11/14/16 Aviation Weather Services AC 00-45H 3-61 imagery does not really “see” clouds, but “sees” high-level water vapor instead.
The most useful information to be gained from the water vapor images is the locations and movements of weather systems, jet streams, and thunderstorms.
Another useful tidbit is aided by the color scale used on the images. In general, regions displayed in shades of red are very dry in the upper atmosphere and may correlate to crisp, blue skies from a ground perspective.
On the contrary, regions displayed in shades of blue or green are indicative of a lot of high-level moisture and may also indicate cloudiness. This cloudiness could simply be high-level cirrus types or thunderstorms. That determination cannot be ascertained from this image by itself, but could easily be determined when used in conjunction with corresponding visible and IR satellite images.
A major advantage of the water vapor channel is that it can sense energy at night, so this imagery is available 24 hours per day.
Figure 3-33. Water Vapor Satellite Image—U.S. Example
The scale is in degrees Celsius. Blue/green colors indicate moisture and/or clouds in the mid/upper troposphere, while dark gray/orange/red colors indicate dry air in the mid/upper troposphere.
3.4.2.3.1 Water Vapor Image Data Legend.
The data legend (see Figure 3-34, Water Vapor Satellite Image Data Legend) on water vapor images is calibrated to temperature expressed in degrees Celsius. The actual data values on the water vapor images are not particularly
11/14/16 Aviation Weather Services AC 00-45H 3-62 useful. Interpretation of the patterns and how they change over time is more important. The legend may vary depending on the satellite image provider.
Figure 3-34. Water Vapor Satellite Image Data Legend
The colors (values) represent temperature in degrees Celsius
3.5 Upper-Air Observations.
3.5.1 Radiosonde Observations.
Since the late 1930s, the NWS has taken upper air observations with radiosondes attached to weather balloons. Weather data from the radiosonde are foundational to all computer model forecasts produced by the NWS.
The radiosonde is a small, expendable instrument package (weighing 100 grams (g) to 500 g) that is suspended below a large balloon inflated with hydrogen or helium gas. As the radiosonde rises at about 300 m per minute (about 1,000 ft per minute), sensors on the radiosonde measure profiles of pressure, temperature, and moisture. These sensors are linked to a battery-powered radio transmitter that sends the sensor measurements to a ground tracking antenna. Wind speed and direction aloft are also obtained by tracking the position of the radiosonde in flight using Global Positioning Satellites (GPS).
3.5.1.1 Issuance.
Weather balloons with radiosondes are launched twice a day worldwide from designated locations (see Figure 3-35, U.S. Radiosonde Network, for U.S. locations) at around 1100 UTC and 2300 UTC. It takes approximately 90 minutes for the balloon to reach an altitude of 100,000 ft. The weather data collected is assigned the observation times of 1200 UTC and 0000 UTC.
Special radiosondes may be launched at select times for various reasons, including when severe weather is expected in a region.
11/14/16 Aviation Weather Services AC 00-45H 3-63 Figure 3-35. U.S. Radiosonde Network
3.6 Graphical Depiction of Surface Observations.
3.6.1 AAWU Surface Observations.
A display of color-coded station plots (see Figure 3-36, AAWU Surface Observations Example) from Alaska is produced by the NWS’ AAWU. This product shows: surface temperature, dewpoint, ceiling, visibility, and wind direction/speed which have been taken from the METAR report. A key to the station plots is found on each map.
11/14/16 Aviation Weather Services AC 00-45H Figure 3-36. AAWU Surface Observations Example 10 999 100P9B9- _PASC39 -48 150 CYUB 999 A999.
52 PAKP PAOT, 54 1999 ı20 PATC 19 PADE 18, 110 58 - PABT APF YU 20 PAEM 100 PAUN $50 PAS. 100 PAHL 999 59 PATA "80;
1207 190220ª PAHV 4550999a5M PATK 19001457 CYDB 999 PASV PANE / 42 TOOPAIL LO " 57 PACV SPAYA SPAGY _PAHO- P 70 PASN 999 190 999 58 PAKN 50 % PAPB 999 152 480 999 T PAPH IMP(deg F)USTNII SURFACE OBSERVATIONSED DEWPT (deg F) : CIG 172 07/14/2005 IFR MYFRAYFR - 19°999
3.6.1.1 Issuance.
The charts are issued hourly and can be found on the AAWU Web site.
3.6.1.2 Legends.
AAWU Surface Observations depict numerous parameters, including the flying category, sky cover, and wind.
3.6.1.2.1 Flying Category.
Each station plot is color-coded according to the weather flying category reported (see Table 3-8, AAWU Surface Observations Flying Categories and Criteria). Red indicates IFR, blue indicates Marginal Visual Flight Rules (MVFR), and black is plotted for stations reporting Visual Flight Rules (VFR) ceiling and/or visibility conditions.
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11/14/16 Aviation Weather Services AC 00-45H 3-65 Table 3-8. AAWU Surface Observations Flying Categories and Criteria FLYING CATEGORY CEILING (feet) VISIBILITY (miles) VFR (black) Greater than 3,000 feet Greater than 5 miles MVFR (blue) 1,000 to 3,000 feet 3 to 5 miles IFR (red) Less than 1,000 feet Less than 3 miles
3.6.1.2.2 Station Plot.
METAR elements are plotted for each station in the form of a station plot (see Figure 3-41, Surface Observations—Aviationweather.gov METAR Static Display Example).
Figure 3-37. AAWU Surface Observation Plot Legend
3.6.1.2.3 Sky Cover.
The sky cover symbol is plotted at the station location and is filled according to the summation total of the sky condition element from the METAR report.
For example, if the METAR sky condition element was SCT030 BKN060 OVC090, the sky cover would be overcast and have a completely filled circle.
3.6.1.2.4 Station Identifier (STNID).
The four-letter ICAO station identifier is entered to the upper right of the station.
3.6.1.2.5 Wind.
Wind is plotted the same as the NWS surface analysis charts.
3.6.1.2.6 Temperature (TMP deg F).
Temperature in degrees Fahrenheit is plotted to the upper left of the sky cover symbol.
3.6.1.2.7 Visibility (VIS).
Visibility in statute miles is plotted to the left of the sky cover symbol.
Decimals are used to represent tenths of miles, when necessary.
3.6.1.2.8 Dewpoint Temperature (DEWPT deg F).
Dewpoint temperature, in degrees Fahrenheit, is plotted to the lower left of the sky cover symbol.
11/14/16 Aviation Weather Services AC 00-45H 3-66
3.6.1.2.9 Ceiling (CIG).
Ceiling is the height from the base of the lowest layer aloft covering more than one-half of the sky. Additionally, vertical visibility into a total surface-based obscuration is defined as a ceiling. For a METAR report, the first broken (BKN) or overcast (OVC) layer is the ceiling. For example, if the METAR sky condition element is SCT030 BKN060 OVC090, the ceiling is 6,000 ft.
For a total surface-based obscuration, no ceiling is plotted and the METAR must be consulted.
If the sky cover is clear, few, or scattered, no ceiling is plotted.
The ceiling is plotted to the lower right of the station circle. Ceilings are reported as hundreds of feet AGL. If no ceiling is present, the code 999 will be plotted.
3.6.1.2.10 Present Weather (WX).
Present weather symbols are entered to the left of the station. Note that the older Surface Aviation Observation (SAO) code is used instead of surface analysis chart symbols or the modern METAR code.
11/14/16 Aviation Weather Services AC 00-45H 3-67 Table 3-9. AAWU Surface Observation Precipitation Symbols Symbol Meaning T Thunderstorm R Rain RW Rain Shower L Drizzle ZR Freezing Rain ZL Freezing Drizzle A Hail IP Ice Pellets IPW Ice Pellet Showers S Snow SW Snow Showers SP Snow Pellets SG Snow Grains IC Ice Crystals
11/14/16 Aviation Weather Services AC 00-45H 3-68 Table 3-10. AAWU Surface Observation Obstruction to Visibility Symbols Symbol Meaning BD Blowing Dust BN Blowing Sand BS Blowing Snow BY Blowing Spray D Dust F Fog GF Ground Fog H Haze IF Ice Fog K Smoke Table 3-11. AAWU Surface Observation Precipitation Intensity Symbols Symbol Meaning
- Light (No symbol) Moderate + Heavy
3.6.1.3 Use.
AAWU Surface Observations provide an overview of weather flying categories and other adverse weather conditions for the valid time shown on the graphic display. This product often does not completely represent the en route conditions because of terrain variations and the possibility of weather occurring between reporting stations. This product should be used in addition to the latest METAR/SPECIs, PIREPs, and radar and satellite imagery for a complete look at the latest flying conditions.
11/14/16 Aviation Weather Services AC 00-45H
3.6.2 METARs on Aviationweather.gov.
3.6.2.1 Issuance.
Surface observations (METAR and SPECI) are provided on http://www.aviationweather.gov in several formats. The formats range from traditional text (referred to as the raw format) to different types of map displays. The two types of map displays are interactive (see Figure 3-38, Surface Observations-Aviationweather.gov METAR Interactive Display Example) and static (see Figure 3-41).
3.6.2.1.1 Interactive Display.
The interactive display includes zoom and pan. Increasing the zoom allows more METARs to be displayed.
Figure 3-38. Surface Observations-Aviationweather.gov METAR Interactive Display Example
ADDS METARS INFO 1ETAR Display at valid at 0129 UTC 17 Apr 2015 permalinI 997 CW NZ 025 52 PART 182 006 ITM r5845 Map: Plot Options: Data Options: Overlays:
OLight All • Temp aWind Alt Latest • Time • Highways Dark • Vis •Wx •Cover Ceil Decoded • Top Jetroutes
•Simple None Dew aWgst DId OTAF •ARTCC/FIR Bounds Data layers:
Satellite Data density
•Radar Metric OHover (1 • Scale Temp 26966--Alt W×- Vis-O.! KRFD~~-Id -Ceil Wind @Calm 15knt 060knt FIt Cat: •MVFR • IFRO LIFR Dew- Windbarb 3-69
11/14/16 Aviation Weather Services AC 00-45H 3-70 “Plot Options” allows users to choose the weather elements displayed
(e.g., view only ceilings and visibilities). The following parameters can be displayed using a station plot model (see Figure 3-39, Interactive Display Station Plot Example):
Figure 3-39. Interactive Display Station Plot Example
• Temp: Temperature in Fahrenheit. Celsius is also available, if metric is selected.
• Alt: Atmospheric pressure reduced to MSL.
o Where expressed in inches of mercury (Hg) or altimeter setting, the value is expressed in hundredths of an inch where the leading 2 or 3 is omitted (e.g., 982 would be 29.82 in, 025 would be 30.25 in). When trying to determine whether to add a 2 or 3, use the number that will give you a value closest to 30.00 in.
o Where expressed in millibars, the value is expressed in tenths of a millibar with the leading 9 or 10 omitted (e.g., 998 would be 999.8 mb, 164 would be 1,016.4 mb). When trying to determine whether to add a 9 or 10, use the number that will give you a value closest to 1,000 mb.
• Ceil: Cloud ceiling in hundreds of feet (BKN, OVC, and OVX only).
• Id: The four-letter ICAO identifier for the airport.
• Windbarb: The stick points in the direction the winds are coming from.
Wind speed is in knots. Red represents wind gusts.
• Dew: Dewpoint in Fahrenheit. Celsius is also available, if metric is selected.
• Vis: Visibility in miles. Kilometers (km), if metric is selected. Visibility value on the chart may not represent actual value reported in the METAR or SPECI, and may be fractionally lower than shown. (e.g., 2 ½ in the METAR is shown as 3 on the surface plot) (see Table 3-12, Rounding of Actual METAR Visibility Values for Depiction on Aviationweather.gov/AWC METAR Display).
• Wx: Weather symbol (see Appendix F, Area Forecasts (FA)—Continental United States (CONUS) and Hawaii) representing current weather.
11/14/16 Aviation Weather Services AC 00-45H 3-71
• Sky: Cloud cover symbol (see Figure 3-40, Station Plot Sky Symbols) is plotted in the center. The amount the circle is filled represents the amount of cloud cover. The color of the circle represents one of the following weather categories (see Table 3-13, Station Plot Weather Categories):
o Green: VFR, o Blue: MVFR, o Red: IFR, or o Purple: Low instrument flight rules (LIFR).
A click on the station circle displays the raw METAR in a separate text window. When the Hover option is selected, the raw METAR text window appears by merely placing the cursor over the station circle. If the Decoded option is set, the METAR appears in plain language instead of METAR code.
If the TAF option is set, the latest Terminal Aerodrome Forecast (TAF) will be appended to the information in the text window.
Table 3-12. Rounding of Actual METAR Visibility Values for Depiction on Aviationweather.gov/AWC METAR Display Visibility Reported in METAR Visibility, as depicted on surface plot 0, 1/16, 1/8, 3/16, 1/4, 3/8 0.3 ½ 0.5 ¾ 0.8 1 ½, 1 ¾, 2 2 ½, 3 3
11/14/16 Aviation Weather Services AC 00-45H 3-72 Table 3-13. Station Plot Weather Categories Category* Color Ceiling Visibility LIFR (Low IFR) Magenta Below 500 feet AGL and/or Less than 1 mile IFR Red 500 to below 1,000 feet AGL and/or 1 mile to less than miles 3 MVFR (Marginal VFR) Blue 1,000 to 3,000 feet AGL and/or 3 to 5 miles VFR Green Greater than 3,000 feet AGL and Greater than 5 miles *These categories are not flight rules and should not be confused with the flight rules provided in Part 91, including those for Basic VFR Weather Minimums. Rather, these categories were created for weather charts as a means to visually enhance the products.
Figure 3-40. Station Plot Sky Symbols
Automated stations report “CLR” when clouds may exist above 12,000 ft, so a square is used to represent this uncertainty, whereas an unfilled circle is used for “SKC”, when a human reports the sky is completely clear overhead. The abbreviation “OVX” is unofficial, but AWC uses it to indicate the sky is obscured, which is the case when a METAR reports vertical visibility and no cloud information. “Miss” means the sky condition is missing.
3.6.2.1.2 Static Display.
Static display products are provided on the AWC’s Web site for 19 select regions plus the CONUS. Each product is updated every 5 minutes using the latest METAR or SPECI information. The Web page automatically refresh every 15 minutes.
The static display has fewer features than the interactive display. Primarily, the static display does not provide ceiling height. Also, the static display does not have the functionality to display separately the raw METAR or TAF, or change units to metric.