Stability

Figure 5.2

pressure vs. height pressure is a force per unit area pressure at sea level is around 1000 millibars pressure decreases with height exponentially 50% of the mass of Earth’s atmosphere is below an altitude of 5-6 km (the 500 millibar height)

adiabatic temperature change

Figure 5.3

lifting mechanisms

orographic lifting

frontal wedging

convergence

3-d storm

convection over the Florida peninsula

convection

Figure 5.4

Figure 5.5

wet and dry adiabatic temperature change

absolute stability

absolute instability

conditional instability

Table 5.1 Summary of categories of atmospheric layer stability   Environmental lapse rate (ELR) Stability ELR > 10C/km Unstable 6C/km < ELR < 10C/km Conditionally unstable (Unstable if saturated, stable if unsaturated) ELR < 6C/km Stable ELR = 10C/km Neutral if unsaturated, unstable if saturated ELR = 6C/km Neutral if saturated, stable if unsaturated  

environment < dry

environment > dry

dry < environment < dry

Figure 5B

Table 5.2 Table 5.2 Stability categories and likelihood of severe convective storms for various ranges of the Lifted Index (LI), Showalter Index (SI), Convective Available Potential Energy (CAPE), Total Totals (TT) index and SWEAT index. Stability LI SI CAPE TT SWEAT   Very stable > +3 (no significant activity) Stable 0 to +3 > +2 < 0 (Showers possible; isolated T’showers unlikely) Marginally unstable 2 to 0 0 to 2 0 to 1000 45 to 50 (T’showers possible) Moderately unstable 4 to –2 3 to 0 1000 to 2500 50 to 55 250 to 300 (Thunderstorms possible) Very unstable 6 to –4 6 to –3 2500 to 3500 55 to 60 300 to 400 (Severe T’storms possible) Extremely unstable < 6 < 6 > 3500 > 400 (Severe T’storms probable; tornadoes possible)