SKEW-T ANALYSIS METHODS

 

A legend explaining the isopleths on the Skew-T is here.
A sample sounding is here.

 

1. Moisture content (RH, w, ws, e, es)
2. Forecasting surface temps (Tmax, Tmin)
3. Theoretical temps (Tw, theta, thetaw, thetae, Tv)
4. Thickness (method, snow index)
5. Mechanical lift (LCLp, LFCp, ELp)
6. Convective lift (CCLp, Tp, LFCp, ELp)
7. Convective lift (CCLML, TCML, LFCCML, ELCML)
8. Turbulent lift (MCLML)
9. Inversions (radiation, subsidence, frontal, turbulent)
10. Fog dissipation (method)

11. Stratus dissipation (method)
12. Contrails (method)
13. Stability indices (definitions, lifted layer)
14. Stability indices (SSI, LI)
15. Stability indices (FMI, KI)
16. Stability indices (TTI, SWEAT)
17. TRW elements (T1, T1A for gusts)
18. TRW elements (T2 for gusts; hailsize)

 


 

MOISTURE CONTENT

 

Relative Humidity (RH) =

  1. w/ws x 100 %
  2. e/es x 100 %

 

Mixing Ratio (w)

  1. At p, find Td.
  2. Read w scale.
  3. Label in g/kg.

Vapor Pressure (e)

  1. At p, find Td.
  2. Extend a line isothermally to 622 millibars.
  3. Read w scale; label in millibars.

Saturation Mixing Ratio (ws)

  1. At p, find T.
  2. Read w scale.
  3. Label in g/kg.

Saturation Vapor Pressure (es)

  1. At p, find T.
  2. Extend a line isothermally to 622 millibars.
  3. Read w scale; label in millibars.

 


 

FORECASTING SURFACE
TEMPERATURES

 

Maximum Temperature (TMAX)

  1. At 850 millibars, find T.
  2. CLR - SCT: Extend line dry-adiabatically to surface.
  3. BKN - OVC: Extend line moist-adiabatically to surface.
  4. Read temperature scale.


If there is an inversion present with top between 4000 and 6000 ft AGL:

  1. Find T at warmest point in inversion.
  2. Extend a line dry-adiabatically to surface.
  3. Read temperature scale.

Minimum Temperature (TMIN)

  1. At 850 millibars, find Td.
  2. Extend line moist-adiabatically to surface.
  3. Read temperature scale.




Alternate method:

  1. Find max temp; read Td.
  2. Td will be following morning's min T.
  3. Most accurate with CLR-SCT sky and light and variable wind.

 


 

THEORETICAL TEMPERATURES

 

Wet-Bulb Temperature (Tw)

  1. At p, find LCL.
  2. Extend a line moist-adiabatically to p.
  3. Read temperature scale.

Potential Temperature (theta)

  1. At p, find T.
  2. Extend a line dry-adiabatically to 1000 millibars.
  3. Read temperature scale.

Equivalent Temperature (Te)

1.      At p, find LCL

  1. Extend a line upward moist adiabats until moist and dry adiabats become parallel
  2. Then follow a dry adiabat to the original level p.
  3. Read temperature scale.

Wet-Bulb Potential Temperature (thetaw)

1.      At p, find LCL or Tw.

2.      Extend a line moist-adiabatically to 1000 millibars.

3.      Read temperature scale.

Virtual Temperature (Tv)

  1. If T is in Kelvin and w in kg/kg
  2. At p, find T and w.
  3. Use equation: Tv = T(1 + .6w).
  1. If T is in C and w in gm/kg
  2. At p, find T and w.
  3. Use equation: Tv = T + w/6

4. Label on temperature scale.

Equivalent Potential Temperature (thetae)

  1. At p, find LCL (or find Te and go to 3)
  2. Extend a line upward moist adiabats until moist and dry adiabats become parallel
  3. Then follow a dry adiabat to 1000 mb level. Read temperature scale.

 

also qe q e(2675w/TLCL) where

 

2675 is an empirical constant, w is mixing ration in kg/kg and TLCL is the temperature at the LCL of level p.

 


 

THICKNESS
(DELTA-Z)

 

Method:

  1. Compute and plot Tv curve.
  2. Find mean Tv using equal area method (Tv and isotherm.
  3. Where mean Tv crosses thickness scale, read value.
  4. Label in feet or meters.

Snow Index:

y + 2x = N



y = 850 to 700 mb delta-z
x = 1000 to 850 mb delta-z


Scale:

  1. More than 4179 meters = Rain.
  2. Equal to 4179 meters = Mixed.
  3. Less than 4179 meters = Snow.

 


 

MECHANICAL LIFT
(PARCEL METHOD)

 

Diagrams illustrating this are here and here.

 

Lifted Condensation Level (LCL)

  1. At p, find Td.
  2. Extend upward parallel to the mixing ratio line.
  3. At p, find T.
  4. Extend a line dry-adiabatically to intersect line from step 2.
  5. Read p, label in millibars.

Level of Free Convection (LFC)

  1. From surface LCL, extend a line moist-adiabatically to T profile.
  2. Read LFC in millibars.

 

Equilibrium Level (EL)

  1. From LFC, extend a line moist-adiabatically to T profile.
  2. Read EL in millibars.

 


 

CONVECTIVE LIFT
(PARCEL METHOD)

 

A diagram illustrating this is here.

 

Convective Condensation Level (CCL)

  1. At surface, find Td.
  2. Extend a line parallel to constant mixing ratio line and intersect T profile.
  3. Read pressure scale.

Level of Free Convection (LFC)

  1. Find CCL
  2. Read LFC in millibars. (Note: CCL will be LFC for convective lift)

Convective Temperature (Tc)

  1. Find CCL.
  2. Extend a line dry-adiabatically to surface, read T.

Equilibrium Level (EL)

  1. From LFC, extend a line moist-adiabatically to T profile.
  2. Read EL in millibars.

 


 

CONVECTIVE LIFT
(MOIST LAYER METHOD)

 

Convective Condensation Level (CCL)

  1. Determine top of moist layer: Dew-point depression greater than 6 degrees Celsius; if greater than 6000 feet, use lowest 150 millibars as moist layer.
  2. Find mean mixing ratio using equal area method: Td and w or isotherm.
  3. Extend a line parallel to constant mixing ratio line and intersect T profile.

Level of Free Convection (LFC)

  1. Bisect Tw curve in moist layer using w or isotherm and equal area method.
  2. From bisection point, extend a line moist-adiabatically to T profile.
  3. Read LFC in millibars.

Convective Temperature (Tc)

  1. Find CCL.
  2. Extend a line moist-adiabatically to surface, read T.

Equilibrium Level (EL)

  1. From LFC, extend a line moist-adiabatically to T profile.
  2. Read EL in millibars.

 


 

TURBULENT LIFT

 

Mixing Condensation Level (MCL)

  1. Find top of mixed layer.
  2. Find mean mixing ratio using equal area method (mixing ratio line and Td).
  3. Extend a a line parallel to constant mixing ratio line to top of mixed layer.
  4. Find mean potential temperature using equal area method.
  5. Extend a line dry-adiabatically from the mean potential temperature to the top of the mixed layer.
  6. If line from step 3 and line from step 5 intersect within layer, read p in millibars. If not, there is no MCL.

 


 

INVERSIONS

 

Radiation Inversion (illustration here)

  1. Surface based.
  2. Often T = Td or T ~ Td at surface.
  3. Tdwill be almost parallel to mixing ratio line within the inversion.
  4. T and Td cools above the inversion.

Frontal Inversion (illustration here)

  1. T is shallow isothermal or stable in inversion.
  2. Td increases within inversion.

Subsidence Inversion (illustration here)

  1. T increases in inversion.
  2. Td rapidly decreases in inversion.
  3. T cools approximately dry-adiabatically above inversion.

Turbulence Inversion

  1. T is dry-adiabatic below inversion.
  2. Td is parallel to mixing ratio lines below the inversion.
  3. T is isothermal in the inversion.

 


 

FORECASTING RADIATION
FOG DISSIPATION

 

Legend

  1. wa = Surface mixing ratio.
  2. wb = Inversion-top mixing ratio.
  3. w' = Mean mixing ratio =

(wa + wb)/2

 

  1. Tx = Intersection temp (w and T).
  2. dT = Fig dissipation temp.
  3. DT = dT - Tx.

Method

  1. Determine wa and wb.
  2. Determine w' using formula; plot.
  3. Read Tx at lowest intersection of w and T.
  4. From Tx extend a line dry-adiabatically to surface: Read dT.
  5. Find DT; multiply by 328 feet for fog depth.

 

DO NOT USE HEIGHT SCALE

 

 


 

FORECASTING STRATUS
DISSIPATION

 

Legend

  1. wa = Surface mixing ratio.
  2. wb = Inversion-base mixing ratio.
  3. w' = Representative mixing ratio =

(((wa + wb)/2)+Wa)/2

 

  1. Tx1 = First intersection w' and temp (base of stratus).
  2. Tx2 = Second intersection w' and temp (top of stratus).
  3. dT1 = Stratus dissipation beginning temperature.
  4. dT2 = Stratus dissipation ending temperature.
  5. DT1 = dT1 - Tx1.
  6. DT2 = dT2 - Tx2.

Method

  1. Determine wa and wb.
  2. Determine w' using formula; plot.
  3. Read Tx1 and Tx2.
  4. From Tx1 and Tx2, extend lines dry-adiabatically to surface.
  5. Read dT1 and dT2.
  6. Find DT1 and DT2.
  7. Stratus base = DT1 x 328 feet.
  8. Stratus top = DT2 x 328 feet.

 

DO NOT USE HEIGHT SCALE

 

 


 

FORECASTING CONDENSATION
TRAILS (CONTRAILS)

 

Method

  1. If forecasting clear-scattered conditions at cirrus levels, draw in 40 % RH line.
  2. If forecasting cloud deck in vicinity of tropopause, or broken-overcast clouds at cirrus levels, draw in 70 % RH line.
  3. Using temperature profile, forecast contrails to the left of the 40 % or 70 % RH line, depending on cloud forecast.
  4. Label contrail area boundaries in millibars.

 


 

STABILITY INDICES

 

Definitions (illustration here)

  1. Absolutely stable: lapse rate .lt. moist adiabatic rate .lt. dry adiabatic rate.
  2. Absolutely unstable: moist adiabatic rate .lt. dry adiabatic rate .lt. lapse rate.
  3. Conditional states: moist adiabatic rate .lt. lapse rate .lt. dry adiabatic rate.

DPD = 0 -> Conditionally unstable
DPD > 0 -> Conditionally stable

Stability of a lifted layer

  1. Lift layer desired number of millibars by lifting both base and top the same distance.
  2. Lift temperature dry-adiabatically until saturation, then moist adiabatically remainder of distance.
  3. Construct new temperature curve by connecting base and top of lifted layer.
  4. Check stability indices.




Showalter Stability Index (SSI)

  1. Find LCL for 850 mb.
  2. From LCL, extend a line moist-adiabatically to 500 mbs.
  3. Read T'.
  4. Use formula: SSI = T500 - T'.

Lifted Index (LI)

  1. In lowest 100 mb, bisect Td with w to form equal areas. w becomes wm.
  2. Extend wm past 500 mbs.
  3. Extend a line dry-adiabatically from T850 to wm.
  4. From intersection, extend a line moist-adiabatically to 500 mbs.
  5. Read T'.
  6. Use formula: LI = T500 - T'.

SCALE

+2 to +3

RASH/SNSH possible

-2 to +1

TS possible

-5 to -3

TS+ possible

.le. -6

Tornado possible

SCALE

0 to -2

RASH/SNSH possible

-3 to -5

TS possible

-6

TS+ possible

.le. -7

Tornado possible




Fawbush-Miller Index (FMI)

  1. Determine moist layer.
  2. In moist layer, bisect Tw curve with w or isotherm to form equal areas.
  3. From bisection point, extend a line moist adiabatically to 500 mbs; read T'.
  4. Use formula: FMI = T500 - T'.

K Index (KI)

KI = (T + Td)850 - (T + Td)700 - T500.

 

SCALE

.gt. 1

Relatively stable

0 to -2

Slightly unstable

- 2 to -6

Moderately unstable

.le. -7

Strongly unstable

SCALE (Airmass TS Probability)

.lt. 15

0 %

15 to 20

20 %

21 to 25

20 to 40 %

26 to 30

40 to 60 %

31 to 35

60 to 80 %

36 to 40

80 to 90 %

.gt. 40

~ 100 %




Total-Totals Index TTI)

TTI = (T + Td)850 - (2 x T500).

 

Severe Weather Threat (SWEAT) Index

SWEAT = 12D + 20(T - 49) + 2f8 + f5 + VT

 

  1. D = 850 mb Td. If .le. 0, D = 0.
  2. T = TTI. If .le. 49, TTI = 0.
  3. f8 = 850 mb wind speed.
  4. f5 = 500 mb wind speed.
  5. VT = Veering Term = 125(S - 0.20), where S = sine of the angle between 850 mb and 500 mb winds.

Note: VT = 0 if any of the following apply:
Either 850 mb or 500 mb wind speed is less than 15 knots.
850 mb wind NOT between 130 degrees to 250 degrees inclusive.
500 mb wind not between 210 degrees and 310 degrees inclusive.
850 mb to 500 mb wind direction doesn't veer.

SCALE (Severe Weather Probability)

.ge. 49

Weak

50 to 55

Moderate

.ge. 56

Strong

SCALE

.gt. 400

Tornadoes

.ge. 250 to .lt. 400

TS+

.lt. 250

Disregard

 


 

FORECASTING THUNDERSTORM
ELEMENTS

 

T1 Method for TS Gusts (Inverson Present)

  1. From warmest point of inversion, extend a line moist-adiabatically to 600 mbs.
  2. Label B' on isotherm scale. B = T600.
  3. See AWSTR 200, pg. 10-4 for uncorrected gust speed (v') (see below).
  4. Add 1/3 mean wind speed, surface to 5000 feet (v) to v' for max gust speed.
  5. Direction is wind direction between 10,000 and 14,000 feet, or use VRB.

T1 Method for TS Gusts (No Inverson)

  1. Forecast TMAX using clear-scattered method.
  2. Extend a line moist-adiabatically to 600 mbs and label B' on isotherm scale. B = T600.
  3. See AWSTR 200, pg. 10-4 for uncorrected gust speed (v') (see below).
  4. Add 1/3 mean wind speed, surface to 5000 feet (v) to v' for max gust speed.
  5. Direction is wind direction between 10,000 and 14,000 feet, or use VRB.

 

Table from AWSTR 200 Page 10-4
Use of T
1 Method for Maximum Wind Gusts

 

T1 values in dC

Maximum gust speed (v') in knots

3

17

4

20

5

23

6

26

7

29

8

32

9

35

10

37

11

39

12

41

13

45

14

47

15

49

16

51

17

53

18

55

19

57

20

58

21

60

22

61

23

63

24

64

25

65




T2 Method for TS Gusts

  1. Find downrush temperature (TDR) by extending a line moist-adiabatically from welt-bulb zero (WBZ) to surface.
  2. Find TMAX using clear-scattered method.
  3. T2 = TMAX - TDR.
  4. See AWSTR 200, pg. 10-5 for max gust (see below). Note 3 values, but forecast the highest.
  5. Direction is wind direction between 10,000 and 14,000 feet, or use VRB.

Hailsize

  1. Draw two reference lines: millibar-level of CCLML and millibar-level of -5 dC on T profile.
  2. At intersection of T and CCLML, extend a line moist-adiabatically to second reference line. Label B'.
  3. Label -5 dC point as B. Base - B' - B.
  4. From 0 dC isotherm on second reference line, extend a line dry-adiabatically to first reference line. Label H' on isotherm scale.
  5. Label point on second reference line as H. Altitude = H' - H.
  6. See AWSTR 200, pg. 9-2 for uncorrected hail size (see below).
  7. If WBZ .ge. 10,000 ft AGL, see See AWSTR 200, pg. 9-4 for corrected hail size (see below).
  8. If WBZ .lt. 10,000 ft AGL, no correction is needed.

 

 

Figure from AWSTR 200 Page 10-5
Use of T
2 Alternate Method for Maximum Wind Gusts

 


 

 

Figure from AWSTR 200 Page 9-2
Uncorrected Hail Size

 


 

 

Figure from AWSTR 200 Page 9-4
Corrected Hail Size

 


 

SOURCES

 

Miller, Robert C., 1975. Notes on Analysis and Severe-Storm Forecasting Procedures of the Air Force Global Weather Central (Technical Report 200 -- Revised). Air Weather Service (MAC): United States Air Force.

U.S. Air Force's weather forecasting school lecture notes, Chanute Air Force Base, Rantoul, Illinois, 1984.