SkewT Plots

A Skew T plot is a standard plot used by meteorologists to analyze data from
a balloon sounding.  This is a plot of temperature with height as denoted by
pressure.  Standard pressure levels and approximate heights:

   Pressure     Approximate Height    Approximate Temp
   Sea level        0 m       0 ft       15 C   59 F
   1000 mb        100 m     300 ft       15 C   59 F
    850 mb       1500 m    5000 ft        5 C   41 F
    700 mb       3000 m   10000 ft       -5 C   23 F
    500 mb       5000 m   18000 ft      -20 C   -4 F
    300 mb       9000 m   30000 ft      -45 C  -49 F
    200 mb      12000 m   40000 ft      -55 C  -67 F
    100 mb      16000 m   53000 ft      -56 C  -69 F

The pressure lines are plotted horizontally in blue and are also on an inverse
log scale.  The concept of Skew T means that the temperature is not plotted 
vertically but angles off to the right at a 45 degree angle.  The temperature 
lines of the Skew T are in blue.  The green lines are called dry adiabats.
The light blue dashed lines are saturation adiabats.  The yellow dashed
lines are lines of constant mixing ratio.

The sounding is plotted as two white lines.  The right line is the temperature
profile.  The left line is the dewpoint profile.  The winds are plotted as
wind barbs (See Weather Symbols Legend) with height on the right edge of the
plot.  Specific sounding related parameters are plotted in the upper right.  
These values are:

    TP:   Tropopause level as pressure in mb
    MW:   Maximum wind level as pressure in mb
    LCL:  Surface lifted condensation level in mb
    PW:   Precipitable water in inches
    TH:   1000-500 mb thickness
    LI:   Lifted index
    SI:   Showalter index
    TT:   Total totals index
    KI:   K index
    SW:   Sweat index

Sounding diagrams provide a important means for determining the stability
of the atmosphere above a specific location.   By using the concept of an
air parcel, lifting it or lowering it and comparing the resulting parcel 
conditions to the conditions of the surrounding environment as defined by
the balloon sounding.

Skew T are in blue.  The green lines are called dry adiabats.  Parcels of 
unsaturated air tend to follow the dry adiabat lines (green lines) as they 
ascend or descend.  The saturation adiabats (light blue) lines show how 
parcels saturated with water vapor will ascend or descend.  Descending 
parcels will tend to unsaturate immediately.  The mixing ratio lines (yellow)
relate to the amount of water vapor in a parcel in grams of water vapor per 
kilograms of dry air.  Parcels of air attempt to maintain a constant mixing 
ratio as they ascend or descend.  Generally, a parcel will rise, following
the dry adiabat until it saturates.  This occurs when dry adiabat crosses
the initial mixing ratio line.  This is considered the LCL.  If lifting
continues, the parcel cools following the saturation adiabat.  If the parcel
descends, it will always follow the dry adiabat as it will immediately
unsaturate if saturated.  By then comparing the parcel temperature to
the environment, you can determine whether it is stable (parcel cooler)
or unstable (parcel warmer).  An unstable parcel will accelerate upwards and
is the primary means for thunderstorm development.  A stable parcel will
decelerate and eventually descend.  This is the typical atmospheric condition
and it the primary condition in high pressure areas.  The descending air
desaturates the atmosphere and leads to clearing skies and calm conditions.

ATMOSPHERIC STABILITY INDICIES
To measure the instability of the atmosphere, several stability indices have
been developed.  They involve differing means for assessing the stability
and result in the following criteria:

Lifted Index (LI)
LI > 2		No significant activity
0 < LI < 2	Showers probable, isolated thunderstorms possible
-2 < LI < 0	Thunderstorms probable
-4 < LI < -2	Severe thunderstorms possible
LI < -4		Severe thunderstorms probable, tornadoes possible

Showalter Index (SI)
SI > 3		No significant activity
1 < SI < 3	Showers probable, isolated thunderstorms possible
-2 < SI < 1	Thunderstorms probable
-6 < SI < -2	Severe thunderstorms possible
SI < -6		Tornadoes possible

Total Totals Index (TT)
44-45		Isolated moderate thunderstorms
46-47		Scattered moderate / few heavy thunderstorms
48-49		Scattered moderate / few heavy / isolated severe 
		thunderstorms
50-51		Scattered heavy / few severe thunderstorms and isolated 
		tornadoes
52-55		Scattered to numerous heavy / few to scattered severe 
		thunderstorm / few tornadoes
> 55		Numerous heavy / scattered severe thunderstorms and 
		scattered tornadoes 

K Index (KI)
K < 15		0% Air mass thunderstorm probability
15-20		<20% Air mass thunderstorm probability
21-25		20-40% Air mass thunderstorm probability
26-30		40-60% Air mass thunderstorm probability
31-35		60-80% Air mass thunderstorm probability
36-40		80-90% Air mass thunderstorm probability
K > 40		>90% Air mass thunderstorm probability

SWEAT ( Severe Weather thrEAT ) Index (SW)
SW < 300	No activity expected
300 < SW < 400	Isolated moderate to heavy thunderstorms
400 < SW < 500	Severe thunderstorms and tornadoes probable
SW > 500	Severe thunderstorms and tornadoes likely