• Sky cover—the station model depicts total sky cover and is shown as clear, scattered, broken, overcast, or obscured/partially obscured.
• Sea level pressure—given in three digits to the nearest tenth of a millibar (mb). For 1,000 mbs or greater, prefix a 10 to the three digits. For less than 1,000 mbs, prefix a 9 to the three digits.
• Pressure change/tendency—pressure change in tenths of mb over the past 3 hours. This is depicted directly below the sea level pressure.
• Dew point—given in degrees Fahrenheit.
• Present weather—over 100 different standard weather symbols are used to describe the current weather.
• Temperature—given in degrees Fahrenheit.
• Wind—true direction of wind is given by the wind pointer line, indicating the direction from which the wind is blowing. A short barb is equal to 5 knots of wind, a long barb is equal to 10 knots of wind, and a pennant is equal to 50 knots.
Figure 13-10. Surface analysis chart.
Figure 13-11. Sample station model and weather chart symbols.
Weather Depiction Chart
A weather depiction chart details surface conditions as derived from METAR and other surface observations. The weather depiction chart is prepared and transmitted by computer every 3 hours beginning at 0100Z time and is valid data for the forecast period. It is designed to be used for flight planning by giving an overall picture of the weather across the United States. [Figure 13-12]
The weather depiction chart also provides a graphic display of IFR, VFR, and marginal VFR (MVFR) weather. Areas of IFR conditions (ceilings less than 1,000 feet and visibility less than three 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 nonhatched area outlined by a smooth line. Areas of VFR (no ceiling or ceiling greater than 3,000 feet and visibility greater than five miles) are not outlined. Also plotted are fronts, troughs, and squall lines from the previous hours surface analysis chart.
Weather depiction charts show a modified station model that provides sky conditions in the form of total sky cover, ceiling height, 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.
Significant Weather Prognostic Charts
Significant weather prognostic charts 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 630 (25,000 to 63,000 feet). The primary concern of this discussion is the low-level significant weather prognostic chart.
The low-level chart is is a forecast of aviation weather hazards, primarily intended to be used as a guidance product for briefing the VFR pilot. The forecast domain covers the 48 contiguous states, southern Canada and the coastal waters for altitudes below 24,000 ft. Low altitude Significant Weather charts are issued four times daily and are valid at fixed times: 0000, 0600, 1200, and 1800 UTC. Each chart is divided on the left and right into 12 and 24 hour forecast intervals (based on the current NAM model available).
Figure 13-12. Weather depiction chart.
Effective September 1, 2015, the four-panel Low Level SFC-240 chart was replaced with a two-panel chart. The new two-panel chart will be the same as the top two panels in the former four-panel chart, depicting the freezing level and areas of IFR, MVFR, and moderate or greater turbulence. The bottom two panels of the chart have been removed. In lieu of these bottom two panels, an enhanced surface chart that includes fronts, pressure, precipitation type, precipitation intensity, and weather type, is displayed. The green precipitation polygons will be replaced by shaded precipitation areas using the National Digital Forecast Database (NDFD) weather grid.
Figure 13-13 depicts the new two-panel significant weather prognostic chart, as well as the symbols typically used to depict precipitation. The two panels depict freezing levels, turbulence, and low cloud ceilings and/or restrictions to visibility (shown as contoured areas of MVFR and IFR conditions). These charts enable the pilot to pictorially evaluate existing and potential weather hazards they may encounter. Pilots can balance weather phenomena with their aircraft capability and skill set resulting in aeronautical decision-making appropriate to the flight. Prognostic charts are an excellent source of information for preflight planning; however, this chart should be viewed in light of current conditions and specific local area forecasts.
The 36- and 48-hour significant weather prognostic chart is an extension of the 12- and 24-hour forecast. This chart is issued twice a day. It typically contains forecast positions and characteristics of pressure patterns, fronts, and precipitation. An example of a 36- and 48-hour surface prognostic chart is shown in Figure 13-14.
ATC Radar Weather Displays
Although ATC systems cannot always detect the presence or absence of clouds, they can often determine the intensity of a precipitation area, but the specific character of that area (snow, rain, hail, VIRGA, etc.) cannot be determined. For this reason, ATC refers to all weather areas displayed on ATC radar scopes as “precipitation.”
ARTCC facilities normally use a Weather and Radar Processor (WARP) to display a mosaic of data obtained from multiple NEXRAD sites. There is a time delay between actual conditions and those displayed to the controller. The precipitation data on the ARTCC controller’s display could be up to 6 minutes old. The WARP processor is only used in ARTCC facilities. All ATC facilities using radar weather processors with the ability to determine precipitation intensity, describe the intensity to pilots as:
• Light
• Moderate
• Heavy
• Extreme
When the WARP is not available, a second system, the narrowband Air Route Surveillance Radar (ARSR) can display two distinct levels of precipitation intensity that is described to pilots as “MODERATE and “HEAVY TO EXTREME.”
ATC facilities that cannot display the intensity levels of precipitation due to equipment limitations describe the location of the precipitation area by geographic position or position relative to the aircraft. Since the intensity level is not available, the controller states “INTENSITY UNKNOWN.”
ATC radar is not able to detect turbulence. Generally, turbulence can be expected to occur as the rate of rainfall or intensity of precipitation increases. Turbulence associated with greater rates of precipitation is normally more severe than any associated with lesser rates of precipitation. Turbulence should be expected to occur near convective activity, even in clear air. Thunderstorms are a form of convective activity that imply severe or greater turbulence. Operation within 20 miles of thunderstorms should be approached with great caution, as the severity of turbulence can be much greater than the precipitation intensity might indicate.
Figure 13-13. Significant weather prognostic chart.
Figure 13-14. 36- (top) and 48-hour (bottom) surface prognostic chart.
Weather Avoidance Assistance
To the extent possible, controllers will issue pertinent information on weather and assist pilots in avoiding such areas when requested. Pilots should respond to a weather advisory by either acknowledging the advisory or by acknowledging the advisory and requesting an alternative course of action as follows:
• Request to deviate off course by stating the number of miles and the direction of the requested deviation.
• Request a new route to avoid the affected area.
• Request a change of altitude.
• Request radar vectors around the affected areas.
The controller’s primary function is to provide safe separation between aircraft. Any additional service, such as weather avoidance assistance, can only be provided to the extent that it does not detract from the primary function. It’s also worth noting that the separation workload is generally greater than normal when weather disrupts the usual flo
w of traffic. ATC radar limitations and frequency congestion may also be a factor in limiting the controller’s capability to provide additional service.
Electronic Flight Displays (EFD)/Multi-Function Display (MFD) Weather
Many aircraft manufacturers now include data link weather services with new electronic flight display (EFD) systems. EFDs give a pilot access to many of the data link weather services available.
Products available to a pilot on the display pictured in Figure 13-15 are listed as follows. The letters in parentheses indicate the soft key to press in order to access the data.
• Graphical NEXRAD data (NEXRAD)
• Graphical METAR data (METAR)
• Textual METAR data
• Textual terminal aerodrome forecasts (TAF)
• City forecast data
• Graphical wind data (WIND)
• Graphical echo tops (ECHO T,,,OPS)
• Graphical cloud tops (CLD TOPS)
• Graphical lightning strikes (LTNG)
• Graphical storm cell movement (CELL MOV)
• NEXRAD radar coverage (information displayed with the NEXRAD data)
• SIGMETs/AIRMETs (SIG/AIR)
• Surface analysis to include city forecasts (SFC)
• County warnings (COUNTY)
• Freezing levels (FRZ LVL)
• Hurricane track (CYCLONE)
• Temporary flight restrictions (TFR)
Pilots must be familiar with any EFD or MFD used and the data link weather products available on the display.
Weather Products Age and Expiration
The information displayed using a data link weather link is near real time but should not be thought of as instantaneous, up-to-date information. Each type of weather display is stamped with the age information on the MFD. The time is referenced from Zulu when the information was assembled at the ground station. The age should not be assumed to be the time when the FIS received the information from the data link.
Two types of weather are displayed on the screen: “current” weather and forecast data. Current information is displayed by an age while the forecast data has a data stamp in the form of “__ / __ __ : __.” [Figure 13-16]
The Next Generation Weather Radar System (NEXRAD)
The NEXRAD system is comprised of a series of 159 Weather Surveillance Radar–1988 Doppler (WSR-88D) sites situated throughout the United States, as well as selected overseas sites. The NEXRAD system is a joint venture between the United States Department of Commerce (DOC), the United States DOD, as well as the United States Department of Transportation (DOT). The individual agencies that have control over the system are the NWS, Air Force Weather Agency (AFWA) and the FAA. [Figure 13-17]
NEXRAD data for up to a 2,000 mile range can be displayed. It is important to realize that the radar image is not real time and can be up to 5 minutes old. The NTSB has reported on 2 fatal accidents where in-cockpit NEXRAD mosaic imagery was available to pilots operating near quickly-developing and fast-moving convective weather. In one of these accidents, the images were from 6 to 8 minutes old. In some cases, NEXRAD data can age significantly by the time the mosaic image is created. In some extreme latency cases, the actual age of the oldest NEXRAD data in the mosaic can exceed the age indication in the cockpit by 15 to 20 minutes. Even small-time differences between the age indicator and actual conditions can be important for safety of flight, especially when considering fast-moving weather hazards, quickly developing weather scenarios, and/or fast-moving aircraft. At no time should the images be used as storm penetrating radar nor to navigate through a line of storms. The images display should only be used as a reference.
Figure 13-15. Information page.
NEXRAD radar is mutually exclusive of Topographic (TOPO), TERRAIN and STORMSCOPE. When NEXRAD is turned on, TOPO, TERRAIN, and STORMSCOPE are turned off because the colors used to display intensities are very similar.
Lightning information is available to assist when NEXRAD is enabled. This presents a more comprehensive picture of the weather in the surrounding area.
In addition to utilizing the soft keys to activate the NEXRAD display, the pilot also has the option of setting the desired range. It is possible to zoom in on a specific area of the display in order to gain a more detailed picture of the radar display. [Figure 13-18]
What Can Pilots Do?
Remember that the in-cockpit NEXRAD display depicts where the weather WAS, not where it IS. The age indicator does not show the age of the actual weather conditions, but rather the age of the mosaic image. The actual weather conditions could be up to 15 to 20 minutes OLDER than the age indicated on the display. You should consider this potential delay when using in-cockpit NEXRAD capabilities, as the movement and/or intensification of weather could adversely affect safety of flight.
Figure 13-16. List of weather products and the expiration times of each.
Figure 13-17. NEXRAD radar display.
Figure 13-18. NEXRAD radar display (500 mile range). The individual color gradients can be easily discerned and interpreted via the legend in the upper right corner of the screen. Additional information can be gained by pressing the LEGEND soft key, which displays the legend page.
• Understand that the common perception of a “5-minute latency” with radar data is not always correct.
• Get your preflight weather briefing! Having in-cockpit weather capabilities does not circumvent the need for a complete weather briefing before takeoff.
• Use all appropriate sources of weather information to make in-flight decisions.
• Let your fellow pilots know about the limitations of in-cockpit NEXRAD.
NEXRAD Abnormalities
Although NEXRAD is a compilation of stations across the country, there can be abnormalities associated with the system. Some of the abnormalities are listed below.
• Ground clutter
• Strobes and spurious radar data
• Sun strobes, when the radar antenna points directly at the sun
• Interference from buildings or mountains that may cause shadows
• Military aircraft that deploy metallic dust and may reflect the radar signature
NEXRAD Limitations
In addition to the abnormalities listed, the NEXRAD system does have some specific limitations.
Base Reflectivity
The NEXRAD base reflectivity does not provide adequate information from which to determine cloud layers or type of precipitation with respect to hail versus rain. Therefore, a pilot may mistake rain for hail.
In addition, the base reflectivity is sampled at the minimum antenna elevation angle. With this minimum angle, an individual site cannot depict high altitude storms directly over the station. This leaves an area of null coverage if an adjacent site does not also cover the affected area.
Resolution Display
The resolution of the displayed data poses additional concerns when the range is decreased. The minimum resolution for NEXRAD returns is 1.24 miles. This means that when the display range is zoomed in to approximately ten miles, the individual square return boxes are more prevalent. Each square indicates the strongest display return within that 1.24 mile square area.
AIRMET/SIGMET Display
AIRMET/SIGMET information is available for the displayed viewing range on the MFD. Some displays are capable of displaying weather information for a 2,000 mile range. AIRMETS/SIGMETS are displayed by dashed lines on the map. [Figure 13-19]
The legend box denotes the various colors used to depict the AIRMETs, such as icing, turbulence, IFR weather, mountain obscuration, and surface winds. [Figure 13-20] The great advantage of the graphically displayed AIRMET/SIGMET boundary box is the pilot can see the extent of the area that the advisory covers. The pilot does not need to manually plot the points to determine the full extent of the coverage area.
Graphical METARs
METARs can be displayed on the MFD. Each reporting station that has a METAR/TAF available is depict
ed by a flag from the center of the airport symbol. Each flag is color coded to depict the type of weather that is currently reported at that station. A legend is available to assist users in determining what each flag color represents. [Figure 13-21]
The graphical METAR display shows all available reporting stations within the set viewing range. By setting the range knob up to a 2,000 mile range, pilots can pan around the display map to check the current conditions of various airports along the route of flight.
By understanding what each colored flag indicates, a pilot can quickly determine where weather patterns display marginal weather, IFR, or areas of VFR. These flags make it easy to determine weather at a specific airport should the need arise to divert from the intended airport of landing.
Data Link Weather
Pilots now have the capability of receiving continuously updated weather across the entire country at any altitude. No longer are pilots restricted by radio range or geographic isolations, such as mountains or valleys.
In addition, pilots no longer have to request specific information from weather briefing personnel directly. When the weather becomes questionable, radio congestion often increases, delaying the timely exchange of valuable inflight weather updates for a pilot’s specific route of flight. Flight Service Station (FSS) personnel can communicate with only one pilot at a time, which leaves other pilots waiting and flying in uncertain weather conditions. Data link weather provides the pilot with a powerful resource for enhanced situational awareness at any time. Due to continuous data link broadcasts, pilots can obtain a weather briefing by looking at a display screen. Pilots have a choice between FAA-certified devices or portable receivers as a source of weather data.
Pilot's Handbook of Aeronautical Knowledge (Federal Aviation Administration) Page 57
