Other UKAWC Stability Indices:
| Lifted Index (LI) | K-Index | CIN | Showalter Index | SWEAT | Total Totals |
|
|
The SWEAT Index evaluates the potential
for severe weather by combining several parameters into one index.
These parameters include low-level moisture (850 mb dewpoint),
instability (Total Totals Index), lower and middle-level (850
and 500 mb) wind speeds, and warm air advection (veering between
850 and 500 mb). Therefore, an attempt is made to incorporate
kinematic and thermodynamic information into one index. As such,
the SWEAT index should be utilized to assess severe weather potential,
not ordinary thunderstorm potential.
SWEAT
= 12 [Td(850 mb)] + 20 (TT - 49) + 2 (f8) + f5 + 125 (S + 0.2)
where TT represents the total
totals index value, f8 and f5 represent the 850 mb and 500 mb
wind speed in knots, respectively, and S = sin (500 mb minus 850
mb wind direction), i.e., the sine of the angle between the 500
and 850 mb wind directions (the shear term).
The last term in the equation (the shear term) is set to zero
if any of the following criteria are not met: 1) 850 mb wind direction
ranges from 130 to 250 degrees, 2) 500 mb wind direction ranges
from 210 to 310 degrees, 3) 500 mb wind direction minus the 850
mb wind direction is a positive number, and 4) both the 850 and
500 mb wind speeds are at least 15 kts. No term in the equation
may be negative; if so, that term is set to zero.
SWEAT over 300: Potential for severe
thunderstorms.
SWEAT over 400: Potential for tornadoes.
These are guidance values developed
by the U.S. Air Force. Severe storms may still be possible for
SWEAT values of 250-300 if strong lifting is present. In addition,
tornadoes may occur with SWEAT values below 400, especially if
convective cell and boundary interactions increase the local shear
which would not be resolved in this index. The SWEAT value can
increase significantly during the day, so low values based on
1200 UTC data may be unrepresentative if substantial changes in
moisture, stability, and/or wind shear occur during the day. Finally,
as with all indices, the SWEAT only indicates the potential for
convection. There must still be sufficient forcing for upward
motion to release the instability before thunderstorms can develop.
Other UKAWC Stability Indices:
| Lifted Index (LI) | K-Index | CIN | Showalter Index | SWEAT | Total Totals |
|
|
|