SkewT Diagrams: New Tools For Vertical Analysis

1 1 February 2004 National Weather ServiceVolume 3, Number 1 Mission StatementTo enhance aviation safety byincreasing the pilot s knowledge ofweather systems and processes andNational Weather Service productsand Editor : Michael GrafContributing Editor: Craig SandersEditor/Layout: Melody Magnus this Issue: SkewT Diagrams: NewTools For VerticalAnalysisUtility of a Near-Realtime AircraftTracking Programin a CWSUWind Shears Within2,000 Feet of theAtmosphereFlying the weather map to avoid orreduce exposure to hazardous weatherconditions is an art based on experience,careful planning and attention to the decades, pilots have used astandard suite of weather products todevelop a three dimensional view of theatmosphere. These products include thesurface Analysis chart with its highs, lows,fronts, isobars and plotted to this suite upper air charts likethe 850 Mb or 500 Mb charts, Winds Aloft(FD) forecasts, Pilot Reports (PIREPs),and now graphics from the AviationWeather Center (AWC).

2 Pilots rely on thesetools to prepare for hazards that could im-pact their flight, especially the Diagrams: New ToolsFor Vertical AnalysisPilots can now add another tool to theirarsenal, the SkewT diagram shown in Fig-ure 1. The SkewT is not new, but has onlyrecently become available to pilots have used SkewTs for yearsto forecast everything from thunderstormsto freezing precipitation. Now pilots can useSkewTs to glean valuable information thatwill make flights SkewT , or sounding chart, is a ver-tical snapshot of temperature, dew pointand winds above a point on the first glance, a SkewT appears to becomplex, but pilots only need to knowabout a few lines and their relative positions,Figure 1. A forecast sounding valid at 20z January 25th for Springfield, MO158By Craig Sanders, Senior Forecaster at WFO Duluth(Continued on Page 2)2or orientations, to obtain valuable infor-mation about cloud bases and tops, freez-ing temperatures, icing zones, fog, inver-sions, and s take a recent weather situation andsee how the SkewT might help.

3 OnJanuary 24, 2004 an arctic air mass had de-scended into the nation s midsection asdepicted in Figure 2. A stationary front laypressed against the Rockies. The southernedge of the cold dome of air had stalledand the frontal zone stretched from the cen-tral plains east to the mid Atlantic were in the low 40s to low50s south of the front in Oklahoma andArkansas and in the 20s to low 30s northof the front in Kansas and the next 24 hours, a low pres-sure area formed and a warm front wasdepicted in the central plains. Warm airmoved north from Oklahoma, and over-running along the warm front began pro-ducing freezing precipitation in Kansas the morning of January 25, theAWC had AIRMETs for icing in place, andWhen two air masses of vastly differ-ent temperatures collide, the colder, moredense air slides beneath the warmer, lessdense air. This is exactly what happens nearwarm fronts and stationary fronts from latefall through early spring.

4 The result is a coldlayer of air near the surface and a warmerlayer above resulting in the atmospheredropping below freezing from the top ofthe warm layer on up. The SkewT is theonly tool that will show you where a warmlayer might be and one in which you couldfind relief from the SkewT diagram is a rather uncon-ventional looking graph on which tempera-ture, dew point and wind data are Vertical axis is pressure in Millibars. Fortechnical reasons, meteorologists use thisvertical coordinate rather than altitude be-cause weather computer models deal bestwith pressure as the Vertical the pressure lines are close to-gether near the bottom of the chart, theearth s surface. The lines get progressivelyfarther apart as you go up. This Logarith-mic Pressure, (LogP) scale represents theactual pressure structure in the is an approximate relationship ofpressure versus altitude under ideal or stan-dard atmospheric Flight Level(Mbs)(Ft)10053,083FL53015044,647FL4 5020038,662FL39025033,999FL34030030,065F L30040023,574FL240500 18,289 FL18060013,801700 9,882800 6,394850 4,781900 3,243925 2,512950 1,773 1000 364 0 The SkewT s temperature scale is notat right angles to the pressure scale as aremost conventional graphs, but skewed tothe right at 45 degrees.

5 Again, technical rea-sons dictate this layout. Thus, the nameFigure 3. Map Symbols that indicatefreezing precipitation and possible dangerousaircraft ADDS sur-face chart verifiedthat threat withfreezing or mixedprecipitation. Thesymbols shownin Figure 3 ap-peared on hourlysurface the coldair at the surface isdeep enough,about 2,000 feetor more, rain re-freezes into sleetor ice pellets. Ifthe cold air is shal-lower than this,the rain stands a good chance of remainingliquid until it hits an object whose tem-perature is below freezing, yielding freez-ing rain or clear icing on the particles reach the AWOS andASOS sensors as some other combinationof partially melted and partially frozen pre-cipitation, they are labeled as UnknownPrecipitation (UP) by the weather get out of an icing situation likethat of January 25, you have three options. First, you could remove yourself fromthat environment by climbing abovethe clouds or into air colder than you know where those levelsare by using the conventional briefingcharts, or if Flight Service had noPIREPs in the area yet?

6 Probably not. A second option is to descend into airnear the ground that is above s normally how the atmosphereis structured. But if the ASOSs andAWOSs below you are reporting-FZRA, -FZDZ or UP, this option maybe worse than the conditions you realready in. A third option is to find a layer ofwarmer air above freezing. This is wherethe SkewT excels. Conventional fore-cast maps and products won t tell youhow to do this; they merely suggestthe lowest freezing level. The SkewTon the other hand completes the threedimensional picture by portraying at-mospheric temperature from the sur-face to about 40000 feet or 2. Surface map of a developing freezing precipitation and icingsituation. A SkewT diagram is a valuable tool to diagnose the condition andaid the pilot to make good, well informed from Page 13 SkewT -LogP diagram or simplySkewT relative distance between tempera-ture and dew point lines represent humid-ity.

7 In Figure 1, temperature is red and dewpoint is closer these two lines are at anypoint, the higher the relative these lines are three degrees C or lessapart, expect clouds and IMC. Dry air ispresent when the temperature and dewpoint lines are farther apart. The greater thedistance, the drier the conditions are likely when thedifference is between 3 and 5 degrees C. Sothe SkewT can help you choose the altitudeat which to file or the altitude at which youcan expect to fly in VMC between icing is a concern, the SkewT willshow where the clouds are likely to be andwhere the temperature is conducive to ic-ing. A major benefit of the SkewT is that itcan show this even if the ASOS is report-ing a low overcast obscuring higher cloudlayers and no PIREPs are available to helpadd wind pole in Figure 5 is a useful,visual tool for finding an altitude to takeadvantage of a tailwind or minimize theeffect of a headwind.

8 The long staff ofeach symbol indicates the direction fromwhich the wind is blowing. Barbs on thesestaffs indicate speed in knots. Each longbarb is 10 knots, a half barb is 5 knots; apennant is 50 5 has three examples of howto interpret these wind flags. The wavy blueline to the right of the wind flags repre-sents the speed in knots. This line makes iteasy to find the max wind speed and thelevel it is at. You can plan your proposedaltitude with this wind tool in more detailthan with the FD Winds Aloft table, andyou can locate areas of possible low levelturbulence due to directional wind that you have a basic understand-ing of the SkewT s layout, let s go back tothe Midwest storm. By January 25, stormsat 2000 UTC freezing or mixed precipita-tion were underway in Kansas and your destination was Springfield, MO,you would want to create a forecast sound-ing specifically for that airport and have itvalid for the time of your descent andarrival premier Website for generatingforecast soundings is NOAA sForecast Systems Laboratory,or FSL at: The sitewill open up to a selection boxshown in Figure either the MAPS orthe RUC model option.

9 Ifeither model output is notavailable, you ll be greeted bya solid red screen. Go back andselect the alternate the UTC time foryour arrival time or the timeyou will be flying over if thiswill be one of your en routeway points. Next type in thethree-letter ID of the can also enter a latitudeand longitude if your airport doesn t havean ID; however, with the dense networkof ASOSs and AWOSs, it s usually easy topick the nearest airport FSL site does not recognize fiveletter FAA intersections. It s helpful to gettwo more versions of the SkewT , so leavethe Number of Hours at It maytake a few seconds longer to process butyou can use these versions to see how theconditions depicted in the sounding mightchange over a three hour sounding that was generated forthis storm is shown in Figure 1 and isalso inserted into the 3D view of theJanuary 25 weather map in Figure 6.

10 Theclassic warm frontal structure is shown:cold air near the surface with a warm layerabove and then cold air above temperature line crosses the zerodegree C reference line twice and demon-strates a multiple freezing level. When theFigure 5. Wind flags represent speed and direction similar to those found on surface 4. FSL opening page for selecting temperature line is rightof the zero degree referenceline, the air is above , the air is that in the cold(blue) arctic air mass, the tem-perature line is below freez-ing, , to the left of the zerodegree reference line. Abovethis blue area, the tempera-ture line is to the right and isabove freezing. If you wereapproaching Springfield at2000 UTC, you would en-counter a warm layer asshown. Your OAT wouldverify this. This warm layerwould be a good choice ifyou were icing up. You couldchoose to stay at that altitudeand fly south toward thewarm front in an increasinglydeep warm layer until youwere away from the front andin surface-based layer ofwarmer tops are also in-dicated on this forecastsounding.