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Additional Weather Information Skew-T Plots
Most weather information is derived from radiosondes, or weather balloons, that are released from over 100 stations in the United States twice daily (00Z and 12Z). The observations of temperature and humidity at various pressure altitudes are transmitted back to the releasing station; the radiosondes are also tracked by radar in order to determine wind speed and direction. This information is plotted to create a diagram known as a Skew-T/Log-P plot, commonly referred to as a Skew-T. These plots can be found at many different online
4-35
Figure 4-35. Example of a Skew-T plot.
weather sites; perhaps the easiest to use is rucsoundings.noaa.gov. The appropriate plot for any of the reporting sites can be found by typing in the closest reporting station identifier (usually an airport), and allowing the graphic to load.
There is a wealth of information that may be derived from the Skew-T plot (or “sounding,” as it may be referred to), but this discussion will be limited to those features of immediate interest to the average balloon pilot.
Some of the information that may be derived from the Skew-T, using the example in Figure 4-35:
• The two lines running vertically through the center of the graphic (red and blue) show the temperature and dew point at ascending pressure altitudes. The temperature is always plotted to the right of the dew point because temperature is almost always greater than the dew point temperature.
• The right side margin shows wind speed and direction, using the standard “barbed” system common in weather reporting. The scale (in this case, 0 to 40 knots) can be changed, but this setting provides the best resolution.
• The left margin of the chart indicates the pressure altitude for a particular reading. Pressure altitude readings correspond generally to certain altitudes. For example, the reading at 850 mb equates roughly to an altitude of 5,000 feet above MSL. That may not appear to be useful; however, this is where the dynamics of the application come into play. If the computer cursor is moved over the graphic part of the diagram, indicators as to the specific information for that altitude may be seen (not depicted in Figure 4-35). A pilot may be able to get information for varying altitudes as close at 125 feet apart, depending on the resolution of the original sounding information.
There is a tutorial available to fully explain the data interpretation of the Skew T plot at www.met.tamu.edu/class/ATMO151/tut/sound/soundmain.html.Velocity Azimuth Display (VAD)Winds
Velocity Azimuth display (VAD) winds are derived from the output of the 160 WSR-88 radar sites located throughout the United States. These radar systems are used by weather professionals to produce many different products, including the weather radar depictions found on many of the web sites previously discussed, as well as various television station weather reporting. [Figure 4-36]
The WSR-88 radar systems can be configured to produce radar returns from dust and other particulate matter that may be in the air. These radar returns can be processed to indicate wind direction and speed at different altitudes. VAD winds are generally reported in 1,000 foot increments, although at
4-36
Figure 4-36. VAD wind graphic.
times reports may be as small as 150 feet between altitudes. Standard “wind barb” depictions are used to represent the wind direction and speed at different altitudes.
Plymouth State University in Plymouth, New Hampshire operates a Weather Center with a website for locating VAD winds. This user-friendly website offers pilots a convenient selection of reporting stations at http://vortex.plymouth.edu/lnids_conus.html. Aviation Forecasts
Observed weather condition reports are often used in the creation of forecasts for the same area. A variety of different forecast products are produced and designed to be used in the preflight planning stage. Pilots need to be familiar with the following printed forecasts: wind and temperature aloft forecasts (FD reports), the terminal aerodrome forecast (TAF), aviation area forecast (FA), and inflight weather advisories (SIGMET, AIRMET).Wind and Temperature Aloft Forecast (FD Report)
Wind and temperature aloft forecasts provide wind and temperature forecasts for specific locations in the contiguous United States, plus network locations in Hawaii and Alaska. The forecasts are made twice a day based on the radiosonde upper air observations taken at 0000Z and 1200Z. Up through 12,000 feet are true altitudes; at and above 18,000 feet are pressure altitudes. Wind direction is always in reference to true north, and wind speed is given in knots. The temperature is given in degrees Celsius; no winds are forecast when a given level is within 1,500 feet of the station elevation. Similarly, temperatures are not forecast for any station within 2,500 feet of the station elevation. If the wind speed is forecast to be greater than 100 knots but less than 199 knots, the computer adds 50 to the direction and subtracts 100 from the speed. A sample FD report is shown in Figure 4-37.
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Balloon Flying Handbook(61)
