AWSITR=791006 .1. Revised THE USE OF SKEW T, LOG P DIAGRAM ANALYSIS AND FORECASTING DECEMBER 1979 Revised March 1990 (This June 1995 reprintingincludes allerrataand revisions to date, includinga minor change to the convective gust potentialforecast procedure on Page 5-41) . APPROVED FOR PUBLIC RELEASE; DISTRIBUTION IS UNLIMITED — AIR WEATHER SERVICE Scott Air Force Base, Illinois 62225-5008 Rovlsod March 1990 REVIEW AND APPROVAL STATEMENT . FX)R THE (-{)hl MANDFi R v 4 —- II Revkted March 1990 REPORT DOCUMENTATION PAGE 2. Report DJte: December 1979 (Revised March 1990) 3. Report Type : Technical Report 4. Title: I“he U.~eof /he Skew T, Log P Diagrom in Analvsis and Forecasting 7, Performing Organization Name and Address: }IQ AWS/XT, SCONAFB, IL 62225-5(X)X 8. Performing Organization Report Number: AWSnR-79/(X)6 (Revised) 11. Supplementary No[es: Reprinted in March 1990 m incorpx+tc the I’ollowing’ Novcmlx’r 1987 revision th:~l updates obsolete references and adds new stability indices and I[wmulas; AugusI IWM and March 1989 crra(a: and a March 1990 revision Lhal adds new stabili[y indices and log forecasting techniqu~$s Supcr$cdc$ AD-A 195862. 12, Dis(ribu[ion/Avtiilabiiity Slatement: Approved for public release: distribution is unlimi(ed. 13, Abstract: Describes the DoD Skew T, Log P diagram and provides instruclionq 011 il~ usc lnslruclton$ include how to plol data on the (iiagram and how to usc the dakr to (ieterminc unreported mc[~wr(ll{)gical clclncnls and a(mosphcric stabi lily, Also tells how to analym disco n[inu ities, stublc Iaycrs, ami cl Iluds, a~ wt’11 as how to (ISC [he plot(cd diagram to help Iorecust meteorological phcnonmna such us oircra~t icing an{! SC!crr corrvcclivc st{mns. Revised in November 1987 to add several new stabilily indices and provide instructions for plotting and wsing pressure allitu(lc curves. ‘1’hc March i9YU revision adds several new stabili(y indices and I’tnxasling uxhniqucs currently displwed on (he Satellite D8ta Handling System (SDHS), (he Aulornatcci Wet!lhcr Distrihulion Syslem (AWDS ), and microcomputer Skew T programs. 14, Subject Tcrrns: METF.OROL(W’I’,” WEAT}l ER, WEATHER FOREC’AS”I”l N(~, WI;4”l’til’.R ANAi. }’SIS. UPPER AIR, SOLINDING, atmcxphc’ric wmdmg, :Iuimphc’ric suihilit>, s[ahillly II I(IC’I. 111~’1~’(lrt)l{~~lc:iiwund Ing, n~c[c(m)loglcal anal >si.s,Skew “r. [ (Jg i’ diagrani 15: Number of Pages: 15~ 17. Sccurily Classtlictrhun ol Rt>jw)rt L’nciassi(icd IX. Sccurily CIassl(ication of thisPage: ( Inclasw!ic(i ][), Security C’lusslfication 0[ Atwuxl: Uncixsllicd 20, Limil~tion 01 Abstract: (11. Standard Form 298 .. Ill Revised March 1990 .- PREFACE This rcpm ([JrigitTiIlly issued (m I %plcmbcr IY52 as AWSM 105- [ 24) was converted (o an Air Wealher Scrvicc ‘lc~hnical Report (A WS[TR-79/(N)6) in Dcccmbcr 1‘J79. Surprisingly, lhcrc have been only minor chwrgcs in the lxrsic pr(~cdurcs for plotting, analyzing, and using [hc Skew T, Log P diagram over (he pas[ 35 years. The (WO rcccnt revisions (November 1987 wrd March 1990) arc primarily cmwerncd with ncw skrbilily indices. This rcprin[, which im orp(~ratcs al! r(cvisions and crra~:t (() {ia[c. was rwccssary [o replenish stock and fill outstanding orders for !his venerable document. Many (JI the chtmgc~ in (he 19X7 rcvlsl(m had to do wilt) updating ohsolctc rclcrcncc:s, but ins[ruc(i(ms l~)r plotting and using IIW pressure altitude curve were also a(klcd, as paragraph 3.7, A major change was the addi(ion (in Chap[cr 5) of several new sktbili(y indlccs, ahmg wilh lhcir descriptions and formulas. For Ihc Ia[(cr, AWS thanks Bill Henry of [he National Wca[hcr Scrvicc Training (’enter, whose pamphlet (’k .Wcw T, Log f’, Diu,grmn) provided mos[ ol [hc ma{crial. Tht. 1990 rcvisi(m Mhls scvcrai new slability indicts currently displayed on (he Satcllilc Datu Handling Syslcm (SDHS ), [hc Automa[cd Weather Diswihuiion Sys[crn (AWDS ), and micr(~omputcr Skew “r programs. For (hc microcnmpu[c’r in{iices, AWS/XTX used the vc>ry rcc(. n[ly puhlisbcd SLcw-/’ Profc.crion(ll, Version 2.3, 2 Novcmt)cr 1989. Thi~ revision also irrcludcs procedures [or dclcrmining a Fog S~~bili[y index, a Fog Point value, tind a Fog Thrcal value. This printing (March 1990) also incorporates August 1988 and March 1989 errata, Readers may note that many of the cquipmmrl rcfcrcnccs in Chaplcr 7 seem obsolete, These references (along with some of [hc equipment) arc being reviewed Ior possible updating in subscqucrrt revisions. Revleed Mnrch 1990 contents Page Chapter I-Introduction Organization of the Manual . , . 1-1 Reason for Choosing the Skew T, Log P Diagram . 1-1 Different Versions of the AWS Skew T, Log P Diagram . 1-2 Chapter 2-Description of tho Skew +7 Chart Isobars . ...2-1 Isotherms . ...2-1 Dry Adtabats, . ...2-1 Saturation Adiabats . 2-1 Saturation Mixing-Ratio Lines . 2-2 Thickness Scales . ...2-3 The 1000-mb Height Nomogram . 2-4 Standard Atmosphere Data. 2-5 Wind Scale . ...2-6 Contrail- Formation Curves . 2-6 Analysis Blocks . ...2-6 Chap/or 3-Plotting Sounding Data on tho Skew-l Chart General . ...3-1 Number of Soundings Plotted Per Chart . 3-1 Chotce of Color . ...3-1 Plotting Individual Elements . 3-1 Plotting Wind Data . ...3-2 Legend . ...3-2 Construction and Use of the Pressure Altitude Curve 3-3 Chapter 4-Determination of lJnroported Meteorological C)uantitios From Pi8ttod Soundings Introduction . ...4-1 Mixing Ratio . ...4-1 Saturation Mixing Ratio. 4-1 Relative Humidity . ...4-1 Vapor Pressure . .. o.. ...4-3 Saturation Vapor Pressure . 4-4 Page Comments on Temperature Parameters . 4-5 Potential Temperature . 4-5 Wet-Bulb Temperature . .. 4-5 . Wet-Bulb Potential Temperature . , . , . 4-5 Equivalent Temperature . 4-6 Equivalent Potential Temperature. , . 4-8 Virtual Temperature ...,. 4-9 Thickness of a Layer . 4-10 Height of the 1000- mb Surface . 4-11 Pressure Altitude . 411 Density Altitude, . 4-11 Convection Corfdensation ~evel . 4-12 Convection Temperature . 4-12 Lifting Condensation Level . 4-13 Mixing Condensation Level . 4-13 Level of Free Convection . 4-14 Positive and Negative Areas . 4-15 Equilibrium Level . 4-16 Energy Determinations on the Skew-T Chart . 4-19 Chapter 5–Determination of Stability The “Parcel” Theory as a Basis for Determining Stability and Instability . 5-1 General Comment on the Effect of the Assumptions in the Parcel Theory on the Utility of Parcel-Method Techniques . 5-2 The Parcel- Theory Assumptions Used in the AWS Skew- T Chart . 5-3 Identifying the Basic Types of Stability and Instability for Small Parcel Displacements in a Sounding Plotted on — the Skew- T Chart . 5-3 Description of Stability Criteria. 5-5 Discussion of Superadiabatic Lapse Rates . 5-6 Oscillation of a Parcel as a Function of Stability . 5-7 Note on Errors Caused by Use of the Observed-Tempera- ture Curve for the Virtual-Temperature Curve in Stability Determinations . 5-7 An Example of Stability Determinations on a Skew-T Chart . 5-9 Processes Which Change .i)~ Lapse Rate. , . 5-10 Non-Adiabatic Heating and Cooling Effects . 5-10 Instability from Surfa( Heatllug . 5-10 Stability from Surface Cooling . 5-13 Advection Effects . 5-13 Preliminary Remarks on the Effects of Vertical Motion . 5-15 Effects of Convergence and Divergence . 5-15 Cases Without Vertical Shrinking or Stretching . 5- lfj Cases With Conver~cr; (’e but Without Vertical Motion . 5-17 Cases With Divergence but Without V(rtical Motion . 5-17 The N()-Divergel)ce Case. 5- Ii’ In the Lower Atmosphere . 5-17 Inthe Upper Levels . 5- 17 HOVJ to Assess the Vertical-Motion Field In Practirc . 5- 18 Penetrative’ Conv~ction . “5- 20 ‘J i WAsed Mmrch 1990 Page Stability-Instability Criterta for Large Verticai Displacements . 5-20 Latent Instability . 5-21 Validity of Assumptions . 5-23 Procedure for Finding Whether Any Latent Instability Is Present . ..O 5-23 The Value of the Distinction Between Pseud* and Real- Latent Instability . 5-24 Utility of Latent-Instability Analysis in General . 5-25 Potential InstabUlty . 5-26 Effects of Lifting Potentially Unstable and Stable Layers . 5-28 Does the Whole Layer Become Unstable ? . 5-28 Superadtabatic Lapse Rates from Layer Lifting . , . 5-31 Effects of Divergence . 5-31 How Much Lift Is Needed for Release of Instability? . 5-31 Lifting’ of Potentially-Stable Layers . 5-31 Processes Which Change the Potential Instability . 5-31 Relation Between Potential and Latent Instability . 5-31 Slice Method . ;:;; The Stability Indexes ................................................................................. ......... The Showalkr Index ......................................................................................... 5’35 The Lifled Index ................................................................................................ 5-35 The Modified Likd Index ................................................................................. 5-36 The Fawbush-Miller Stability index .................................................................. 5-36 The Marlin Index ............................................................................................... 5-37 The Besl Lifted Index ........................................................................................ 5-37 The Model Lifkd in&x ..................................................................................... 5-38 The NGM LifmdIndex...................................................................................... 5-38 The “K”Index .................................................................................................... 5-38 The “KO”