|INTERPRETATION OF SKEW-T INDICES
METEOROLOGIST JEFF HABY
To the right of most Skew-T diagrams on the web there will be a list a parameters and indices.
To a first timer, these letters and numbers look as a collection of abbreviations and random numbers. With
experience in examining Skew-T data on a daily basis, you will be able to instantly interpret the abbreviations
and numbers. The numbers will also have meaning since they can be classified into categories.
This section interprets most of those values and gives operational significance to the values.
Each parameter or indice will be broken down one by one. In a
severe weather situations and during inclement
weather, these indices come in handy.
The indices should be used as guides. Often, indices will contradict each other and can change
rapidly in the course of a couple of hours. An experienced meteorologist is well informed to how
a sounding will change throughout the day and why some sounding indices are better than others in
certain situations. Soundings are most notably changed through
and evaporative cooling.
Modified soundings should be studied along with the standard 12 Z and 00Z sounding.
Interpretation of Skew- T Log-P Indices
WMO: 4-letter station identification number
TP: Tropopause Level
Location in millibars of the tropopause, generally near 150 millibars
FRZ: Pressure level at which the environmental sounding is exactly zero
Find intersection of 0-degree isotherm with environmental sounding
WBO: Wet bulb zero temperature. Value at
which the sounding is at zero degrees
Celsius due to
evaporative cooling. Value is given as a pressure level. This value will always be at a higher
pressure (closer to the surface) than the FRZ level unless the sounding is saturated.
1. Pick a pressure level
- Value found through computer algorithms (once the wet bulb is found for every pressure level,
the wet bulb zero can then be located.)
- Wet bulb temperature can be found by the following sequence.
2. Find LCL from that pressure level
3. From LCL go back down the sounding at the wet adiabatic lapse rate to the original pressure
4. This temperature is the wet bulb temperature
PW: Value of precipitable water in inches
This is the amount of liquid water on the surface after all water in all three phases is
brought to the surface
Greater than 1.75 inches represents a water loaded sounding
less than 0.75 inches represents a fairly dry sounding
RH: The average relative humidity between the surface and 500 millibars.
0 to 40% very low
41 to 60% low
61 to 80% moderate
81 to 100% Moist
Relative humidity is a good measure of the evaporational drying power of the air and how close
the atmosphere is to saturation. It does not, however, tell you how much moisture mass there is in the air.
MAXT: Estimated maximum afternoon temperature. Most relevant
when using a morning sounding. Most accurate on days with clear skies and moderate winds.
L57: 700 to 500 millibar lapse rate
Less than 5.5 stable
5.5 to 9 conditionally unstable (unstable if moist in
9 or greater Incredibly unstable
Atmosphere is stable when the environmental lapse rate is less than the moist adiabatic lapse rate.
Especially true if large inversion is present.
Atmosphere is conditional unstable when the environmental lapse rate is between the moist
and dry adiabatic lapse rate
Atmosphere is absolutely unstable if environmental lapse rate is greater than the dry adiabatic
LCL: Lifted condensation level in millibars using surface data. This is
the level in the atmosphere clouds will form if forced lifting takes place.
LCL is found by the following process
draw a dry adiabat from the surface temperature
draw a mixing ratio line from the dewpoint
intersection is the LCL
LI: Lifted Index. This is the temperature difference between the environmental
and parcel temperatures at the 500 mb level.
500 mb envir. Temp - 500 mb parcel Temp = LI
0 or greater= stable
-1 to -4= marginal instability
-5 to -7= large instability
-8 to -10= extreme instability
-11 or less = ridiculous instability
SI: Showalter index. Same as LI, except parcel is lifted from 850 mb.
Use SI instead of LI in the cool season especially when surface is
capped by a cool front.
TT: Total totals index. (T850 - T500) + (Td850 - T500)<44 convection not likely
44 to 50 convection likely
51- 52 isolated severe storms
53- 56 widely scattered severe storms
Greater than 56 scattered severe storms
Vertical totals + cross totals
Limitations: large lapse rates can produce large TT values with little
low level moistureTT is region specific
KI: K index. (T850 - T500) + (Td850 - Tdd700)Lapse rate + available moisture
less than 15 Convection not likely
15 to 25 Small potential for convection
26 to 39 Moderate potential for convection
40+ High potential for convection
Favors non severe convection. This index is a measure of thunderstorm potential but has nothing to do
with severity of storms.
Can not be used in mountain areas
SW: Sweat Index. Severe WEAther Threat index . Indice combining many
thermodynamic and wind values.150 to 300 Slight severe
300 to 400 Severe storms possible
400+ Tornadic severe storms possible
If TT less than 49, then that term of equation is set to zero
low level moisture,
low level jet,
upper level jet,
warm air advection
EI: Energy Index
CAPE: Convective Available Potential Energy. This is the positive area on
a sounding (the area between the parcel and environmental temperature)
1 to 1,500 Positive CAPE
1,500 to 2,500 Large CAPE
2,500 + Extreme CAPE
Max upward vertical velocity =
(2*CAPE)^1/2, does not take into consideration water loading,
CINH: Convective Inhibition. This is the negative area on a sounding. A
large cap or a dry
planetary boundary layer will lead to high values of CINH and stability
CAP: Cap strength in degrees Celsius. Values above 2 indicate convection
will not occur within at least the next couple of hours. Cap needs to be less than 2 in general before it can
EL: Equilibrium level. The pressure value at the top of the positive
MPL: Maximum parcel level. Highest level a parcel can rise in the atmosphere.
This value is above the EL due to the updrafts momentum.
STM: Estimated storm motion. Storm will be moving from X and X knots.
HEL: Helicity Amount of streamwise vorticity available for ingestion
into a storm. Streamwise vorticity is a function of low level inflow and horizontal vorticity generated by
speed shear with height or directional shear with height in the
150 to 300= possible supercell
300 to 400= supercell severe storms
400+ = Tornadic supercells possible
SHR: Positive shear in the 0 to 3000m above ground level. Units are in
time to the negative 1. Dividing the change in vertical wind speed by the change in the distance derives these
units. Km/hr divided by km = hr-1. Value is found by finding the change in wind speed from the surface to 3000m
and dividing that value by 3000m (3 km).
0 to 3 weak
4 to 5 moderate
6 to 8 large
9+ very large
SRDS: Storm relative directional shear
EHI: Energy helicity index. = (SR HEL * CAPE) / 160,000
EHI > 1 Supercells likely
EHI from 1 to 5 F2, F3 tornadoes possible
EHI 5+ F4, F5 tornadoes possible
BRN: Bulk Richardson Number = (CAPE / 0-6km shear)
less than 45 Supercells
less than 10 Environment too sheared
Teens Optimum for severe storms, good balance of
BSHR: Bulk shear value (magnitude of shear over layer)
CCL: Level at which condensation will occur if sufficient afternoon heating
causes rising parcels of air to reach saturation. The CCL is greater than or equal in height (lower or equal
pressure level) than the LCL. The CCL and the LCL are equal when the atmosphere is saturated. found at the intersection of the saturation mixing ratio line (through the surface dewpoint) and
the environmental temperature.
Level of Free Convection (LFC)- The level at the bottom of the
area of positive CAPE. If a parcel reaches this level it will begin to accelerate in the vertical.
Relative Humidity- Found by dividing the mixing ratio by the saturation
mixing ratio or the vapor pressure divided by the saturation vapor pressure.Find the saturation mixing ratio value that runs through the dewpoint and the temperature.
Next, divide the dewpoint mixing ratio by the temperature mixing ratio.
Potential Temperature- Temperature found by lifting or descending a
parcel to the 1000 mb level from the pressure level of interest.
Equivalent Potential Temperature- Also known as THETA-E. Temperature
of a parcel after all latent heat energy is released in a parcel then brought to the 1000 mb level.From pressure of interest (typically the surface) find the LCL, lift the parcel wet
adiabatically to 100 mb. Next, descend the parcel dry adiabatically to the 1000 mb level. The temperature
at 1000 mb of this parcel is the THETA-E.
Wet Bulb potential temperature- Found the same as the wet bulb. When
the wet bulb value is found, keep descending wet adiabatically to the 1000 mb level.
Convective instability- Occurs when a dry layer overlays a warm and
humid layer. Lifting of atmosphere causes the lapse rate to increase since the lower layer cool at the
WALR while the dry layer cools at the DALR.
Hydrolapse- Rapid increase or decrease in dewpoint with height