Aviation Weather Services, Advisory Circular 00-45F, Change 2 (March 2009)
Figure 5-21. Unified Surface Analysis Chart Symbols
5.1.5.3 Analyses Isobars, pressure systems, and fronts are the most common analyses depicted on the surface analysis charts.
5.1.5.3.1 Isobars An isobar is a line of equal or constant pressure commonly used to analyze pressure patterns.
On a Surface Analysis Chart, isobars are solid lines usually spaced at intervals of 4 millibars (mb). Each isobar is labeled. For example, 1032 signifies 1032.0 mb and 992 signify 992.0 mb.
METAR/SPECI (Section 2.1) reports pressure in hectopascals. However, one millibar is equivalent to one hectopascal, so no conversion is required.
5.1.5.3.2 Pressure Systems On a Surface Analysis Chart, a High (H) is a maximum of atmospheric pressure while a Low (L) is a minimum of atmospheric pressure. Central pressure is the atmospheric pressure at the center of a High or Low -- the highest pressure in a High and the lowest pressure in a Low. The central pressure is denoted near each pressure center. Tropical storms, hurricanes, and typhoons (See Figure 5-21) are low-pressure systems with their names and central pressures denoted.
On a Surface Analysis Chart, a trough is an elongated area of relatively low atmospheric pressure, while a ridge is an elongated area of relatively high atmospheric pressure. Troughs are denoted by dashed lines and identified with the word TROF. Ridges are denoted by sawtoothed symbols. Ridges are rarely denoted on charts produced by the HPC.
5.1.5.3.3 Fronts The analysis shows positions and types of fronts by the symbols in Figure 5-21. The symbols on the front indicate the type of front and point in the direction toward which the front is moving.
Two short lines across a front indicate a change in front type.
AC 00-45F, CHG 2 Section 5: Graphical Observations and Derived Products Page 5-18
Aviation Weather Services, Advisory Circular 00-45F, Change 2 (March 2009)
5.1.6 AAWU Surface Charts The Alaskan Aviation Weather Unit (AAWU) in Anchorage, Alaska is responsible for the state of Alaska. The surface analysis chart is located at:
• http://aawu.arh.noaa.gov/surtace.php
5.1.6.1 Issuance The AAWU issues Surface Analysis Charts 4 times daily for the state of Alaska. The valid times are shown in Table 5-5.
Table 5-5. AAWU Surface Analysis Issuance Schedule Valid Time 00 06 12 18 (UTC)
5.1.6.2 Analysis Symbols The symbols (Figure 5-22) used on the Alaskan Surface Analysis Chart are similar to those found on the HPC Surface Analysis chart. However, since the Alaskan Surface Analysis chart is in black and white all of the symbols are black and white as well.
A Key to Symbols used on the AAWU Graphic Products
- Cold Front H 1032 - High Pressure Center Pressure in millibars
- Warm Front L 988 - Low Pressure Center
- Occluded Front Pressure in millibars
- Stationary Front
- Occasional or greater
- Trough Precipitation
- Ridge
- Fog - Freezing Rain * - Mixed Rain/Snow
- Haze - Freezing Drizzle - Rain Showers
- Smoke - Light Icing - Snow Showers ‡→ - Drifting Snow - Moderate Icing - Rain/Snow Showers
- Sandstorm - Severe Icing K - Thunderstorm
• - Drizzle * - Snow A - Light Turbulence
• - Rain › - Ice Crystals A - Moderate Turbulence A - Ice Pellets A - Severe Turbulence Figure 5-22. AAWU Surface Analysis Chart Symbols AC 00-45F, CHG 2 Section 5: Graphical Observations and Derived Products Page 5-19
Aviation Weather Services, Advisory Circular 00-45F, Change 2 (March 2009)
Figure 5-23. AAWU Alaskan Surface Chart Example
5.1.6.3 Station Plot Models Land, ship, buoy, and C-MAN stations are plotted on the chart to aid in analyzing and interpreting the surface weather features. These plotted observations are referred to as station models. Some stations may not be plotted due to space limitations. However, all reporting stations are used in the analysis.
Figures 5-24 and 5-25 show the most commonly used station plot models used in surface analysis charts.
Figure 5-24. AAWU Surface Analysis Chart Station Plot Model
AC 00-45F, CHG 2 Section 5: Graphical Observations and Derived Products Page 5-20
Aviation Weather Services, Advisory Circular 00-45F, Change 2 (March 2009)
Figure 5-25. AAWU Surface Analysis Chart Ship/buoy Plot Model
5.1.6.3.1 Station Identifier The format of the station identifier depends on the observing platform.
• Ship -- Typically 4 or 5 characters. If 5 characters, then the fifth will usually be a digit.
• Buoy -- Whether drifting or stationary, a buoy will have a 5-digit identifier. The first digit will always be a 4.
• C-MAN -- Stands for Coastal-Marine Automated Network, and are usually close to coastal areas. Their identifier will appear like a 5-character ship identifier, however the 4th character will identify off which state the platform is located.
• Land -- Land stations will always be 3 characters, making them easily distinguishable from ship, buoy, and C-MAN observations.
5.1.6.3.2 Temperature Air temperature is plotted in whole degrees Celsius on large-scale charts. Hourly surface charts may have temperatures using whole degrees Fahrenheit.
5.1.6.3.3 Dew Point Dew point temperature is plotted in whole degrees Celsius on large-scale charts. Hourly surface charts may have dew point temperatures using whole degrees Fahrenheit.
5.1.6.3.4 Wind Wind is plotted in increments of 5 knots (kts). The wind direction is in “true” degrees and is depicted by a stem (line) pointed in the direction from which the wind is blowing. Wind speed is determined by adding the values of the flags (50 kts), barbs (10kts), and half barbs (5 kts) found on the stem.
A single circle over the station with no wind symbol indicates a calm wind.
Figure 5-26 shows some sample wind symbols.
AC 00-45F, CHG 2 Section 5: Graphical Observations and Derived Products Page 5-21
Aviation Weather Services, Advisory Circular 00-45F, Change 2 (March 2009)
Figure 5-26: AAWU Surface Analysis Chart Wind Plotting Model
5.1.6.3.5 Ceiling Ceiling is plotted in hundreds of feet above ground level.
5.1.6.3.6 Visibility Surface visibility is plotted in whole statute miles.
5.1.6.3.7 Pressure Sea-level pressure is plotted in tenths of millibars (mb), with the first two digits omitted (generally a 10 or 9). For reference, 1013 mb is equivalent to 29.92 inches of mercury. Below are some sample conversions between plotted and complete sea-level pressure values:
410: 1041.0 mb 103: 1010.3 mb 987: 998.7 mb 872: 987.2 mb
5.1.6.3.8 Pressure Trend The pressure trend has two components, a number and symbol, to indicate how the sea-level pressure has changed during the past three hours. The number provides the 3-hour change in tenths of millibars, while the symbol provides a graphic illustration of how this change occurred.
Figure 5-27 shows the meanings of the pressure trend symbols.
AC 00-45F, CHG 2 Section 5: Graphical Observations and Derived Products Page 5-22
Aviation Weather Services, Advisory Circular 00-45F, Change 2 (March 2009) Continuously falling Continuously rising Falling, then steady Rising, then steady Falling before a lesser rise Falling before a greater rise 1 Rising before a greater fall Rising before a lesser fall Steady Figure 5-27. AAWU Surface Analysis Chart Pressure Trends
5.1.6.3.9 Sky Cover The approximate amount of sky cover can be determined by the circle at the center of the station plot. The amount the circle is filled reflects the amount of sky covered by clouds. Figure 5-28 shows the common cloud cover depictions:
Clear Scattered cloud:
(approximately 25% cloud cover (apply sinatly 50% cloud cover) (apst xinately 75% cloud cover) Overcast Sky Obscured Sky Cover Missing Figure 5-28. AAWU Surface Analysis Chart Sky Cover Symbols
5.1.6.3.10 Water Temperature Water temperature is plotted in whole degrees Fahrenheit.
AC 00-45F, CHG 2 Section 5: Graphical Observations and Derived Products Page 5-23
Aviation Weather Services, Advisory Circular 00-45F, Change 2 (March 2009)
5.1.6.3.11 Swell Information Swell direction, period, and height are represented in the surface observations by a 6-digit code.
The first two digits represent the swell direction, the middle digits describe the swell period (in seconds), and the last two digits are the swell's height (in half meters).
090703 09 - The swell direction is from 90 degrees (i.e. it is coming from due east).
07 - The period of the swell is 7 seconds.
03 - The height of the swell is 3 half meters.
271006 27 - The swell direction is from 270 degrees (due west).
10 - The period is 10 seconds.
06 - The height of the swell is 6 half meters.
5.1.6.3.12 Wave Information The period and height of waves are represented by a 5-digit code. The first digit is always 1.
The second and third digits describe the wave period (in seconds), and the final two digits give the wave height (in half meters). Below are two examples:
10603 1 - A group identifier. The first digit will always be 1.
06 - The wave period is 6 seconds.
03 - The wave height is 3 half meters.
10515 1 - The group identifier again.
05 - The wave period is 5 seconds.
15 - Wave height is 15 half meters.
5.1.6.4 Analyses Isobars, pressure systems, and fronts are the most common analyses depicted on surface analysis charts.
5.1.6.4.1 Isobars An isobar is a line of equal or constant pressure commonly used to analyze pressure patterns.
On a Surface Analysis Chart, isobars are solid lines usually spaced at intervals of 4 millibars (mb). Each isobar is labeled. For example, 1032 signifies 1032.0 mb and 992 signify 992.0 mb.
METAR/SPECI (Section 2.1) reports pressure in hectopascals. However, one millibar is equivalent to one hectopascal, so no conversion is required.
5.1.6.4.2 Pressure Systems On a Surface Analysis Chart, a High (H) is a maximum of atmosphere pressure, while a Low (L) is a minimum of atmospheric pressure. Central pressure is the atmospheric pressure at the center of a High or Low – the highest pressure in a High and the lowest pressure in a Low. The central pressure is denoted near each pressure center.
AC 00-45F, CHG 2 Section 5: Graphical Observations and Derived Products Page 5-24
Aviation Weather Services, Advisory Circular 00-45F, Change 2 (March 2009) A trough or an elongated area of relatively low atmospheric pressure is denoted by dashed lines and identified with the word TROF. A ridge or an elongated area of relatively high atmospheric pressure is denoted by saw-toothed symbols. Ridges are rarely denoted on charts produced by the AAWU.
5.1.6.4.3 Fronts The analysis shows positions and types of fronts by the symbols in Figure 5-22. The symbols on the front indicate the type of front and point in the direction toward which the front is moving.
Two short lines across a front indicate a change in front type.
AC 00-45F, CHG 2 Section 5: Graphical Observations and Derived Products Page 5-25
Aviation Weather Services, Advisory Circular 00-45F, Change 2 (March 2009)
5.2 Constant Pressure Charts Constant pressure charts are maps of selected conditions along specified constant pressure surfaces (pressure altitudes) and depict observed weather.
Constant pressure charts help to determine the three-dimensional aspect of depicted pressure systems. Each chart provides a plan-projection view at a specified pressure altitude.
Figure 5-29. Constant Pressure Chart Example
5.2.1 Issuance Constant pressure charts are issued twice per day from observed data obtained at 00Z and 12Z. Charts are available at the NWS Fax Chart web site at:
http://weather.noaa.gov/fax/barotrop.shtml.
5.2.2 Observational Data Observational data is plotted according to priority with some data deleted to prevent overlap.
The retention priority is:
AC 00-45F, CHG 2 Section 5: Graphical Observations and Derived Products Page 5-26
Aviation Weather Services, Advisory Circular 00-45F, Change 2 (March 2009) y Radiosonde observations (see Figure 5-30) y Weather reconnaissance aircraft observations y Aircraft observations on-time and on-level y Aircraft observations off-time or off-level y Satellite wind estimates
Many other data sources are used in the analysis but are not plotted. These include:
y Ships y Buoys y Tide gauges y Wind profilers y WSR-88D weather radar VAD wind profile s y Satellite sounder
Figure 5-30. U.S. Radiosonde Network
AC 00-45F, CHG 2 Section 5: Graphical Observations and Derived Products Page 5-27
Aviation Weather Services, Advisory Circular 00-45F, Change 2 (March 2009)
Table 5-6 Features of Constant Pressure Charts Height Plot Decoding Examples of Height Plotting Constant Pressure Chart Pressure Altitude (MSL) Isotachs Contour Interval (meters) Prefix Suffix Plotted Height (meters, MSL) 850MB 5,000 ft 1,500 m No 30 m 1 - 530 1,530 m 700MB 10,000 ft 3,000 m No 30 m 2 or 3* - 180 3,180 m 500MB 18,000 ft 5,500 m No 60 m - 0 582 5,820 m 300MB 30,000 ft 9,000 m Yes 120 m - 0 948 9,480 m 200MB 39,000 ft 12,000 m Yes 120 m 1 0 164 11,640 m Note: Pressure altitudes are rounded to the nearest 1,000 for feet and 500 for meters.
* Prefix a “2” or “3,” whichever brings the height closer to 3,000 meters.
Table 5-7. Constant Pressure Chart Plotting Models Radiosonde Reconnaissanc e Aircraft (RECCO) Aircraft Report (AIREP) Satellite Wind Estimate
SYMBOL MEANING
Wind plotted in standard notation. The stem points in the direction from which the wind is blowing plotted to 36 compass points, relative to true north.
Wind speed is denoted by a combination of flags (50 knots), barbs (10 knots), and half barbs (5 knots).
TT Temperature rounded to the nearest whole degree Celsius, with minus sign prefixed if negative.
hhh Height of the constant pressure surface in meters MSL. See the Table 5-8 to see how each chart abbreviates height.
PaPaPa Pressure altitude in hundreds of feet MSL.
DD Temperature-dew point spread (depression of the dew point temperature) rounded to the nearest whole degree Celsius.
When DD is less than or equal to 5 degrees Celsius, the station circle is darkened so a region of high moisture content will stand out. If DD is greater than 5 degrees Celsius, the station circle is not shaded. If the DD is too large to measure, an X is plotted. For RECCO reports, DD will be missing when the temperature is colder than -41oC.
R Reconnaissance aircraft type.
AC 00-45F, CHG 2 Section 5: Graphical Observations and Derived Products Page 5-28
Aviation Weather Services, Advisory Circular 00-45F, Change 2 (March 2009)
Table 5-8. Radiosonde Plotting Model Examples
Plotting Model 850MB 700MB 500MB 300MB 200MB
Wind* 190o/20 kt 190o/25 kt 270o/15 kt 240o/70 kt 250o/115 kt TT Temperature 20oC 8 oC -11 oC -36 oC -56 oC hhh Height 1,504 m 3,156 m 5,820 m 9,560 m 12,140 m DD Temperature -Dew Point Spread 5oC 6 oC Too dry to measure 21oC 7 oC hchc Height Change -10 m -20 m +10 m -50 m -20 m * Wind direction assumes that north is at the top of the page. Latitude and longitude lines, as well as other geographical references, must be used to determine actual compass direction.
5.2.3 Analyses All constant pressure charts contain analyses of height and temperature. Selected charts have an analysis of wind speed as well.
5.2.3.1 Height Heights are analyzed with contours. Contours are lines of constant height in MSL and are used to map height variations of constant pressure surfaces. They identify and characterize pressure systems on constant pressure charts.
Contours are drawn as solid lines labeled with 3-digit numbers in decameters. Intervals at which the contours are drawn at: 30 meters for the 850 mb and 700 mb charts, 60 meters for the 500-mb chart, and 120 meters for the 300-mb and 200-mb charts. The location of a High or Low is marked with a 9 symbol together with a larger H or L, and the central value in decameters printed under the center location.
Contour gradient is the amount of height change over a specified horizontal distance. Gradients identify slopes of constant pressure surfaces that fluctuate in altitude. Strong gradients are denoted by closely-spaced contours which identify steep slopes. Weak gradients are denoted by widely-spaced contours which identify shallow slopes.
Wind speeds are directly proportional to contour gradients. Faster wind speeds are associated with strong contour gradients and slower wind speeds are associated with weak contour gradients. In mountainous areas, winds are often variable on constant pressure charts with altitudes near terrain elevation due to friction.
5.2.3.2 Temperature Temperature is analyzed with isotherms which are lines of constant temperature. They are drawn as long dashed lines at intervals of 5° Celsius. They are given a two-digit label in whole degrees Celsius and are preceded with a + (positive) or – (negative) sign. The zero degree isotherm denotes the freezing level.
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