December 1999 6-3 Total sky obscuration and the vertical visibility into the obscuration is 300 feet, 1/4 visibility ¼, fog, bracket indicates fog was determined by an automated system 3 FEW sky coverage (no cloud height is indicated with FEW) 5 SCT sky coverage, cloud height 3,000’ AGL, visibility 5 miles, haze 30 3 BKN sky coverage, ceiling height 2,000’ AGL, visibility 3 miles, continuous rain 20 1 OVC sky coverage, ceiling height 500’ AGL, visibility 1 mile, intermittent snow 5 SCT sky coverage, cloud height 25,000’ AGL 250 .1½ BKN sky coverage, ceiling height 1,000’ AGL, visibility 1½ mile, thunderstorm with rain shower 10 Missing cloud (or sky cover) observation or partial obscuration Figure 6-2. Examples of Plotting on the Weather Depiction Chart.
December 1999 6-4
ANALYSIS The chart shows observed ceiling and visibility by categories as follows:
IFR - Ceiling less than 1,000 feet and/or visibility less than 3 miles; hatched area outlined by a smooth line.
MVFR (Marginal VFR) - Ceiling 1,000 to 3,000 feet inclusive and/or visibility 3 to 5 miles inclusive;
non-hatched area outlined by a smooth line.
VFR - No ceiling or ceiling greater than 3,000 feet and visibility greater than 5 miles; not outlined.
The three categories are also explained in the lower right portion of the chart for quick reference.
In addition, the chart shows fronts and troughs from the surface analysis for the preceding hour (with one exception being that fronts and troughs are omitted on the 10Z and 23Z charts). These features are depicted the same as the surface chart.
Because space on the chart is limited, only about half the METAR reports are plotted on the chart. The areas for each flight category are determined using all available reports whether or not they are plotted.
USING THE CHART The weather depiction chart is an ideal place to begin preparing for a weather briefing and flight planning.
From this chart, one can get a “bird’s eye” view of areas of favorable and adverse weather conditions for chart time. This chart may not completely represent the en route conditions because of variations in terrain and possible weather occurring between reporting stations. Due to the delay between data and transmission time, changes in the weather could occur. One should update the chart with current METAR reports. After initially sizing up the general weather picture, final flight planning must consider forecasts, progs, and the latest pilot, radar, and surface weather reports.
December 1999 01Z THU 08 JAN 1998 01+49 THAN CONTOURÉD WITHOUT SHADING... MVFR AREAS WITH CIG GREATE 1000 TO HESS THAN OR EQUAL TO_3059 FT ESSO HAN OK EQUAL ... VER AREAS MITH CIG GREATER THAN IFR WITH CIG L 1000 FT AND/OR VSBY LESS THAN 3 MI AND/OR VSBY GREATER THAN OR EQUAL TO,3 TO 3000 FT AND VSBY GREATER THAN 5 MI
THAN OR EQUAL T COLD WKCONTOUR
• O Figure 6-3. Weather Depiction Chart.
®½10 O] IS AUTOMATIC WÝ DE'S LE US DEPT OF COMMERCS WASHINGTON WEATHER DEPICTION® 08 JAN 1998" D018 TOTAL STATIONS PAVAILABLE FOR ANAL'USTS 1133 OUT OF 1363 +01Z THU
December 1999 7-1 Section 7
RADAR SUMMARY CHART A radar summary chart (Figure 7-1) is a computer-generated graphical display of a collection of automated radar weather reports (SDs). This chart displays areas of precipitation as well as information about type, intensity, configuration, coverage, echo top, and cell movement of precipitation. Severe weather watches are plotted if they are in effect when the chart is valid. The chart is available hourly with a valid time of H+35; i.e., 35 minutes past each hour. Figure 7-2 depicts the WSR-88D radar network from which the radar summary chart is developed.
ECHO (PRECIPITATION) TYPE The types of precipitation are indicated on the chart by symbols located adjacent to precipitation areas on the chart. Table 7-1 lists the symbols used to denote types of precipitation. Note that these symbols do not reflect the change to METAR. Since the input data for the radar summary chart are the automated SDs, the type of precipitation is determined by computer models and is limited to the ones listed in Table
7-1.
Table 7-1 Key to Radar Chart Symbols Used on Chart Symbol Meaning Symbol Meaning R Rain Cell movement to the northeastat 35 knots RW Rain shower LM Little movement S Snow Severe thunderstorm watch SW Snow shower number 999 T Thunderstorm Tornado watch number 210 NA Not available 8/10 or greater coverage NE No echoes in a line OM Out for maintenance Line of echoes 35 WS999 WT210 SLD
December 1999 7-2 450
INTENSITY The intensity is obtained from the amount of energy returned to the radar from the target and is indicated on the chart by contours. Six precipitation intensity levels are reduced into three contour intervals as indicated in Table 7-2. In Figure 7-1, over central Montana is an area of precipitation depicted by one contour. The intensity of the precipitation area would be light to possibly moderate. Whether there is moderate precipitation in the area cannot be determined. However, what can be said is that the maximum intensity is definitely below heavy. When determining intensity levels from this chart, it is recommended that the maximum possible intensity be used. To determine the actual maximum intensity level, the SD for that time period should be examined. It should also be noted that intensity is coded for frozen precipitation (i.e., snow or snow showers). This is due to the fact that the WSR-88D is much more powerful and sensitive than previous radars. Finally, it is very important to remember that the intensity trend is no longer coded on the radar summary chart.
Table 7-2 Precipitation Intensities Digit Precipitation Intensity Rainfall Rate in./hr. Stratiform Rainfall Rate in./hr. Convective 1 Light Less than 0.1 Less than 0.2 2 Moderate 0.1-0.5 0.2-1.1 3 Heavy 0.5-1.0 1.1-2.2 4 Very heavy 1.0-2.0 2.2-4.5 5 Intense 2.0-5.0 4.5-7.1 6 Extreme More than 5.0 More than 7.1 Highest precipitation top in area in hundreds of feet MSL (45,000 feet MSL).
ECHO CONFIGURATION AND COVERAGE The configuration is the arrangement of echoes. There are three designated arrangements: a LINE of echoes, an AREA of echoes, and an isolated CELL. (See Radar Weather Reports in Section 3 for definitions of the three configurations.)
Coverage is simply the area covered by echoes. All the hatched area inside the contours on the chart is considered to be covered by echoes. When the echoes are reported as a LINE, a line will be drawn through them on the chart. Where there is 8/10 coverage or more, the line is labeled as solid (SLD) at both ends. In the absence of this label, it can be assumed that there is less than 8/10 coverage. For example, in Figure 7-1, there is a solid line of thunderstorms with intense to extreme rain showers over central Georgia.
December 1999 7-3
ECHO TOPS Echo tops are obtained from both radar and, on occasion, satellite data and displayed for precipitation tops. Echo tops are the maximum heights of the precipitation in hundreds of feet MSL. They should be considered only as approximations because of radar wave propagation limitations. Tops are entered above a short line, with the top height displayed being the highest in the indicated area.
Examples:
220: maximum top 22,000 feet 500: Maximum top 50,000 feet It is assumed that all precipitation displayed on the chart is reaching the surface. Some examples of top measurements in Figure 7-1 include a top of 15,000 feet MSL over northeast Washington; 23,000 feet over north-central Texas; and 32,000 feet MSL in central Georgia.
ECHO MOVEMENT Individual cell movement is indicated by an arrow with the speed in knots entered as a number at the top of the arrow head. Little movement is identified by LM. For example, in Figure 7-1, the precipitation over north-central Texas is moving southwest at 8 knots. The precipitation in New England area is moving east-northeast at 25 knots. Line or area movement is no longer indicated on the chart.
SEVERE WEATHER WATCH AREAS Severe weather watch areas are outlined by heavy dashed lines, usually in the form of a large rectangular box. There are two types - tornado watches and severe thunderstorm watches. Referring to Figure 7-1 and Table 7-1, the type of watch and the watch number are enclosed in a small rectangle and positioned as closely as possible to the northeast corner of the watch box. For example, in Figure 7-1, the boxed “WS0005” in northeast Georgia and western South Carolina is a severe thunderstorm watch and is the 5th severe thunderstorm watch issued so far in the year. The watch number is also printed at the bottom of the chart (in Mexico) together with the issuance time and expiration time.
USING THE CHART The radar summary chart aids in preflight planning by identifying general areas and movement of precipitation and/or thunderstorms. This chart displays drops or ice particles of precipitation size only; it does not display clouds and fog. Therefore, the absence of echoes does not guarantee clear weather, and cloud tops will most likely be higher than the tops of the precipitation echoes detected by radar. The chart must be used in conjunction with other charts, reports, and forecasts.
Examine chart notations carefully. Always determine location and movement of echoes. If echoes are anticipated near the planned route, take special note of echo intensity. Be sure to examine the chart for missing radar reports before assuming “no echoes present.” For example, the Rapid City (RAP) radar report in western South Dakota is shown as “not available (NA).”
Suppose the planned flight route goes through an area of widely scattered thunderstorms in which no increase in area is anticipated. If these storms are separated by good VFR weather, they can be visually sighted and circumnavigated. However, widespread cloudiness may conceal the thunderstorms. To avoid these embedded thunderstorms, either use airborne radar or detour the area.
Remember that the radar summary chart is for preflight planning only and should be updated by current WSR-88D images and hourly reports. Once airborne, the pilot must evade individual storms by inflight
December 1999 7-4 observations. This can be done by using visual sighting or airborne radar as well as by requesting radar echo information from Automated Flight Service Station (AFSS) Flight Watch. The AFSS Flight Watch has access to current WSR-88D imagery.
There can be an interpretation problem concerning an area of precipitation that is reported by more than one radar site. For example, station A may report RW with cell movement toward the northeast at 10 knots. For the same area, station B may be reporting TRW with cell movement toward the northeast at 30 knots. This difference in reports may be due to a different perspective and distance of the radar site from the area of echoes. The area may be moving away from station A and approaching station B. The rule of thumb is to use that plotted data associated with the area that presents the greatest hazard to aviation. In this case, the station B report would be used.
5/98 ŠDOz Ň¿Âu 08 JAN 1998 RADAR SUMMARY 340 AS0005 20đ NA US DEPT OF COMMERCEN NOÀA/NAS/NMC WASHINGTON .. REPOR RU -02352 THU° 30 MINUTES SLOW.. EQUIP PBLMS. TRC 76382 AND VẸR AT GSO. „DELETED WINDS AT MZT 160 200 NE 460 NE Figure 7-1. Radar Summary Chart.
RUNNING ABOUT KNEAS/SD//NCEP/NCO AND ABV AT 250 180 WAL.. HAD NE NE__230 250 200. NE 0802407_002 ETA TO SOONBAMA. UNR. DELETED AGTS 100MB" NE NE NA 100 NE NA 200 +55 RAOBS. NA SUU< NE NE NE WEATHER WAICH AREAS 0300Z 06002 0800Z H770 RADAR QUAKY GGH. NA NE NE START MS0004 20331 US0006 02167 NE NE -AREA PRECIP TYPE AND CHANGE OF INTENSITY. MSO005 NO DO CAN DUMP TIME 0245Z START 0252Z. LOCAL NING HAS ME OBS ARX -NE NA /NE NE NE NE NE NE -NE NA NE SHADING--ECHO AREAS CONTOURS AT INTENSITIES 1-3-5 TOPS 999--BASES 999--100S QF PREFIX-A--ACFT RPTD TOP- MUMT--AREAS+LINES--PENNANT'S MUMT--CELLS--ARRONS WITH SPEED * IS NEW OR INCRG; - IS DCRG. 1752 LEGEND. bỏ54
December 1999 COMPLETED WSR-88D INSTALLATIONS March 17, 1998 139 132 ·80 128 *88 11 + 176 56153 189 Ťž5 +64 27+ 160 Ť29 28+ 40 1145714 58159 +127 7T + 18 - 50+ 4152+ 25 L 162 +100 ~52 163+ 90 20 65_ 70+19 140 + 143 +111 45 · 7102+103 105 150 16č ALASKA ≤+ 165_ 43/126 AZORES SOUTH KOREA 50/121 {130 . & 163 \162
OKINAWA 152
GUAM HAWAII
PUERTO RICO KAUAI
MQLOKAI 60/116 HAWAII
OPERATIONAL SUPPORT FACILITY WSR-88D NORMAN, OKLAHOMA Figure 7-2. WSR-88D Radar Network.
7-6
December 1999 8-1 Section 8
CONSTANT PRESSURE ANALYSIS CHARTS Weather information for computer generated constant pressure charts is observed primarily by balloonascending radiosonde packages. Each package consists of weather instruments and a radio transmitter.
During ascent instrument data are continuously transmitted to the observation station. Radiosondes are released at selected observational sites across the USA at 00Z and 12Z. The data collected from the radiosondes are used to prepare constant pressure charts twice a day.
Constant pressure charts are prepared for selected values of pressure and present weather information at various altitudes. The standard charts prepared are the 850 mb (hPa), 700 mb (hPa), 500 mb (hPa), 300 mb (hPa), 250 mb (hPa), and 200 mb (hPa) charts. Charts with higher pressures present information at lower altitudes, and charts with lower pressures present information at higher altitudes. Table 8-1 lists the general altitude (pressure altitude) of each constant pressure chart.
PLOTTED DATA Data from each observation station are plotted around a station circle on each constant pressure chart.
The circle identifies the station position. The data plotted on each chart are temperature, temperaturedew point spread, wind, height of the surface above sea level, and height change of the surface over the previous 12-hour period. The temperature and spread are in degrees Celsius, wind direction is relative to true north, wind speed is in knots, and height and height change are in meters. The station circle is shaded black when the spread is 5 degrees or less (moist atmosphere), and open when spread is more than 5 degrees (dry atmosphere). Figure 8-1 illustrates a station model of the radiosonde data plot. Table 8-2 gives station data plot examples for each constant pressure chart.
Aircraft and satellite observations are also used as information sources for constant pressure charts. A square is used to identify an aircraft reporting position. Data plotted are the flight level of the aircraft in hundreds of feet, temperature, wind, and time to the nearest hour UTC. A star is used to identify a satellite reporting position. Satellite information is determined by identifying cloud drift and height of cloud tops. Data plotted are the flight level in hundreds of feet, time to the nearest hour UTC, and wind.
Aircraft and satellite data are plotted on the constant pressure chart closest to their reporting altitudes.
Aircraft and satellite information are particularly useful over sparse radiosonde data areas.
ANALYSIS All constant pressure charts contain analyses of height and temperature variations. Also, selected charts have analyses of wind speed variations. Variations of height are analyzed by contours, variations of temperature by isotherms, and variations of wind speed by isotachs.
CONTOURS Contours are lines of constant height, in meters, which are referenced to mean sea level. Contours are used to map the height variations of surfaces that fluctuate in altitude. They identify and characterize pressure systems on constant pressure charts.
December 1999 8-2 Contours are drawn as solid lines on constant pressure charts and are identified by a three-digit code located on each contour. To determine the contour height value, affix "zero" to the end of the code. For example, a contour with a "315" code on the 700 mb/hPa chart identifies the contour value as 3,150 meters. Also, affix a "one" in front of the code on all 200 mb/hPa contours and on 250 mb/hPa contours when the code begins with zero. For example, a contour with a "044" code on a 250 mb/hPa chart identifies the contour value as 10,440 meters.
The contour interval is the height difference between analyzed contours. A standard contour interval is used for each chart. The contour intervals are 30 meters for the 850 and 700 mb (hPa) charts, 60 meters for the 500 mb (hPa) chart, and 120 meters for the 300, 250, and 200 mb (hPa) charts.
The contour gradient is the distance between analyzed contours. Contour gradients identify slopes of surfaces that fluctuate in altitude. Strong gradients are closely spaced contours and identify steep slopes.
Weak gradients are widely spaced contours and identify shallow slopes.
The contour analysis displays height patterns. Common types of patterns are lows, highs, troughs, and ridges. Contours have curvature for each of these patterns. Contour patterns can be further characterized by size and intensity. Size represents the breadth of a system. Sizes can range from large to small. A large pattern is generally more than 1,000 miles across, and a small pattern is less than 1,000 miles across. Intensities can range from strong to weak. Stronger systems are depicted by contours with stronger gradients and sharper curvatures. Weaker systems are depicted by contours with weaker gradients and weaker curvatures. For example, a chart may have a large, weak high, or a small, strong low.
Contour patterns on constant pressure charts can be interpreted the same as isobar patterns on the surface chart. For example, an area of low height is the same as an area of low pressure.
Winds respond to contour patterns and gradients. Wind directions parallel contours. In the Northern Hemisphere, when looking downwind, contours with relatively lower heights are to the left and contours with relatively higher heights are to the right. Thus, winds flow counterclockwise (cyclonically) around lows and clockwise (anticyclonically) around highs. (In the Southern Hemisphere these directions are reversed.) Winds that rotate are termed circulations. Wind speeds are faster with stronger gradients and slower with weaker gradients. In mountainous areas, winds are variable on pressure charts with altitudes at or below mountain crests. Contours have the effect of "channeling" the wind.
ISOTHERMS Isotherms are lines of constant temperature. An isotherm separates colder air from warmer air.
Isotherms are used to map temperature variations over a surface.
Isotherms are drawn as bold, dashed lines on constant pressure charts. Isotherm values are identified by a two-digit block on each line. The two digits are prefaced by "+" for above-freezing values as well as the zero isotherm and "-" for below-freezing values. Isotherms are drawn at 5-degree intervals on each chart. The zero isotherm separates above-freezing and below-freezing temperatures.
Isotherm gradients identify the magnitude of temperature variations. Strong gradients are closely spaced isotherms and identify large temperature variations. Weak gradients are loosely spaced isotherms and identify small temperature variations.
December 1999 8-3
ISOTACHS Isotachs are lines of constant wind speed. Isotachs separate higher wind speeds from lower wind speeds.
Isotachs are used to map wind speed variations over a surface. Isotachs are analyzed on the 300, 250, and 200 mb (hPa) charts.
Isotachs are drawn as short, fine dashed lines. Isotach values are identified by a two- or three-digit number followed by a "K" located on each line. Isotachs are drawn at 20-knot intervals and begin at 10 knots.
Isotach gradients identify the magnitude of wind speed variations. Strong gradients are closely spaced isotachs and identify large wind speed variations. Weak gradients are loosely spaced isotachs and identify small wind speed variations.
Zones of very strong winds are highlighted by hatches. Hatched and unhatched areas are alternated at 40-knot intervals beginning with 70 knots. Areas between the 70- and 110-knot isotachs are hatched.
Areas between the 110- and 150-knot isotachs are unhatched. This alternating pattern is continued until the strongest winds on the chart are highlighted. Highlighted isotachs assist in the identification of jet streams.
THREE-DIMENSIONAL ASPECTS It is important to assess weather in both the horizontal and vertical dimensions. This not only applies to clouds, precipitation, and other significant conditions, but also pressure systems and winds. The characteristics of pressure systems vary horizontally and vertically in the atmosphere.
The horizontal distribution of pressure systems is depicted by the constant pressure charts and the surface chart (Section 5.) Pressure systems appear on each pressure chart as pressure patterns. Pressure charts identify and characterize pressure systems by their location, type, size, and intensity.
The vertical distribution of pressure systems must be determined by comparing pressure patterns on vertically adjacent pressure charts. For example, compare the surface chart with the 850 mb/hPa chart, 850 mb/hPa with 700 mb/hPa, and so forth. Changes of pressure patterns with height can be in the form of position, type, size, or intensity.
The three-dimensional assessment of pressure systems infers the assessment of the three-dimensional variations of wind.
USING THE CHARTS Constant pressure charts are used to provide an overview of selected observed en route flying conditions.
Use all pressure charts for a general overview of conditions.
Select the chart closest to the desired flight altitude for assessment of en route conditions. Review the winds along the route. Consider their direction and speed. For high altitude flights, identify jet stream positions. Note whether pressure patterns cause significant wind shifts or speed changes. Determine if these winds will be favorable or unfavorable (tailwind, headwind, crosswind.) Consider vertically adjacent charts and determine if a higher or lower altitude would have a more desirable en route wind.
Interpolate winds between charts for flights between chart levels. Review other conditions along the