SHW Ch. 5 - Atmospheric Stability

• Stable: if a parcel is displaced vertically, it will return to its original position

• Neutral: if a parcel is displaced vertically, it will remain in its new position

• Unstable: if a parcel is displaced vertically, it will accelerate away (upward or downward) from its original position; Figure 5.1

• Adiabatic Process: a process in which there is no exchange of heat between a parcel and its environment; a dry parcel will expand and cool as it rises and will compress and warm as it sinks; Figure 5.2

• Dry Adiabatic Lapse Rate: a rising (sinking) parcel will cool (warm) at a constant 10° C/km (5.5° F/1000 feet)

• Moist Adiabatic Lapse Rate: once a rising parcel becomes saturated (RH=100%), further rising will create condensation and the release of latent heat will offset the adiabatic cooling somewhat; in the lower troposphere, it is about 6° C/km; in the middle troposphere, it is about 8° C/km; in the upper troposphere near the tropopause, it is essentially the same as the dry adiabatic lapse rate

• Environmental Lapse Rate (ELR): it varies from 4° C/km to 9° C/km; it varies from location to location, from day to day at a fixed location and even from one layer of the atmosphere to another at a particular location and time

• Inversion: where temperature increases with height; a negative lapse rate

• Standard Atmosphere: a lapse rate of 7° C/km from the surface to the tropopause and -5° C/km above the tropopause in the stratosphere; Figure 5.3

• Determining Stability: subject a parcel to dry and moist adiabatic motions and compare to the existing environmental lapse rate; Figures 5.4, 5.5 & 5.6

  Atmospheric Layer Stability

  ELR > 10° C/km absolutely unstable

  6° C/km < ELR < 10° C/km conditionally unstable

  (stable if unsaturated; unstable if saturated)

  ELR < 6° C/km stable; (inversion = absolutely stable)

  ELR = 10° C/km neutral if unsaturated; unstable if saturated

  ELR = 6° C/km neutral if saturated; stable if unsaturated

(SHW Ch. 5 - Atmospheric Stability Continued)

• Primary Mechanisms That Cause Air to Rise (Figure 5.9):

  • Solar Heating

  • Frontal Lifting (including Sea Breeze Front)

  • Local Convergence in Low Pressure Systems

  • Lifting by Mountains (upslope)

• Lifting Condensation Level (LCL): the level (mb) at which a parcel of air lifted dry adiabatically becomes saturated (RH=100%); where cloud bases should form caused by mechanical lifting

• Level of Free Convection (LFC): the level (mb) at which a lifted saturated air parcel first becomes buoyant and rises freely (i.e., its temp is warmer than the ambient air temp)

• Equilibrium Level (EL_LFC & EL_CCL ): the level (mb) at which a buoyant air parcel loses its buoyancy upon rising (i.e., its temp is colder than the ambient air temp)

• Convective Condensation Level (CCL): the level (mb) at which a parcel of air, when heated sufficiently from below, rises and becomes saturated (RH=100%); where convective clouds should form bases

• Convective Temperature (T_C): the surface temp required to sufficiently heat air parcels to reach the CCL; graphically determined by following the dry adiabat from the CCL down to the surface pressure

• Convective Available Potential Energy (CAPE): a measure of the amount of potential energy (j/kg) available for convection; it is the integrated area on a sounding enclosed by the environmental temp profile and the path of the rising buoyant (warmer) air parcel; also known as the Positive Energy Area (PEA); sometimes marked by "+" signs; shaded RED on a sounding; Figure 5.7

• Convective INHibition (CINH_LFC): a measure of the amount of potential energy (j/kg) needed to initiate convection through mechanical lifting; it is the integrated area on a sounding enclosed by the environmental temp profile and the path of the lifted (cooler) air parcel; also known as the Negative Energy Area (NEA); sometimes marked by "-" signs; shaded BLUE on a sounding

• Convective INHibition (CINH_CCL): a measure of the amount of potential energy (j/kg) needed to initiate convection through heating; it is the integrated area on a sounding enclosed by the environmental temp profile and the dry adiabat used to graphically determine the Convective Temp (T_C); also known as the Negative Energy Area (NEA); sometimes marked by "-" signs; shaded BLUE on a sounding

(SHW Ch. 5 - Atmospheric Stability Continued)

• Stability Indices (Table 5.2):

  • K Index (KI): gives a probability of thunder; does not provide information about severity; the larger the KI, the greater the probability of thunderstorms; warm, moist air at 850 mb & 700 mb, along with cold air at 500 mb, will result in a large KI.

  • Lifted Index (LI): gives an indication of severity, based on graphically lifting a surface air parcel (based on average temp & dew point in the lowest 50 mb) to its LCL and then moist adiabatically to 500 mb and comparing it to the ambient 500 mb temp; a graphically warmer temp by definition, indicates buoyancy and is unstable (negative number); conversely, a graphically cooler temp, indicates stability (positive number).

  • Showalter Index (SI): gives an indication of severity, based on graphically lifting a surface air parcel from 850 mb to its LCL and then moist adiabatically to 500 mb and comparing it to the ambient 500 mb temp; a graphically warmer temp by definition, indicates buoyancy and is unstable (negative number); conversely, a graphically cooler temp, indicates stability (positive number).

  • Total Totals Index (TT): gives an indication of severity and is best used when plotted on a map and analyzed regionally; is the sum of the Vertical Total (T₈₅₀ - T₅₀₀) and the Cross Total (TD₈₅₀ - T₅₀₀); TT less than 50 are considered weak indicators of severity; TT greater than 55 are considered strong indicators of severity; gives a more accurate picture of potential instability as the air becomes colder and drier

  • Severe Weather ThrEAT Index (SWEAT): gives an indication of potential severity and tornadoes; developed by the Air Force; is a complex formula that incorporates instability, wind shear and wind speeds; a SWEAT of 300 or greater is the threshold for potential severe weather; a SWEAT of 400 or greater is the threshold for potential tornadoes; a low SWEAT value (under 100) almost certainly means that there is no severe weather occurring; a high SWEAT value for a given time (observed or predicted) does not mean that severe weather is occurring or will occur, as some sort of "triggering mechanism" is necessary to release the potential.