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ATMOSPHERIC STABILITY INDICIES
Lifted Index (LI)
This is an index used to determine the stability of the lower half of the
troposphere. An air parcel is lifted from the surface with temperature and
mixing ratios representative of the mean layer values of the lowest 100 mb
of the atmosphere. This is done in order to capture low level boundary layer
temperature and moisture conditions while reducing diurnal effects. This
hypothetical parcel is then lifted dry adiabatically to the LCL and
pseudo-adiabatically to 500 mb. The value of this index is the temperature
of the environment subtracted from the temperature of the parcel at 500 mb.
The risk of thunderstorms and severe weather activity is defined as follows:
LI > 2 No significant activity
0 < LI < 2 Showers/thunderstorms possible with other source of lift -2 < LI < 0 Thunderstorms possible -4 < LI < -2 Thunderstorms more probable, but few, if any severe LI < -4 Severe thunderstorms possible Showalter Index (SI)
This is an index used to determine the stability of the lower half of the
troposphere. An air parcel is lifted from an initial position at 850 mb where
localized low level influences are greatly reduced. It is lifted dry
adiabatically to its LCL and then pseudo-adiabatically to 500 mb. The
environmental temperature is then subtracted from the parcel temperature
to obtain the value of the Showalter index. The risk of severe weather activity
is defined as follows: SI > 3 No significant activity
1 < SI < 3 Showers possible with other source of lift -2 < SI < 1 Thunderstorms possible (generally weak) -3 < SI < -2 Thunderstorms more probable (possibly strong) -6 < SI < -4 Strong or severe thunderstorms possible SI < -6 Any thunderstorms likely to be strong or severe Total Totals Index (TT) The Total Totals index is a simple index derived from the temperature lapse rate between 850 mb and 500 mb and moisture content at 850 mpse rate between 850 mb and 500 mb and moisture content at 850 mb. It is defined as follows: TT = T850 + Td850 - 2T500
The risk of severe weather activity is defined as follows: 44-45 Isolated moderate thunderstorms
46-47 Scattered moderate / few heavy thunderstorms 48-49 Scattered moderate / few heavy / isolated severe thunderstorms 50-51 Scattered heavy / few severe thunderstorms and isolated tornadoes 52-55 Scattered to numerous heavy / few to scattered severe thunderstorm / few tornadoes > 55 Numerous heavy / scattered severe thunderstorms and scattered tornadoes K Index (KI) The K index is similar to Total Totals index except that it takes into account moist air at 700 mb contributing to air mass thunderstorm development. The K index is defined as follows: K = T850 - T500 + Td850 - (T700 - Td700)
The risk of air mass thunderstorms is defined as follows: K < 15 near 0% Air mass thunderstorm probability
15-20 <20% Air mass thunderstorm probability 21-25 20-40% Air mass thunderstorm probability 26-30 40-60% Air mass thunderstorm probability 31-35 60-80% Air mass thunderstorm probability 36-40 80-90% Air mass thunderstorm probability K > 40 >90% Air mass thunderstorm probability SWEAT ( Severe Weather thrEAT ) Index
This is an United States Air Force index that takes into account thermal
instability, low level moisture content, vertical wind shear and horizontal
wind speeds. The risk of severe weather activity is defined as follows: SW < 300 No severe storms expected
300 < SW < 400 Possible severe thunderstorms SW > 400 Severe thunderstorms likely and possible tornadoes Bulk Richardson Number (BRN)
Ratio between the CAPE and the wind shear vector difference. A valuable tool in
determining if potential storms will be supercells. MUST
be used with other thunderstorm parameters when making thunderstorm decisions.
BRN < 10 Probably too much shear for thunderstorms
BRN > 10 < 45 Supercells possible BRN > 45 Storms more likely to be multicells rather than supercells Convective Available Potential Energy (CAPE):
CAPE + represents the vertically integrated positive buoyancy of an
adiabatically rising parcel. Proportional to the amount of kinetic energy that
a parcel gains while it is warmer than the surrounding environment. Parcel ascent
is enhanced by this instability. CAPE- represents the cumulative effect of
atmospheric layers that are warmer than the parcel moving vertically along an adiabat.
These stable layers often inhibit low-level parcel ascent.
CAPE < 1000 instability is WEAK
CAPE > 1000 < 2500 MODERATE instability CAPE > 2500 STRONG instability
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