The weather depiction typically displays major fronts or areas of high and low pressure. It also provides a graphic display
of Instrument Flight Rules (IFR), VFR, and marginal VFR (MVFR) weather. Areas of IFR conditions (ceilings less than
1,000 feet and visibility less than 3 miles) are shown by a hatched area outlined by a smooth line. MVFR regions (ceilings
1,000 to 3,000 feet, visibility 3 to 5 miles) are shown by a non-hatched area outlined by a smooth line. Areas of VFR (no
ceiling or ceiling greater than 3,000 feet and visibility greater than 5 miles) are not outlined. Weather depiction charts show
a modified station model that provides sky conditions in the form of total sky cover, cloud height or ceiling, weather, and
obstructions to visibility, but does not include winds or pressure readings like the surface analysis chart. A bracket ( ] )
symbol to the right of the station indicates the observation was made by an automated station.
A detailed explanation of a station model is depicted in the previous discussion of surface analysis charts.
Radar Summary Chart (SD)
A radar summary chart [ Figure 4-30 ] is a computer-generated graphical display of a collection of automated radar
weather reports (SDs). The chart is published hourly, 35 minutes past the hour. It displays areas of precipitation as well as
information regarding the characteristics of the precipitation. An SD chart includes:
Figure 4-30. Radar summary chart
• No information—if information is not reported, the chart will read “NA.” If no echoes are detected, the chart will
read “NE.”
• Precipitation intensity contours—intensity can be described as one of six levels and is shown on the chart by three
contour intervals.
• Height of tops—the heights of the echo tops are given in hundreds of feet MSL.
• Movement of cells—individual cell movement is indicated by an arrow pointing in the direction of movement. The
speed of movement in knots is the number at the top of the arrow head. “LM” indicates little movement.
• Type of precipitation—the type of precipitation is marked on the chart using specific symbols.
• Echo configuration—echoes are shown as areas, cells, or lines.
• Weather watches—severe weather watch areas for tornadoes and severe thunderstorms are depicted by boxes
outlined with heavy dashed lines.
A valuable tool for preflight planning, the SD chart has several limitations. Since it depicts only areas of current precipitation,
it will not show areas of clouds and fog with no appreciable precipitation, or the height of the tops and bases of the clouds.
SD charts should be used in conjunction with current METAR and weather forecasts.
Composite Moisture Stability Chart
The composite moisture stability chart is a chart composed of four panels depicting stability, precipitable water, freezing
level, and average relative humidity conditions. This computer-generated chart contains data obtained from upper-air
observations, is updated twice a day, and shows the relative stability of the air mass and the potential for thunderstorms or
thermal activity.
Stability/Lifted Index Chart
A subdisplay of the composition moisture stability chart is the stability or lifted index (LI) chart, a valuable tool for
determining the stability of the atmosphere. The stability or LI chart is the upper left hand panel of the composite moisture
stability chart. Two indexes represent the moisture and stability of the air: the K index (KI) and the LI, with these numbers
composing a fraction. KI (denominator of the fraction) provides moisture and stability information. Values range from high
positive values to low negative values. A high positive KI indicates moist, unstable air. KI values are considered high when
at or above +20, and low when less than +20. [Figure 4-31]
Figure 4-31. Lifted index chart.
The LI (numerator of the fraction) is a common measure of atmospheric stability. It is calculated by hypothetically lifting
a parcel of air to the 500 mb level, approximately 18,000 feet MSL, and analyzing its stability at that level. A positive LI
indicates that a particular parcel of air is stable at that level and resists further upward motion. Large positive values (+8 or
greater) would indicate very stable air. Conversely, a negative LI means that a lifted surface parcel of air is unstable, and
more likely to rise. Large negative values (–6 or more) indicate very unstable air.
The KI and LI can be used together to determine the moisture and stability characteristics of a particular air mass. The air
masses may be classified as moist and stable, moist and unstable, dry and stable, or dry and unstable. This determination
allows the balloon pilot to make an informed decision regarding the likelihood of thermal and potential thunderstorms, and
if a safe flight can be conducted.
Constant Pressure Analysis Charts
A constant pressure analysis chart or isobaric chart is a weather map representing conditions on a surface of equal
atmospheric pressure. [Figure 4-32] For example, a 500 mb chart will display conditions at the level of the atmosphere at
which the atmospheric pressure is 500 mb. The height above sea level at which the pressure is that particular value may
vary from one location to another at any given time, and also varies with time at any one location, so it does not represent
a surface of constant altitude/height.
Figure 4-32. Constant pressure analysis chart.
Constant pressure charts provide the pilot with a clearer picture of how the atmosphere behaves at different altitudes and
pressures. For example, a low pressure system that seems insignificant based on surface observations may prove to be a
major factor in the weather at five or ten thousand feet.
Constant pressure charts are prepared for selected values of pressure and present weather information at various altitudes.
Standard charting values are at 850 mb (approximately 5,000 feet MSL), 700 mb (approximately 10,000 feet MSL), 500
mb (approximately 18,000 feet MSL), as well as higher and lower altitudes. Charts with higher pressure altitudes present
information at lower altitudes, and charts with lower pressure altitudes present information at higher altitudes.
Symbology on the constant pressure analysis chart is the same as that of the surface analysis chart. This chart depicts
the information at a specific pressure altitude. When compared with the surface analysis of the same time frame, a three-
dimensional concept of the atmosphere can be conceptualized, and the pilot can gain a greater understanding of the
atmospheric dynamics involved in weather patterns.
Significant Weather Prognostic Charts
Significant weather prognostic charts [ Figure 4-33] display the observed or forecast significant weather phenomena at
different flight levels that may affect the operation of aircraft. They are available for low-level significant weather from the
surface to FL 240 (24,000 feet), also referred to as the 400 mb level, and high-level significant weather from FL 250 to FL
600 (25,000 to 60,000 feet). This discussion involves the low-level significant weather prognostic chart.
Figure 4-33. Significant weather prognostic chart.
The low-level chart comes in two forms: the 12- and 24-hour forecast chart, and the 36- and 48-hour surface only forecast
chart. The first chart is a four-panel chart that includes 12- and 24-hour forecasts for significant weather and surface weather.
Charts are issued four times a day at 0000Z, 0600Z, 1200Z, and 1800Z. The valid time for the chart is printed on the lower
left corner of each panel. The upper two panels show forecast significant weather, which may include nonconvective
turbulence, freezing levels, and IFR or MVFR weather. Areas of moderate or greater turbulence are enclosed in dashed
lines. Numbers within these areas give the height of the turbulence in hundreds of feet MSL. Figures below the line show
the anticipated base, while figures above the line show the top of the zone of turbulence. Also shown on this panel are
areas of VFR, IFR, and MVFR. IFR areas are enclosed by solid lines, MVFR areas are enclosed by scalloped lines, and
the remaining, unenclosed area is designated VFR. Zigzag lines and the letters “SFC” (surface) indicate freezing levels in
that area are at the surface. Freezing level height contours for the highest freezing level are drawn at 4,000 foot intervals
with dashed lines.
Additional Weather Information
Some additional things you should know about the weather are as follows.
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 weather sites.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 SkewT, using the example in Figure 4-34:
• The two lines running vertically through the center of the graphic (red and blue) show the temperature and dew point
for a specific location 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 4-34). 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.
Figure 4-34. Example of a Skew-T plot.
There is a tutorial available to fully explain the data interpretation of the Skew T plot here.
Velocity Azimuth Display (VAD)Winds
Velocity Azimuth display (V AD) 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 4-35]
Figure 4-35. VAD wind graphic.
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. V AD winds
are generally reported in 1,000 foot increments, although at 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.
The College of Dupage has a website with current weather observations (analysis), satellite and radar information,
including, data from the GOES 16/17, meso sector information, V AD radar data, and numerical models where you can
left click on any model output and get a forecast skew-t for any particular location. This data-filled website is located at:
Additional Weather Information
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 in-flight 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