, below 770 hPa (along the section of the line T-T1-T2) the atmosphere is stable. However after passing Point T2, the parcel is always warmer and less dense than its surroundings and is therefore unstable. We refer to this point (T2) as the level of free convection (LFC)level of free convection (LFC). The air is said to be "conditionally" unstable. "Conditional" means the lifted parcel is stable if unsaturated and unstable (above the LFC) if saturated. In the case of downward displacement, the temperature of both initially saturated and unsaturated parcels would warm following the dry adiabat.