11/14/16 Aviation Weather Services AC 00-45H 3-54 Figure 3-24. WSR-88D Weather Radar Angel Example
This angel was caused by bats departing Selman Bat Cave at Alabaster Caverns State Park, OK around sunset.
3.3.7.6 Anomalous Propagation (AP).
AP (see Figure 3-25, WSR-88D Weather Radar Anomalous Propagation (AP) Example) is an extended pattern of ground echoes caused by superrefraction of the radar beam. Superrefraction causes the radar beam to bend downward and strike the ground. It differs from ground clutter because it can occur anywhere within the radar’s range, not just at ranges close to the radar.
AP typically appears as speckled or blotchy, high-reflectivity echoes. When animating images, AP tends to “bloom up” and dissipate, and has no continuity of motion. AP can sometimes be misinterpreted as thunderstorms;
differentiating between the two is determined by animating the images.
Thunderstorms move with a smooth, continuous motion while AP appears to “bloom up” and dissipate randomly.
11/14/16 Aviation Weather Services AC 00-45H Figure 3-25. WSR-88D Weather Radar Anomalous Propagation (AP) Example
BASE REFLECTIVITY
KTLX - OKLAHOMA CITY, OK ANOMALOUS 05/26/2000 12:15:59 GMT PROPOGATION LAT: 35/19/58 N LON: 97/16/40 W ELEV: 1277.0 FT MODE/VCP: A / 11 ELEV ANGLE: 0.50 ° MAX: 73 dBZ RANGE 248 NM Legend: (Category) dBZ
(15) 75
(14) 70
(13) 65
(12) 60
(11) 55
(10) 50 ЛОАЯ 300108000
3.3.7.7 Other Non-Meteorological Phenomena.
3.3.7.7.1 Wind Farms.
Wind farms can affect the return signal of the radar beam. Depending on the proximity of the wind farm to the radar site (generally within 10 NM), wind farm turbines can result in beam blockage, false echoes, or high reflectivity values (see Figure 3-26, Wind Farms Have Been Known to Make Benign Echoes Appear as Small Storms. This Example Is from the Buffalo Radar).
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11/14/16 Aviation Weather Services AC 00-45H 3-56 Figure 3-26. Wind Farms Have Been Known to Make Benign Echoes Appear as Small Storms. This Example Is from the Buffalo Radar
3.3.8 Use.
Radar mosaic products based on composite reflectivity radar data provide a more complete depiction of precipitation than mosaics based on base reflectivity data.
3.3.9 Future Products.
The NWS is working on two new types of radar displays that are not available for description in time for publication.
3.3.9.1 Product Update Rate.
Today, composite radar images are updated after volume scans are complete (about every 5 minutes). In the future, composite radar products will be updated with every tilt sweep.
3.3.9.2 Mosaicked Composite Reflectivity.
The NWS will add mosaics of composite reflectivity. This is especially important for aviation as the heaviest precipitation at any altitude in the
11/14/16 Aviation Weather Services AC 00-45H 3-57 vertical will be depicted in mosaics. Today, only the base elevation angle
(i.e., closest to the ground) contributes to the mosaic.
3.3.9.3 Hydrometeorological classification.
The implementation of dual polarization radar across the NEXRAD system means more information about detected targets can be observed. For more information on what dual polarization is, refer to Dual-Polarization Radar Training for NWS Partners provided by the NWS Warning Decision Training Division at http://www.wdtb.noaa.gov/courses/dualpol/Outreach/.
Dual polarization allows the NWS to make a best guess at the precipitation type detected from a pre-defined list. The precipitation types of interest to aviation include:
• Graupel (also known as snow pellets). Graupel is ice crystals encased in a layer of ice. When graupel hits the ground, you can sometimes see it bounce. Graupel is formed when small cores of ice crystals fall through a region of super-cooled (below freezing) liquid water droplets. These conditions are also favorable for airframe icing, thus detection of graupel implies potential icing conditions above the freezing level.
• Hail (or hail mixed with rain). Research is continuing to identify hail size from dual-polarization radar information.
3.4 Satellite.
3.4.1 Description.
Satellite is perhaps the single most important source of weather data worldwide, particularly over data-sparse regions, such as countries without organized weather data collection and the oceans.
The National Oceanic and Atmospheric Administration’s (NOAA) Geostationary Operational Environmental Satellite (GOES) imagery can be found on the AWC’s Web site, as well as all NWS WFO Web sites. Additional satellite imagery for Alaska can be found on the NWS Alaska Aviation Weather Unit (AAWU) Web site.
3.4.2 Imagery Types.
Three types of satellite imagery are commonly used: visible, infrared (IR), and water vapor. Visible imagery is only available during daylight hours. IR and water vapor imagery are available day or night.
3.4.2.1 Visible Imagery.
Visible imagery (see Figure 3-27, Visible Satellite Image—U.S. Example) displays reflected sunlight from the Earth’s surface, clouds, and particulate matter in the atmosphere. Visible satellite images, which look like black and white photographs, are derived from the satellite signals. Clouds usually appear white, while land and water surfaces appear in shades of gray or black.
11/14/16 Aviation Weather Services AC 00-45H The visible channel senses reflected solar radiation. Clouds, the Earth's atmosphere, and the Earth's surface all absorb and reflect incoming solar radiation. Since visible imagery is produced by reflected sunlight (radiation), it is only available during daylight.
Figure 3-27. Visible Satellite Image U.S. Example 12 15 18 21 24 27 30 33 36 42 45 48 51 54 57 60 63 66 69 02 05 28 81 84 87 90 03
3.4.2.1.1 Visible Image Data Legend.
The data legend (see Figure 3-28, Visible Satellite Data Legend) on a visible image displays albedo, or reflectance, expressed as a percentage. For example, an albedo of 72 means 72 percent of the sunlight that struck a feature was reflected back to space.
Figure 3-28. Visible Satellite Data Legend 6 9 12 15 18 21 27 30 33 36 39 42 45 48 51 54 57 60 63 66 69 72 75 28 81 84 87 00 93 The gray shades (values) represent albedo, or reflectance, expressed as a percentage.
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11/14/16 Aviation Weather Services AC 00-45H 3-59
3.4.2.2 Infrared (IR) Imagery.
IR images (see Figure 3-29, Infrared (Color) Satellite Image—U.S.
Example, and Figure 3-30, Unenhanced Infrared (Black and White) Satellite Image—U.S. Example) display temperatures of the Earth’s surface, clouds, and particulate matter. Generally speaking, the warmer an object, the more IR energy it emits. The satellite sensor measures this energy and calibrates it to temperature using a very simple physical relationship.
Clouds that are very high in the atmosphere are generally quite cold
(e.g., -50 ºC), whereas clouds very near the Earth’s surface are generally quite warm (e.g., +5 ºC). Likewise, land may be even warmer than the lower clouds
(e.g., +20 ºC). Those colder clouds emit much less infrared energy than the warmer clouds and the land emits more than those warm clouds.
The data measured by satellite is calibrated and colorized according to the temperature. If the temperature of the atmosphere decreases with height (which is typical), cloud-top temperature can be used to roughly determine which clouds are high-level and which are low-level.
When clouds are present, the temperature displayed on the IR images is that of the tops of clouds. When clouds are not present, the temperature is that of the ground or the ocean. A major advantage of the IR channel is that it can sense energy at night, so this imagery is available 24 hours per day.
Figure 3-29. Infrared (Color) Satellite Image—U.S. Example
The scale is in degrees Celsius. Blue/purple colors indicate colder temperatures, while orange/red colors indicate warmer temperatures.
11/14/16 Aviation Weather Services AC 00-45H 3-60 Figure 3-30. Unenhanced Infrared (Black and White) Satellite Image—U.S. Example
The scale is in degrees Celsius. Lighter gray shades indicate colder temperatures, while darker gray shades indicate warmer temperatures.
3.4.2.2.1 Infrared Image Data Legends.
The data legend (see Figure 3-31, Infrared (Color) Satellite Image Data Legend, and Figure 3-32, Unenhanced Infrared (Black and White) Satellite Image Data Legend) on an IR image is calibrated to temperature expressed in degrees Celsius. The legend may vary based on the satellite image provider.
Figure 3-31. Infrared (Color) Satellite Image Data Legend
The colors (values) represent temperature in degrees Celsius.
Figure 3-32. Unenhanced Infrared (Black and White) Satellite Image Data Legend
The gray shades (values) represent temperature in degrees Celsius.
3.4.2.3 Water Vapor Imagery.
The water vapor imagery (see Figure 3-33, Water Vapor Satellite Image— U.S. Example) displays the quantity of water vapor generally located in the middle and upper troposphere within the layer between 700 mb (approximately 10,000 ft MSL) and 200 mb (approximately flight level (FL)
390). The actual numbers displayed on the water vapor images correspond to temperature in degrees Celsius. No direct relationship exists between these values and the temperatures of clouds, unlike IR imagery. Water vapor
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.