SKEW-T

UPPER AIR THERMODYNAMIC

MISCELLANEOUS
Mixing Ratio
Saturation Mixing Ratio
Vapor Pressure
Saturation Vapor Pressure
Relative Humidity
Thickness
1000mb Surface Height
Lifting Condensation Level
Convection Condensation Level - Parcel Method
Convection Condensation Level - Moist Layer Method
Mixing Condensation Level
Contrails
Tropopause
Significant Stabilization Point
Turbulence
Icing

TEMPERATURES
Dew Point Temperature
Potential Temperature
Wet Bulb Temperature
Wet Bulb Potential Temperature
Virtual Temperature
Equivalent Temperature
Convective Temperature - Parcel Method
Convective Temperature - Moist Layer

INVERSIONS
Inversions
Radiation Inversion
Subsidence Inversion
Turbulence Inversion
Frontal Inversion
Stability Determination

ENERGY AREAS
Energy Areas Definition
Energy Areas Procedures For Forced Lift
Energy Area For Surface Heating

INDICES
Showalter Index
K Index
Vertical Total
Cross Total
Total Total
Lifted Index
Modified Lifting Index

FOG
Fog Stability Index
Fog Point
Fog Threat

Dew Point Temp (Td): Temp an air parcel must be cooled at constant pressure and moisture content for saturation occurrence.

1. At given level, find dew point curve.

2. Read isotherm value; label in degree C.

Mixing Ratio (w): Ratio of water vapor mass in grams to dry air mass.

1. At pressure, find dew point.

2. Read saturation mixing ratio line value; label in g/kg.

Saturation Mixing Ratio (w_s): Mixing ratio a parcel has if it were saturated.

1. At pressure, find temperature.

2. Read saturation mixing ratio line value; label in g/kg.

Vapor Pressure (e): Parcel water vapor pressure contributing to total atmospheric pressure shown by Dalton’s Law: Pt = Pn₂ + Po₂ + Pte + Ph₂ o

1. At pressure, find dew point.

2. Follow isotherm to 622mb isobar.

3. Read saturation mixing ratio line value; label in mb.

Saturation Vapor Pressure (e_s): Parcel water vapor pressure contributing to total atmospheric pressure if parcel were saturated.

1. At pressure, find temperature.

2. Follow isotherm to 622mb isobar.

3. Read saturation mixing ratio line value; label in mb.

Relative Humidity (RH): Ratio of water vapor in given parcel to amount that parcel would hold if saturated.

1. Mixing Ratio Method: Find w and w_s.
Solve for RH = (w/w_s)*100%.

2. Vapor Pressure Method: Fine e and e_s.
Solve for RH = )e/e_s)*100%.

Potential Temp (ø): Air sample temp if brought dry adiabatically to 1000mb.

1. At pressure, find temperature.

2. Read dry adiabat value; label in ^oC.

1. At pressure, find temperature.

2. Follow dry adiabat to 1000mb.

3. Read isotherm; label in ^oC.

Wet Bulb Temp (T_w): Lowest temp an air parcel can be cooled by evaporating water into it.

1. At pressure find dew point, extend mixing ratio line.

2. At pressure find temp, extend dry adiabat line until it intersects the first line.

3. From intersection (LCL), follow saturation adiabat to original pressure level.

4. Read isotherm value; label in ⁰C.

Wet Bulb Potential Temp (ø_w): Parcel’s wet bulb temp if brought saturation-adiabatically to 1000mb.

1. At pressure, construct LCL.

2. From LCL follow saturation adiabat to 1000mb.

3. Read isotherm value; label in ⁰C.

Virtual Temperature (T_v): Temp which a dry air parcel would have the same density as moist air parcel at same pressure. Calculated, not measured, temp.

1. At pressure, find mixing ratio.

2. Divide mixing ratio by 6.

3. Add result to the temp: (T_v = T + w/6)

Equivalent Temp (T_e): Air sample temp if moisture was condensed by the psuedo-adiabatic process and returned to it’s original pressure dry adiabatically.

1. At pressure, construct LCL.

2. Follow saturation adiabat until parallel dry adiabat.

3. Follow dry adiabat back to original pressure level.

4. Read isotherm value; label in ^oC.

Thickness (delta Z): Distance between two constant pressure sfc. Depends on mean layer’s virtual temp (T_v). Higher the mean virtual temp, greater the thickness.

1 Construct virtual temp curve for layer of interest.

2. Determine mean virtual temp using equal areas.

3. Read appropriate thickness scale at intersection of mean virtual temp of isotherm. Label in ft or meters.

1000mb Surface Height (AGL)

1. At surface find temp; plot on top of chart temp scale.

2. Plot sfc pressure on left side of chart pressure scale.

3. Lay straight edge across these two plotted points so it intersects height scale to left of the pressure scale.

4. Intersection value is height of 1000mb surface.

5. Label in ft or meters. If pressure is less than 1000mb, height is a negative number (underground).

Lifting Condensation Level (LCL): Height which an air parcel becomes saturated if lifted dry adiabatically.

1. At pressure, find dew point.

2. Draw line up mixing ratio.

3. At pressure, find temp.

4. Draw line up dry adiabat.

5. Intersection of line is LCL.

6. Read isobar; label in mb.

Convection Condensation Level - Parcel Method (CCLp): Height an air parcel rises if heated from below adiabatically until saturation occurs.

1. At surface, find dew point.

2. Follow mixing ratio until intersecting temp curve.

3. Read isobar; label in mb.

Convective Temp - Parcel Method (Tcp): Temp the surface reaches before convection begins.

1. Construct CCLp.

2. Extend line dry adiabatically to surface.

3. Read temp; label in ^oC.

Convection Condensation Level - Moist Layer Method (CCLml): Used when low level moisture is highly available.

1. Ensure sfc dew point depression is less then 6^oC.

2. Determine moist layer top.

A. Where dew point depression becomes > 6^oC.
B. If moist layer >060 feet thick, use lowest 150mb.

3. Find average mixing ratio using equal area method.

4. Extend mean mixing ratio to intersect temp curve.

5. Read isobar; label in mb.

Convective Temp - Moist Layer (Tcml): Temp which convection begins using the moist layer method.

1. Construct CCLml.

2. Extend line dry adiabatically to surface.

3. Read temp, label in ^oC.

Mixing Condensation Level (MCL): Lowest height in a layer mixed by turbulence at which saturation occurs after complete mixing of the layer.

1. Find mixed layer top. Locally established: based on wind speed, terrain, turbulence, and stability.

2. Find layer’s average mixing ratio using equal area method. Use dew point curve and mixing ratio lines as boundaries. Extend to mixing layer’s top.

3. Find layer’s average potential temp using equal area method. Use temp and dry adiabatic as boundaries. Extend to mixing layer’s top.

4. If average mixing ratio and average potential temp lines intersect within layer, read and label in mb.

5. If average mixing ratio and average potential temp lines do not intersect within layer, there is not MCL.

Contrails: Clouds that form behind aircraft.

1. Plot forecasted flight level temp. Read RH using the contrail curves.

2. If flight level is above the trop, the assumed RH is 10%. If within 300meters below, assume RH is 70%. If lower than 300 meters, assume RH is 40%.

3. If RH is less than the assumed RH, forecast contrails.

Forecast contrails if temps and spreads are:

1. Temp <45^oC with any spread.

2. 45^oC<Temp<40^oC with 5-10^oC spread.

3. 40^oC<Temp<37^oC with 0-5^oC spread.

Inversions: Temp normally decrease with altitude in the tropopause. Soundings frequently show layers where temps remain isothermal or increase with height.

I. Inversion base is the first point the temp increases or remains isothermal with height.

2. Inversion top is the point temp decreases with height.

Radiation Inversion: Thermally produced surface based inversion formed by rapid cooling of air in contact with the surface as compared to upper layers.

1. Surface based.

2. Frequently associated with fog.

3. Mixing ratio almost constant with inversion layer.

4. T/Td point decreases with height above inversion.

Subsidence Inversion: Mechanically produced inversion formed by adiabatic heating of sinking air.

1. Temp increase with height through inversion.

2. Dew Point begins decreasing at inversion’s base.

3. T/Td decreases rapidly above inversion.

Turbulence Inversion: Mechanically produced inversion formed by movement of air over uneven surface and the resultant vertical mixing of air.

1. Lapse rate below inversion is dry adiabatic.

2. Mixing ratio constant below inversion.

3. Isothermal or slight warming within inversion.

Frontal Inversion: Transition layer between a cold air mass and the warmer air mass above it.

1. Temp: shallow isothermal layer/ weakening warming.

2. Dew point increases through inversion.

3. Winds through inversion:

A. Backing indicates cold front (counterclockwise).
B. Veering indicates warm front (clockwise).

Stability Determination:

Tpd = Parcel’s temp lifted from layer’s base to top dry adiabatically.

Tpm = Parcel’s temp lifted from layer’s base to top moist adiabatically.

Tenv = Environment’s temp at layer’s top.

Tpd < Tpm < Tenv Absolutely Stable

Tenv < Tpd < Tpm Absolutely Unstable

Tpd < Tenv < Tpm Conditional State

If the base is: Then the layer is:

Unsaturated Conditionally Stable

Saturated Conditionally Unstable

Turbulence:

Horizontal Shear Vertical Shear

25-49KT/90NM MDT 06-09KT/1000FT LGT

50-89KT/90NM SVR 10-15KT/1000FT MDT

>90KT/90NM EXT 06-21KT/1000FT SVR

>21KT/1000FT EXT

Energy Areas Definition:

1. Negative energy areas: area on a skew-t requiring energy to move a parcel.

2. Positive energy areas: area on a skew-t where a parcel is warmer than the environment.

3. Level of free convection (LFC): height a parcel first becomes warmer than the environment.

4. Equilibrium Level (EL): height which the positively buoyant parcel again becomes neutrally buoyant.

Energy Areas Procedures For Forced Lift:

1. Construct LCL.

2. Extend line up saturation adiabat from LCL.

3. Negative energy area bounded by sfc, temp curve to LFC, saturation adiabat to LCL, dry adiabat to sfc.

4. If lifted parcel temp crosses environmental temp curve, there is a positive energy and the LFC is where they intersect.

5. Positive energy area is bounded by temp curve, saturation adiabat above LFC to level where Tp=Te.

6. Equilibrium level (EL) is intersection of parcel and environmental temp at top of positive area.

Energy Area For Surface Heating:

1. Construct CCL.

2. Extend line up saturation adiabat from CCL until it intersects temp curve.

3. Negative engery area is bounded by sfc, temp curve to CCL and dry adiabat line from CCL to sfc.

4. LFC is the same as the CCL.

5. Positive energy area is bounded by temp curve, saturation adiabat line above LFC to where Tp=Te.

6. Equilibrium level (EL) is intersection of parcel and environmental temp at top of positive area.

Showalter Index (SSI): Moist layer must reach 850mb.

1. Follow 850mb LCL saturation adiabat to 500mb.

2. Read isotherm and label as T¹.

3. SSI = T^500mb - T¹.

>+3 Expect showers and some thunderstorms
+3>SSI>-2 Expect thunderstorms to increase rapidly
-2>SSI>-6 Look for severe thunderstorms
-6>SSI Consider possibility of tornadoes

K Index: KI = VT+850Td-700Td_d

K<15 None

K<20 < 20%

K<25 <40%

K<30 <60%

K<35 <80%

K<40 <90%

K>40 >90%

Vertical Total: VT =T^850mb - T^500mb

Cross Total: CT = Td^850mb-T^500mb

Total Total: TT = T^850mb+Td^850mb-2(T^500mb)

CT TT

18 44 Isol Thunderstorms

20 46 Sct ORG, Few GRN

22 48 Sct ORG, Few GRN, Isol BLU

24 50 Sct GRN, Few BLU, Isol RED

24 52 Sct-Num GRN, Sct BLU, Few RED

26 56 Num GRN, Sct BLU, Sct RED

>26 Thunderstorms

Lifted Index (LI): Use moist layer average below 850mb.

1. Determine mean mixing ratio for lowest 3000 feet.

2a. If no significant heating occurs, construct mean potential temp for lowest 3000FT to intersect mean mixing ratio and go to step 3.

2b. If significant heating occurs, forecast maximum temp and follow potential temp from maximum temp to intersection with mean mixing ratio line.

3. From intersection of mean mixing ratio and appropriate potential temp, follow saturation adiabat to 500mb and label value of isotherm as T¹.

4. LI = T^500mb - T¹.

0>LI>-3 Weak indication of severe TS
-3>LI>-6 Moderate indication of severe TS
-6>LI Strong indication of severe TS

Icing:

1. Aircraft icing conditions:

A. 0^oC>Environmental temp<-40^oC.
B. Aircarft surface <0⁰C.
C. Super-cooled water drops in the environment.

2. Factors to consider for icing:

A. Icing rarely occurs at temps less than -22^oC.
B. Higher moisture content, higher icing amount.

3. Cloud type and icing type:

A. Stratiform clouds are made up of small droplets and should be expected to produce rime icing.
B. Cumuliform clouds contain large droplets in lower levels producing clear icing, small droplets at higher levels for rime, and mixed in between.

4. Cumuliform Clouds
0^oC Clear -8^oC Mixed -15⁰C Rime -22^oC
Stratiform Clouds
0^oC Rime -22^oC

WAA, FRZ LVL above clouds:
CU CLDS = SHRA ST CLDS = RA DZ

CAA, FRZ LVL>1200FT AGL:
CU CLDS = SHRA ST CLDS = RA DZ

CAA, FRZ LVL< 600FT AGL:
CU CLDS = SHSN ST CLDS = SN

CAA, 600FT>FRZ LVL<1200FT, warm sfc:
Mixed RA and SN

CAA, Warm layer aloft 600FT-1200FT thick with a deep cold layer near sfc: Ice Pellets and Sleet

CAA, Warm layer >1200FT with FRZ LVL >800FT AGL: FRZG RA

WAA, Cold layer near sfc: FRZG DZ

Modified Lifting Index (MLI) To evaluate the MLI, find LCL. Extend saturation adiabat through LCL to M20^oC pressure level. This is the updraft temp. Threshold are same as LI.

Fog Stability Index (FSI)

FSI=4T_s-2(T₈₅₀+TD_s)+W₈₅₀	FSI	Likelihood of Radiation Fog
T_s is the surface temperature in ^oC.	>55	Low
T₈₅₀ is the 850mb temperature in ^oC.	31-55	Moderate
TD_s is the surface dew-point in ^oC.	<31	High
W₈₅₀ is the 850mb wind speed in knots.

Fog Point This indicates the temperature (^oC) at which radiation fog forms. To determine the fog point, find the LCL pressure level. From the dew-point at this pressure level, follow the saturation mixing ratio to the surface. The isotherm value at this point is the fog point.

Fog Threat	Likelihood of Radiation Fog
>3	Low
0-3	Moderate
<3	High

Fog Threat=850mb wet-bulb potential temp minus fog point.

Tropopause: The height between 500mb and 30mb where the lapse rate decreases to 2^oC or less per kilometer and maintains this average lapse rate for at least two kilometers above the stabilization point. It can occur below the 500mb if it is the only stabilization point satisfying the definition of a sounding reaching 200mb. The lapse rate at any higher layer can not exceed 3^oC per kilometer.

Secondary Tropopause: It is located where the lapse rate above the predominant tropopause exceeds 3^oC per kilometer on average (less stable and getting colder) for at least one kilometer and then meets the conditions for predominant tropopause. The one kilometer interval with an average lapse rate of 3^oC per kilometer can occur at any height above the predominant tropopause.

Significant Stabilization Point: It is located where the lapse rate decreases 3^oC or more per kilometer.

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