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)
|
|
Relative
Humidity (RH) =
- w/ws x 100 %
- e/es x 100 %
|
|
|
Mixing Ratio
(w)
- At p, find Td.
- Read w scale.
- Label in g/kg.
|
Vapor Pressure
(e)
- At p, find Td.
- Extend a line isothermally to 622
millibars.
- Read w scale; label in millibars.
|
|
Saturation
Mixing Ratio (ws)
- At p, find T.
- Read w scale.
- Label in g/kg.
|
Saturation
Vapor Pressure (es)
- At p, find T.
- Extend a line isothermally to 622
millibars.
- Read w scale; label in millibars.
|
|
FORECASTING
SURFACE
TEMPERATURES
|
|
Maximum
Temperature (TMAX)
- At 850 millibars, find T.
- CLR - SCT: Extend line
dry-adiabatically to surface.
- BKN - OVC: Extend line
moist-adiabatically to surface.
- Read temperature scale.
If there is an inversion present with top between 4000 and 6000 ft AGL:
- Find T at warmest point in inversion.
- Extend a line dry-adiabatically to
surface.
- Read temperature scale.
|
Minimum
Temperature (TMIN)
- At 850 millibars, find Td.
- Extend line moist-adiabatically to
surface.
- Read temperature scale.
Alternate method:
- Find max temp; read Td.
- Td will be following
morning's min T.
- Most accurate with CLR-SCT sky and
light and variable wind.
|
|
Wet-Bulb
Temperature (Tw)
- At p, find LCL.
- Extend a line moist-adiabatically
to p.
- Read temperature scale.
|
Potential
Temperature (theta)
- At p, find T.
- Extend a line dry-adiabatically
to 1000 millibars.
- Read temperature scale.
|
|
Equivalent
Temperature (Te)
1. At
p, find LCL
- Extend a line upward moist
adiabats until moist and dry adiabats become parallel
- Then follow a dry adiabat to the
original level p.
- 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)
- If T is in Kelvin and w in kg/kg
- At p, find T and w.
- Use equation: Tv = T(1
+ .6w).
- If T is in C and w in gm/kg
- At p, find T and w.
- Use equation: Tv = T +
w/6
4. Label on temperature scale.
|
Equivalent Potential Temperature
(thetae)
- At p, find LCL (or find Te and go
to 3)
- Extend a line upward moist
adiabats until moist and dry adiabats become parallel
- 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.
|
|
Method:
- Compute and plot Tv curve.
- Find mean Tv using equal area
method (Tv and isotherm.
- Where mean Tv crosses
thickness scale, read value.
- 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:
- More than 4179 meters = Rain.
- Equal to 4179 meters = Mixed.
- Less than 4179 meters = Snow.
|
|
MECHANICAL
LIFT
(PARCEL METHOD)
|
Diagrams illustrating this are here
and here.
|
Lifted Condensation Level (LCL)
- At p, find Td.
- Extend upward
parallel to the mixing ratio line.
- At p, find T.
- Extend a line
dry-adiabatically to intersect line from step 2.
- Read p, label in
millibars.
|
Level of Free Convection (LFC)
- From surface LCL,
extend a line moist-adiabatically to T profile.
- Read LFC in
millibars.
|
|
|
Equilibrium Level (EL)
- From LFC, extend a
line moist-adiabatically to T profile.
- Read EL in
millibars.
|
|
CONVECTIVE
LIFT
(PARCEL METHOD)
|
A diagram illustrating this is here.
|
Convective Condensation Level (CCL)
- At surface, find Td.
- Extend a line
parallel to constant mixing ratio line and intersect T profile.
- Read pressure scale.
|
Level of Free Convection (LFC)
- Find CCL
- Read LFC in
millibars. (Note: CCL will be LFC for convective lift)
|
|
Convective Temperature (Tc)
- Find CCL.
- Extend a line
dry-adiabatically to surface, read T.
|
Equilibrium Level (EL)
- From LFC, extend a
line moist-adiabatically to T profile.
- Read EL in
millibars.
|
|
CONVECTIVE
LIFT
(MOIST LAYER METHOD)
|
|
Convective
Condensation Level (CCL)
- 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.
- Find mean mixing ratio using
equal area method: Td and w or isotherm.
- Extend a line parallel to
constant mixing ratio line and intersect T profile.
|
Level
of Free Convection (LFC)
- Bisect Tw curve in
moist layer using w or isotherm and equal area method.
- From bisection point, extend a
line moist-adiabatically to T profile.
- Read LFC in millibars.
|
|
Convective
Temperature (Tc)
- Find CCL.
- Extend a line moist-adiabatically
to surface, read T.
|
Equilibrium
Level (EL)
- From LFC, extend a line
moist-adiabatically to T profile.
- Read EL in millibars.
|
|
Mixing
Condensation Level (MCL)
- Find top of mixed layer.
- Find mean mixing ratio using equal
area method (mixing ratio line and Td).
- Extend a a line parallel to constant
mixing ratio line to top of mixed layer.
- Find mean potential temperature
using equal area method.
- Extend a line dry-adiabatically from
the mean potential temperature to the top of the mixed layer.
- If line from step 3 and line from
step 5 intersect within layer, read p in millibars. If not, there is no
MCL.
|
|
Radiation
Inversion (illustration here)
- Surface based.
- Often T = Td
or T ~ Td at surface.
- Tdwill be
almost parallel to mixing ratio line within the inversion.
- T and Td
cools above the inversion.
|
Frontal Inversion (illustration here)
- T is shallow
isothermal or stable in inversion.
- Td
increases within inversion.
|
|
Subsidence Inversion (illustration here)
- T increases in
inversion.
- Td
rapidly decreases in inversion.
- T cools
approximately dry-adiabatically above inversion.
|
Turbulence Inversion
- T is dry-adiabatic
below inversion.
- Td is
parallel to mixing ratio lines below the inversion.
- T is isothermal in
the inversion.
|
|
FORECASTING
RADIATION
FOG DISSIPATION
|
|
Legend
- wa = Surface mixing
ratio.
- wb = Inversion-top
mixing ratio.
- w' = Mean mixing ratio =
(wa
+ wb)/2
- Tx = Intersection temp
(w and T).
- dT = Fig dissipation temp.
- DT = dT - Tx.
|
Method
- Determine wa and wb.
- Determine w' using formula; plot.
- Read Tx at lowest
intersection of w and T.
- From Tx extend a line
dry-adiabatically to surface: Read dT.
- Find DT; multiply by 328 feet for
fog depth.
DO NOT USE HEIGHT SCALE
|
|
FORECASTING
STRATUS
DISSIPATION
|
|
Legend
- wa = Surface mixing
ratio.
- wb = Inversion-base
mixing ratio.
- w' = Representative mixing ratio =
(((wa
+ wb)/2)+Wa)/2
- Tx1 = First
intersection w' and temp (base of stratus).
- Tx2 = Second
intersection w' and temp (top of stratus).
- dT1 = Stratus
dissipation beginning temperature.
- dT2 = Stratus
dissipation ending temperature.
- DT1 = dT1
- Tx1.
- DT2 = dT2
- Tx2.
|
Method
- Determine wa and wb.
- Determine w' using formula;
plot.
- Read Tx1 and Tx2.
- From Tx1 and Tx2,
extend lines dry-adiabatically to surface.
- Read dT1 and dT2.
- Find DT1 and DT2.
- Stratus base = DT1 x
328 feet.
- Stratus top = DT2 x
328 feet.
DO NOT USE HEIGHT SCALE
|
|
FORECASTING
CONDENSATION
TRAILS (CONTRAILS)
|
|
Method
- If forecasting clear-scattered conditions
at cirrus levels, draw in 40 % RH line.
- If forecasting cloud deck in vicinity of
tropopause, or broken-overcast clouds at cirrus levels, draw in 70 % RH
line.
- Using temperature profile, forecast
contrails to the left of the 40 % or 70 % RH line, depending on cloud
forecast.
- Label contrail area boundaries in
millibars.
|
|
Definitions
(illustration here)
- Absolutely stable:
lapse rate .lt. moist adiabatic rate .lt. dry adiabatic rate.
- Absolutely unstable:
moist adiabatic rate .lt. dry adiabatic rate .lt. lapse rate.
- 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
- Lift layer desired
number of millibars by lifting both base and top the same distance.
- Lift temperature
dry-adiabatically until saturation, then moist adiabatically remainder
of distance.
- Construct new
temperature curve by connecting base and top of lifted layer.
- Check stability
indices.
|
|
Showalter
Stability Index (SSI)
- Find LCL for 850 mb.
- From LCL, extend a line
moist-adiabatically to 500 mbs.
- Read T'.
- Use formula: SSI = T500 - T'.
|
Lifted
Index (LI)
- In lowest 100 mb, bisect Td
with w to form equal areas. w becomes wm.
- Extend wm past 500 mbs.
- Extend a line dry-adiabatically
from T850 to wm.
- From intersection, extend a
line moist-adiabatically to 500 mbs.
- Read T'.
- 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)
- Determine moist layer.
- In moist layer, bisect Tw
curve with w or isotherm to form equal areas.
- From bisection point, extend a
line moist adiabatically to 500 mbs; read T'.
- 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
- D = 850 mb Td. If
.le. 0, D = 0.
- T = TTI. If .le. 49, TTI = 0.
- f8
= 850 mb wind speed.
- f5 =
500 mb wind speed.
- 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)
- From warmest point of inversion,
extend a line moist-adiabatically to 600 mbs.
- Label B' on isotherm scale. B = T600.
- See AWSTR 200, pg. 10-4 for
uncorrected gust speed (v') (see below).
- Add 1/3 mean wind speed, surface to
5000 feet (v) to v' for max gust speed.
- Direction is wind direction between
10,000 and 14,000 feet, or use VRB.
|
T1
Method for TS Gusts (No Inverson)
- Forecast TMAX
using clear-scattered method.
- Extend a line moist-adiabatically to
600 mbs and label B' on isotherm scale. B = T600.
- See AWSTR 200, pg. 10-4 for
uncorrected gust speed (v') (see below).
- Add 1/3 mean wind speed, surface to
5000 feet (v) to v' for max gust speed.
- Direction is wind direction between
10,000 and 14,000 feet, or use VRB.
|
Table from AWSTR 200 Page 10-4
Use of T1 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
- Find downrush temperature (TDR)
by extending a line moist-adiabatically from welt-bulb zero (WBZ) to
surface.
- Find TMAX
using clear-scattered method.
- T2 = TMAX
- TDR.
- See AWSTR 200, pg. 10-5 for max gust
(see below). Note 3 values, but forecast the highest.
- Direction is wind direction between
10,000 and 14,000 feet, or use VRB.
|
Hailsize
- Draw two reference lines:
millibar-level of CCLML and millibar-level of -5 dC on
T profile.
- At intersection of T and CCLML,
extend a line moist-adiabatically to second reference line. Label B'.
- Label -5 dC point as B. Base - B' -
B.
- From 0 dC isotherm on second
reference line, extend a line dry-adiabatically to first reference line.
Label H' on isotherm scale.
- Label point on second reference line
as H. Altitude = H' - H.
- See AWSTR 200, pg. 9-2 for
uncorrected hail size (see below).
- If WBZ .ge. 10,000 ft AGL, see See
AWSTR 200, pg. 9-4 for corrected hail size (see below).
- If WBZ .lt. 10,000 ft AGL, no
correction is needed.
|
Figure from AWSTR 200 Page 10-5
Use of T2 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
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.