Chapter 25, Analysis 25-20
25.3.1.1 Examples
See Figure 25-21 for an example of a 500 mb chart.
Figure 25-21. Example of a 500 mb Constant-Pressure Chart
Constant pressure level forecasts are used to provide an overview of weather patterns at specified times and
pressure altitudes and are the source for wind and temperature aloft forecasts.
Pressure patterns cause and characterize much of the weather. Typically, lows and troughs are associated
with clouds and precipitation , while highs and ridges are associated with fair weather, except in winter
when valley fog may occur. The location and strength of the jet stream can be viewed at 300 mb, 250 mb,
and 200 mb levels.
25.3.2 Radiosonde Observation (Weather Balloon) Analysis
A common means of analyzing radiosonde observations is the skew-T diagram (see Figure 25-22). Skew-T
diagrams are primarily intended for, and used by, meteorologists as part of their analyses of the atmosphere.
For example, the skew-T diagram can be used to:
• Determine the freezing level (or levels).
• Determine the stability of the atmosphere.
• Determine the potential for severe weather.
• Determine the height and depth of inversions.
Chapter 25, Analysis 25-21
• Infer cloud bases, tops, and layers.
• Determine soaring conditions.
25.3.2.1 Issuance
Skew-T diagrams are primarily intended for, and used by, meteorologists as part of their analyses of the
atmosphere and formulation of various forecasts.
Skew-T diagrams are available from the NWS NCO. Their “Model Analyses and Guidance ” website
contains a user’s guide that provides descriptions, details, and examples of the various products, including
the skew-T.
25.3.2.2 Format
The skew -T diagram provided on the NWS NCO “Model Analyses and Guidance ” website uses the
following format (other weather providers and websites may have different formats, especially colors):
• Horizontal axis is temperature in degrees Celsius, skewed to the right, labeled -20, 0,
and 20 (Celsius).
• Vertical axis is pressure levels in millibars, labeled 1,000 (near sea level) to 100 (approximately
53,000 ft MSL).
• Bold, solid red line represents the temperature profile over the station taken from the radiosonde
observation (weather balloon).
• Bold, solid green line represents the dewpoint profile.
• Wind aloft is shown on the far-right side.
Figure 25-22. Skew-T Diagram Example
Chapter 25, Analysis 25-22
25.3.2.3 Examples
Two examples are provided below: a multiple freezing level example (see Figure 25-23) and a cloud top
example (see Figure 25-24).
25.3.2.3.1 Multiple Freezing Level Example
Note how the temperature profile (bold red line) crosses the 0 -degree temperature line (also known as an
isotherm) five times (near 900 mb, 860 mb, 775 mb, 725 mb, and 675 mb).
Figure 25-23. Skew-T Diagram—Multiple Freezing Level Example
25.3.2.3.2 Cloud Top Example
Figure 25-24 is the radiosonde observation from Vandenberg Space Force Base (VSFB), California, for
1200 UTC for a typical coastal stratus cloud.
At about 950 mb, the temperature profile (bold red line) and dewpoint profile (bold green line) almost touch
each other. This is the profile of a cloud top. The temperature and dewpoint quickly diverge, representing
a change from the cool, moist air (and associated strat us cloud) to the dry and warmer air (cloud free)
above.
Figure 25-24. Skew-T Diagram—Cloud Top Example
Chapter 25, Analysis 25-23
25.4 Freezing Level Analysis
The freezing level is the lowest altitude in the atmosphere over a given location at which the air temperature
reaches 0°C. This altitude is also known as the height of the 0°C constant-temperature surface.
The initial analysis is updated hourly. The colors represent the height in hundreds of feet above MSL of the
lowest freezing level. Regions with white indicate the surface and the entire depth of the atmosphere are
below freezing. Hatched or spotted regions (if present) represent areas where the surface te mperature is
below freezing with multiple freezing levels aloft.
More information on the freezing level forecast graphics is available on the AWC’s website.
See Section 27.12 for additional information on freezing level forecast graphics.
25.5 Icing Analysis [Current Icing Product (CIP)]
The NWS produces icing products that are derived from NWS computer model data combined with
observations with no forecaster modifications. One of these products is the CIP.
The CIP combines weather satellite, weather radar, METAR, PIREPs and NWS model data to provide an
hourly 3D diagnosis of the icing environment. This information is displayed on a suite of graphics available
for the CONUS, much of Canada and Mexico, and their respective coastal waters.
The CIP [and its forecast counterpart Forecast Icing Product ( FIP) (see Section 27.13)] provide a
broad-brush approach to describing icing intensity using estimated liquid water content , drop size, and
temperature to depict ice accumulation rate. The intensity of ice accumulation rate varies by aircraft wing
shape. Hence, the icing intensity categories depicted in CIP s and FIPs (e.g., none, light, moderate, heavy)
is only a broad -brushed indication of ice accumulation rate, and not necessarily of aircraft performance.
The icing terms used in SIGMETs and AIRMETs do refer to icing impact on aircraft.
CIPs will continue to evolve over the coming years with increased model resolutions, additional horizontal
layers, and improvements to the algorithms and/or data sets used to produce the products. Along with these
improvements may come a change in references to the product update version. Users can find additional
information on these products and any changes on the AWC’s website.
The CIP suite as it appears on the AWC’s website consists of three graphics, including:
• Icing Probability (see Section 27.13.1 for additional information).
• Icing Severity (see Section 27.13.2 for additional information).
• Icing Severity plus SLD (see Section 27.13.3 for additional information).
The CIPs are generated for select altitudes from 1,000 ft MSL to FL300.
The CIPs can be viewed at single altitudes and FLs or as a composite of all altitudes from 1,000 ft MSL to
FL300, which is referred to as the “maximum” or “max.”
The CIP can be used to identify the latest and forecast 3D probability and intensity of ice accumulation rate.
The CIP should be used in conjunction with the report and forecast information contained in an AIRMET
and SIGMET.
The Icing Severity plus SLD product can help in determining the threat of SLD, which is particularly
hazardous to some aircraft.
Icing PIREPs are plotted on a single -altitude CIP graphic if the PIREP is within 1,000 ft of the selected
altitude and has been observed within 75 minutes of the chart ’s valid time. Icing PIREPs for all altitudes
(i.e., 1,000 ft MSL to FL300) are displayed, except negative reports are omitted to reduce clutter. The
PIREP legend is located on the bottom of each graphic.
Chapter 25, Analysis 25-24
Finally, while the “C” in CIP stands for “current,” the product does not show the current conditions, rather
it depicts the computer ’s expected conditions at the valid time shown on the product. This valid time can
be an hour old or more depending on when it is received by the user.
See Section 27.13 for additional information on the FIP.
25.6 Turbulence (Graphical Turbulence Guidance Nowcast (GTG-N))
The GTG Nowcast (GTG-N) product has been developed as a tactical aid for aviation. GTG-N provides a
nowcast of the current turbulent state of the atmosphere over the CONUS in near real-time, updating every
15 minutes. The basis for the nowcast is the most recently available short -term (one-hour) forecast from
the GTG product whose valid time is closest to the current update time. Recent observations of turbulence
are then used to update the GTG forecast and create a blended nowcast (expressed as EDR).
Current inputs include PIREPs, automated in situ EDR reports, and EDR estimated from ground -based
radar observations via the Next-Generation Radar (NEXRAD) Turbulence Detection Algorithm (NTDA).
In future upgrades, lightning, wind/gust observations from METARs, and EDR derived from ADS -B
vertical rate data will be assimilated as well.
GTG-N is available at 100 ft MSL, 1000 ft MSL, then every 1000 ft up to FL500.
See Section 27.14 for additional information on the GTG forecast.
25.7 Real-Time Mesoscale Analysis (RTMA)
RTMA is an hourly analysis system by the NWS ’ Environmental Modeling Center that produces analyses
of surface weather elements. The FAA has determined that RTMA temperature and altimeter setting
information is a suitable replacement for missing temperature and altimeter setting observations for a subset
of airports. RTMA temperature and altimeter setting information is intended for use by operators, pilots,
and aircraft dispatchers when an airport lacks a surface temperature and/or altimeter setting report from an
automated weather system (e.g., ASOS or AWOS sensor) or human observer. Airports with RTMA data
available are located in Alaska, Guam, Hawaii, Puerto Rico, and the CONUS.
RTMA is issued by the NWS every hour, 24 hours a day. Temperatures and altimeter settings are reported
for an airport station including the latitude and the longitude. Temperatures are reported in degrees Celsius.
Altimeter setting is reported in inches of mercury. See Figure 25-25 for an example RTMA temperature
and altimeter setting report.
Chapter 25, Analysis 25-25
25.7.1 Adjustments
The values found at the RTMA site are 95 percent accurate throughout the United States for both
temperature and altimeter setting when using the following mitigations:
• Temperature requires adding 4°C to the RTMA-derived temperature.
• Altimeter setting requires increasing the minimum descent altitude (MDA) or decision height
(DH) value on the approach chart by 100 ft and increasing the required flight visibility minimums
by ½ SM.
*****************************************************************
RTMA 2m-temperature (degrees Celsius) and altimeter setting
(inHg)
COMPUTED: 1239Z 25 Jan 2024
VALID: 1239Z 25 Jan 2024 to 1339Z 25 Jan 2024
*****************************************************************
Station Lat Lon 2m-T ALT
KABE 40.65 -75.44 3.43 30.19
KABI 32.41 -99.68 7.31 30.08
KABQ 35.04 -106.61 1.67 N/A
KABR 45.45 -98.42 -2.06 30.02
KABY 31.54 -84.19 19.07 30.15
KACK 41.25 -70.06 8.98 30.14
KACT 31.61 -97.23 10.00 30.06
KACV 40.98 -124.11 9.74 30.22
KACY 39.46 -74.58 11.59 30.17
KADS 32.97 -96.84 8.69 N/A
KAEX 31.33 -92.55 15.29 29.96
KAFW 32.99 -97.32 8.40 30.06
KAGS 33.37 -81.96 18.85 30.21
KAHN 33.95 -83.33 16.51 30.23
KAIA 42.05 -102.80 -4.11 29.94
KALB 42.75 -73.80 3.18 30.09
KALN 38.89 -90.05 3.02 30.12
KALO 42.56 -92.40 1.03 30.10
Figure 25-25. RTMA Surface Temperature and Altimeter Setting Example