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 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 thermal advection, moisture advection, 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 degrees Celsius

• 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.

• 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.

1. Pick a pressure level
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 PBL)
• 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 lapse rate

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)
Vertical totals + cross totals

• <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

Limitations: large lapse rates can produce large TT values with little low level moisture

TT 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

Limitations: 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.
SWEAT= 12(850Td)+20(TT-49)+2(V850)+(V500)+125(sin(dd500-dd850)+.2) If TT less than 49, then that term of equation is set to zero

• 150 to 300    Slight severe
• 300 to 400    Severe storms possible
• 400+    Tornadic severe storms possible

Formula covers: low level moisture, instability, 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, entrainment

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 be broken.

EL: Equilibrium level. The pressure value at the top of the positive CAPE area

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 PBL.

• 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 CAPE and shear

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