SPC Lower Atmospheric Severity Index is abbreviated as SPC LASI. It is composed of TERM A and TERM B.

TERM A of the SPC LASI is the temperature lapse rate from 500 ft to 5500 ft (1.5 km) of (15 to 165 mb) above the model surface.

TERM B is the dew point depression (T-Td) at 2500 ft above the surface.

AVERAGE HUMIDITY (used in evaluating dry thunderstorm potential) is computed by averaging the lowest two 30 mb deep layers which represent the lowest 2000 ft (60 mb).

The Eta model output temperature, wind and moisture at the mid-point of six-30 mb "Boundary layers" above the model surface at each grid point. Each 30mb layer is approximately 1000 ft thick.

180 mb above surface____________________________________(Top of Layer 6)____________

Layer 6 mid-point - 165 mb (5500 ft) above surface (T,Td, wind).......................................................

150 mb above surface____________________________________(Top of Layer 5)____________

Layer 5 mid-point - 135 mb above surface(T,Td, wind).......................................................................

120 mb above surface____________________________________(Top of Layer 4)____________

Layer 4 mid-point - 105 mb above surface(T,Td, wind).......................................................................

90 mb above surface_____________________________________(Top of Layer 3)____________

Layer 3 mid-point - 75 mb (2500 ft) above surface(T,Td, wind)..........................................................

60 mb above surface_____________________________________(Top of Layer 2)____________

Layer 2 mid-point - 45 mb above surface(T,Td,wind)..........................................................................

30 mb above surface_____________________________________(Top of layer 1)_____________

Layer 1 mid-point - 15 mb (500 ft) above surface (T,Td,wind)............................................................

______________________________________________________________________SURFACE (Botom of Layer 1)___

EXAMPLE: At a grid point, if the model surface is 970 mb, the top of layer 6 is 790 mb.