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Aws/Tr-79/006 the Skew T, Log P Diagram

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Aws/Tr-79/006 the Skew T, Log P Diagram .P AWS/TR-79/006 Revised THE USE OF N THE SKEW T, LOG P DIAGRAM 00 IN ANALYSIS AND FORECASTING ,~ TI,. DECEMBER 1979- Revised March 1990 (This printing Includes all errata and revisions to date) APPROVED FOR PUBLIC RELEASE; DISTRIBUTION IS UNLIMITED. AIR WEATHER SERVICE (MAC) Scott Air Force Base, Illinois, 62225-5008 90 05 24 076 Revised March 1990 REVIEW AND APPROVAL STATEMENT AWS/TR-79/(X)6, December 1979 (revised March 1990), The Use of the Skew T, Log P Diagram in Analysis and Forecasting, has been reviewed and is approved for public release. There is no objection to unlimited distribution of this document to the public at large, or by the Defense Technical Information Center (DTIC) to the National Technical Information Service (NTIS). RO C. WHITON, Lt Col, USAF Dir 'torate of Aerospace Development FOR THE COMMANDER ES W. OVERALL, Col, USAF kS/Technology O ii Revised March 1990 REPORT DOCUMENTATION PAGE 2. Report Date: December 1979 (Revised March 1990) 3. Report Type: Technical Report 4. Title: The Use of the Skew T, Log P Diagram in Analysis and Forecasting 7. Performing Organization Name and Address: HQ AWS/XT, Scott AFB, IL 62225-5008 8. Performing Organization Report Number: AWS/TR-79/006 (Revised) It. Supplementary Notes: Reprinted in March 1990 to incorporate the following: November 1987 revision that updates obsolete references and adds new stability indices and formulas; August 1988 and March 1989 errata; and a March 1990 revision that adds new stability indices and fog forecasting techniques. Supersedes AD-A 195862. 1-2. Distribution/Availability Statement: Approved for public release; distribution is unlimited. 13. Abstract: Describes the DoD Skew T, Log P diagram and provides instructions on its use. Instructions include how to plot data on the diagram and how to use the data to determine unreported meteorological elements and atmospheric stability. Also tells how to analyze discontinuities, stable layers, and clouds, as well as how to use the plotted diagram to help forecast meteorological phenomena such as aircraft icing and severe convective storms. Revised in November 1987 to add several new stability indices and provide instructions for plotting and using pressure altitude curves. The March 1990 revision adds several new stability indices and forecasting techniques currently displayed on the Satellite Data Handling System (SDHS), the Automated Weather Distribution System (AWDS), and microcomputer Skew T programs. ' . 14. Subject Terms: METEOROLOGY, WEATHER, WEATHER FORECASTING, WEATHER ANALYSIS, UPPER AIR, SOUNDING, atmospheric sounding, atmiospheric stability, stability index, meteorological sounding, meteorological analysis, Skew T, Log P diagram. Ac-ession For 15: Number of Pages: 153 DTI TRA& UTI . , 17. Security Classification of Report: Unclassified 18. Security Classification of this Page: Unclassified 19. Security Classification of Abstract: Unclassified .. 20. Limitation of Abstract: UL . e Standard Form 298 I!1 Revised March 1990 PREFACE This report (originally issued on 1 September 1952 as AWSM 105-124) was converted to an Air Weather Service Technical Report (AWS/TR-79/006) in December 1979. Surprisingly, there have been only minor changes in the basic procedures for plotting, analyzing, and using the Skew T, Log P diagram over the past 35 years. The two recent revisions (November 1987 and March 1990) are primarily concerned with new stability indices. This reprint, which incorporates all revisions and errata to date, was necessary to replenish stock and fill outstanding orders for this venerable document. Many of the changes in the 1987 revision had to do with updating obsolete references, but instructions for plotting and using the pressure altitude curve were also added, as paragraph 3.7. A major change was the addition (in Chapter 5) of several new stability indices, along with their descriptions and formulas. For the latter, AWS thanks Bill Henry of the National Weather Service Training Center, whose pamphlet (The Skew T, Log P, Diagram) provided most of the material. The 1990 revision adds several new stability indices currently displayed on the Satellite Data Handling System (SDHS), the Automated Weather Distribution System (AWDS), and microcomputer Skew T programs. For the microcomputer indices, AWS/XTX used the very recently published Skew-T Professional, Version 2.3, 2 November 1989. This revision also includes procedures for determining a Fog Stability Index, a Fog Point value, and a Fog Threat value. This printing (March 1990) also incorporates August 1988 and March 1989 errata. Readers may note that many of the equipment references in Chapter 7 seem obsolete. These references (along with some of the equipment) are being reviewed for possible updating in subsequent revisions. iv Revised Msrch 1990 Contents Chapter I-Introduction Page 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 the Skew ,Y Chart Isobars ........ ....................... .... 2-1 Isotherms ....... ....................... .... 2-1 Dry Adiabats ............. ............... 2-1 SaturationAdiabats........... .............. 2-1 Saturation Mixing-Ratib 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 Chapter 3-Plotting Sounding Data on the Skew-T Chart General ....................... 3-1 Number of Soundings Plotted Per Chart .......... 3- 1 Choice 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 Unreported Meteorological Quantities From Pltted Soundings Introduction .. ... .................... 4-1 Mixing Ratio ...... ..................... 4-1 Saturation Mixing Ratio ...... ................ ... 4-1 Relative Humidity ..... ............... 4-1 Vapor Pressure ................. .. 4-3 Saturation Vapor Pressure....... ............... 4-4 V 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 Timperature . ...... ........... 4-8 Virtual Temperature ........ .................. 4-9 Thickness of a Layer ....... ................. ... 4-10 Height of the 1000-mb Surface ... ............. ..... 4-11 Pressure Altitude ......... .................... 4-11 Density Altitude .......... .............. ... 4-11 Convection Cordensation Level ..... ............. ... 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 S-Determination of Stability The "Parcel" Theory as a Pasis 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 Stabtltty'and "Instabi'lityfor 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 the Lapse Rate ............ .... 5-10 Non-Adiabatic Heating and Cooling Effects ........... .... 5-10 Instability from Surface Heating ... ............. ... 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- 16 Cases With Convergence but Without Vertical Motion . 5-17 Cases With Divergence but Without Vertical Motion .... 5-17 The No-Divergence Case .... ................. .... 5-17 In the Lower Atmosphere .... ............... ..... 5-17 In the Upper Levels ...... ................... ... 5-17 How to Assess the Vertical-Motion Field in Practice .... 5-18 Penetrative Convection ...... ................. ... 5-20 Vi Revised March 1990 Page Stability- Instability Criteria for Large Vertical Displacements. .. .. .... ..... ... 5-20 Latent Instability .. .. ... ...... ... 5-21 Validity of Assumptions. .. ... .. .. .. 5-23 Procedure for Finding Whether Any Latent Instability is Present .. ..... .... ..... .. 5-23 The Value of the
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