METR 361
Spring, 2014
Stability Indices
Assessing stability of a sounding on a thermodynamic diagram is usually too time-consuming a process considering
the daily routine of a weather office. This is especially true during times when thunderstorms are likely to occur. For this
reason indices have been devised to give a rapid means for making stability assessments. The various stability indices
described below have been statistically linked to shower and thundershower activity. In the winter, many of these indices are
used as indicators of the atmosphere’s tendency to create snow squalls or convective clouds in general. When using any of
these indices for forecasting purposes, one must be careful to anticipate changes that might occur in the environmental lapse
rate between the sounding time (usually 12Z) and the expected time of convective activity (usually 5-10 hours after 12Z).
Lifted Index
This is probably the most well-known index. Lift a parcel from the surface to its LCL. Then lift it moistadiabatically to 500 hPa. Subtract the lifted parcel temperature from the environmental 500 hPa temperature.
LI = T500 - Temperature of lifted parcel
The LI works well in severe weather situations with a conditionally unstable atmosphere and a low level trigger for
lifting, such as a cold front or pressure trough.
K Index
Subtract the environmental 500 hPa temperature from the 850 hPa temperature. Add the 850 hPa dew point.
Subtract the 700 hPa dew point depression.
K = T850 - T500 + Td 850 - (T700 - Td 700)
The K Index does not work well in severe weather situations because convectively unstable atmospheres usually
have mid-tropospheric dry tongues at 700 hPa. To maximize the K Index, the low level (850 hPa) should be warm and
moist, the upper level (500 hPa) cold and the 700 hPa dew point depression should be small, indicating deep low level
moisture.
Total-Totals Index
This index consists of two sub-indices, the Vertical Totals and the Cross Totals. The Vertical Totals index is the
850 hPa temperature minus the 500 hPa temperature. The Cross Totals index is the 850 hPa dew point minus the 500 hPa
temperature. The Total-Totals index is the sum of the Vertical and Cross Totals.
TT = (T850 - T500) + (Td 850 - T500) = T850 + Td 850 - 2T500
For the TT to work well, the lower level (850 hPa) should be warm and moist and the upper level (500 hPa) cold. It
is a fairly reliable severe thunderstorm predictor.
Severe Weather Threat Index (SWEAT)
The SWEAT Index was developed by the Air Force Global Weather Center severe storm group. It puts more
information into an index than any of the others listed. Wind speeds and shear are included. To calculate the SWEAT Index,
first multiply the 850 hPa dew point by 12. Then subtract 49 from the Total-Totals Index and multiply the result by 20.
Add twice the 850 hPa speed to the 500 hPa wind speed. Finally, subtract the 850 hPa wind direction from the 500 hPa wind
direction, take the sine, add 0.2 and multiply the result by 125. All negative terms are set to zero for the SWEAT index.
SWEAT = 12Td 850 + 20(TTI - 49) + 2w850 + w500 + 125(S + 0.2)
In SWEAT,
Td 850 = 850 hPa dew point in °C
w850, w500 = 850hPa and 500 hPa wind speeds in knots
S = sin(500 hPa wind direction - 850 hPa wind direction).
This term is set to zero if either w850 or w500 are less than 15 knots. S is not computed
unless the 500 hPa wind direction is within the range 210° to 310° and 850 hPa wind
direction is in the range 130° to 250°.
CAPE (Convective Available Potential Energy)
This index is calculated from a sounding on the thermodynamic diagram. Lift a parcel from the surface to
the tropopause. The parcel will saturate at the LCL and rise moist-adiabatically. Whenever the parcel is warmer
than the sounding, add the area between the parcel and the sounding to CAPE. When the parcel is cooler than the
sounding, subtract the area from CAPE. Values around 1500-2000 are high. CAPE has substantial variability.
CIN (Convective Inhibition)
The convective inhibition is a measure of the cap or lid. As in CAPE, lift a parcel. Before it gets to its level
of free convection (LFC), it will have negative buoyant energy, i.e., negative CAPE. That’s CINH.
LCL (Lifting Condensation Level)
Not an index but important. When lifting a parcel from the surface, this is where condensation first occurs.
CCL (Convective Condensation Level)
This is the level where condensation occurs if a parcel lifts itself from the surface due to positive buoyancy.
You will need to heat the surface air until it breaks the cap. The CCL will be higher than the LCL.
LFC (Level of Free Convection)
This is where negative buoyancy becomes positive. In practical terms, this is where a lifted surface parcel
(not one which was heated as in the CCL) will rise on its own due to buoyancy.
SUGGESTED INDEX THRESHOLD AND CRITICAL VALUES
GENERAL THUNDERSTORMS
UNLIKELY POSSIBLE VERY LIKELY
LI
> -1
K
> 16
TTI < 46
SWEAT < 200
-1 to -2
16 to 36
46 to 50
200 to 300
< -2
> 36
> 50
>300
SEVERE THUNDERSTORMS
UNLIKELY POSSIBLE VERY LIKELY
> -3
NA
< 50
< 300
-3
NA
50 to 55
300 to 500
< -4
NA
> 55
> 500
These are only suggested values that should be applied with caution (or refined) to local areas.
ASSIGNMENT (due next Wednesday)
1. On thermodynamic diagrams, plot the two given soundings for stations AAA and BBB. Plot
temperature, dew point and wind barbs to 100 hPa, using the usual format. For both AAA and BBB, do this:
2. Calculate the LCL and LFC of surface air, the Lifted Index, the K Index, the Total-Totals Index, and the
Severe Weather Threat Index. Write these indices on the top of each sounding. Show your work where possible.
3. Assess the potential for severe weather (poor, fair, good, spectacular, etc.) listing four reasons for your
assessment. Use the indices calculated in part 1 as one of your reasons but be specific about how the individual
indices best predicted the severe weather potential. Examine the sounding itself for the other three reasons. Write
your assessments and reasons on a separate sheet of paper.
4. The last sounding is from Little Rock, Arkansas (KLZK), taken at 18Z (special sounding) on February 5,
2008. Plot the temperature, dew point, wind barbs ONLY. Calculate the LCL of surface air, both LFC’s, the
Lifted Index using surface air, the K Index, the Total-Totals Index, and the Severe Weather Threat Index. You
must also calculate the CCL in hPa and estimate the surface temperature required for air parcels to acquire the
CCL. Assess the potential for severe weather, using the indices and anything else about this sounding (like
CAPE and CIN).
Write the calculated indices on the top of the sounding. On a separate sheet of paper, show your
work where possible.
Station AAA at 12Z
Level T
Td
DDFFF
(hPa) (°C) (°C)
(knots)
100
150
191
200
250
300
317
385
400
444
500
550
700
757
769
850
900
932
992
-64.0
-59.7
-59.1
-63.5
-53.0
-44.6
-40.0
-32.9
-30.1
-27.5
-16.2
-10.0
7.2
12.0
7.3
13.2
16.1
17.8
20.2
-44.7
-33.8
-30.2
-36.2
-20.7
-22.2
-24.8
-18.2
7.2
12.8
15.3
16.2
16.7
24048
24084
23597
23599
23084
23060
22540
21542
19046
Level
(hPa)
T
(°C)
Station BBB at 12Z
Td
DDFFF
(°C)
(knots)
100
150
200
250
300
353
400
458
500
579
700
729
766
850
872
902
1000
1016
-72.1
-62.7
-55.1
-46.3
-39.9
-30.9
-26.3
-17.7
-13.7
-6.5
2.2
2.6
5.6
8.8
10.8
12.6
17.6
18.8
-48.9
-40.9
-56.3
-47.7
-43.7
-36.5
-2.8
1.6
5.1
6.8
6.4
11.0
16.5
17.5
26576
26604
26626
26621
27068
26540
27029
27516
15521
11015
10009
17015
Sounding for Little Rock, AR (KLZK) for 18Z Feb 5, 2008
------------------------------------------------------------------------------LEV PRES HGHT
TEMP
DEWP
RH
DD
DIR
SPD
mb
m
C
C
%
C
deg
knt
------------------------------------------------------------------------------988.0
172
21.0
17.1
78 12.57
180
9
925.0
736
16.6
14.4
87 11.27
195
27
905.9
914
15.3
13.9
92 11.16
200
30
850.0
1452
12.2
11.0
92
9.79
215
43
812.4
1829
10.2
7.9
86
8.30
215
49
754.4
2438
5.9
4.3
90
6.96
215
54
700.0
3053
1.6
0.7
94
5.78
210
61
693.0
3134
0.8
-0.2
93
5.47
211
61
680.0
3287
5.6 -15.4
20
1.71
212
60
663.0
3493
4.4 -17.6
18
1.46
214
60
649.4
3658
2.8 -16.7
22
1.61
215
59
601.6
4267
-3.0 -13.3
45
2.30
215
57
573.0
4655
-6.7 -11.1
71
2.87
212
60
500.0
5700 -15.5 -19.2
73
1.68
210
61
400.0
7360 -25.3 -31.3
57
0.70
220
76
300.0
9370 -42.5 -48.5
52
0.16
230
100
250.0 10590 -48.7 -66.7
11
0.02
235
76
200.0 12050 -47.9 -72.9
4
0.01
150.0 13900 -58.7 -79.7
5
0.00
100.0 16400 -68.5 -86.5
6
0.00