Definitions of Atmospheric Variables

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,br> 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 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 activity expected
300 < SW < 400 Isolated moderate to heavy thunderstorms
400 < SW < 500 Severe thunderstorms and tornadoes probable
SW > 500 SW < 500 Severe thunderstorms and tornadoes probable
SW > 500 Severe thunderstorms and tornadoes likely


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