11/14/16 Aviation Weather Services AC 00-45H 4-5 Figure 4-6. NWS Surface Analysis Chart for Aviation Interests Station Plot Model
4.1.1.3.1 Station Identifier.
The format of the station identifier depends on the observing platform.
• Ship: Typically four or five characters. If five characters, then the fifth will usually be a digit.
• Buoy: Whether drifting or stationary, a buoy will have a five-digit identifier. The first digit will always be a 4.
• C-MAN: Usually located close to coastal areas. Their identifier will appear like a five-character ship identifier; however, the fourth character will identify off which state the platform is located.
• Land: Land stations will always be three characters, making them easily distinguishable from ship, buoy, and C-MAN observations.
4.1.1.3.2 Temperature.
The air temperature is plotted in whole degrees Fahrenheit.
4.1.1.3.3 Dewpoint.
The dewpoint temperature is plotted in whole degrees Fahrenheit.
4.1.1.3.4 Weather.
A weather symbol is plotted if, at the time of observation, precipitation is either occurring or a condition exists causing reduced visibility.
Figure 4-7, NWS Surface Analysis Chart Common Weather Symbols, contains a list of the most common weather symbols.
11/14/16 Aviation Weather Services AC 00-45H 4-6 Figure 4-7. NWS Surface Analysis Chart Common Weather Symbols
4.1.1.3.5 Wind. Wind is plotted in increments of 5 knots (kts). The wind direction is referenced to true north 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 (10 kts) and half-barbs (5 kts) found on the stem.
If the wind is calm at the time of observation, only a single circle over the station is depicted.
Figure 4-8, NWS Surface Analysis Chart Wind Plotting Model, includes some sample wind symbols.
11/14/16 Aviation Weather Services AC 00-45H 4-7 Figure 4-8. NWS Surface Analysis Chart Wind Plotting Model
4.1.1.3.6 Ceiling.
Ceiling is plotted in hundreds of feet above ground level (AGL).
4.1.1.3.7 Visibility.
Surface visibility is plotted in whole statute miles (sm).
4.1.1.3.8 Pressure.
Sea-level pressure is plotted in tenths of millibars (mb), with the first two digits (generally 10 or 9) omitted. For reference, 1013 mb is equivalent to
29.92 inches (in) 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
4.1.1.3.9 Pressure Trend.
The pressure trend has two components, a number and a symbol, to indicate how the sea-level pressure has changed during the past 3 hours. The number provides the 3-hour change in tenths of millibars, while the symbol provides a graphic illustration of how this change occurred.
11/14/16 Aviation Weather Services AC 00-45H 4-8 Figure 4-9, NWS Surface Analysis Chart Pressure Trends, contains the meanings of the pressure trend symbols.
Figure 4-9. NWS Surface Analysis Chart Pressure Trends
4.1.1.3.10 Sky Cover.
The approximate amount of sky cover can be determined by the circle at the center of the station plot. The amount that the circle is filled reflects the amount of sky covered by clouds. Figure 4-10, NWS Surface Analysis Chart Sky Cover Symbols, contains the common cloud cover depictions.
11/14/16 Aviation Weather Services AC 00-45H 4-9 Figure 4-10. NWS Surface Analysis Chart Sky Cover Symbols
4.1.1.3.11 Water Temperature.
Water temperature is plotted in whole degrees Fahrenheit.
4.1.1.3.12 Swell Information.
Swell direction, period, and height are represented in the surface observations by a six-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 (m) 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
4.1.1.3.13 Wave Information.
Period and height of waves are represented by a five-digit code. The first digit
11/14/16 Aviation Weather Services AC 00-45H 4-10 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).
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: A group identifier 05: The wave period is 5 seconds 15: The wave height is 15 half meters In some charts by the OPC, only the wave height (in feet) is plotted.
4.1.1.4 Analyses.
Isobars, pressure systems, and fronts are the most common analyses depicted on surface analysis charts.
4.1.1.4.1 Isobars.
An isobar is a line of equal or constant pressure commonly used in the analysis of pressure patterns.
On a surface analysis chart, isobars are solid lines usually spaced at intervals of 4 mb. Each isobar is labeled. For example, 1032 signifies 1,032.0 mb and 992 signifies 992.0 mb.
4.1.1.4.2 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 located at the center of a High or Low. In general, the central pressure is the highest pressure in the center of a High and the lowest pressure at the center of a Low. The central pressure is denoted near each pressure center.
A trough or an elongated area of low atmospheric pressure is denoted by dashed lines and identified with the word TROF. A ridge or an elongated area of high atmospheric pressure is denoted by saw-toothed symbols. Ridges are rarely denoted on charts produced by the WPC.
Tropical storms, hurricanes, and typhoons are low-pressure systems with their names and central pressures denoted.
11/14/16 Aviation Weather Services AC 00-45H 4-11
4.1.1.4.3 Fronts.
NWS surface analysis charts positions and types of fronts are shown by symbols in Figure 4-3. 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.
4.1.2 Unified Surface Analysis Chart.
The NWS Unified Surface Analysis Chart is a surface analysis product produced collectively and collaboratively by NWS’s WPC, the OPC, the National Hurricane Center (NHC), and Weather Forecast Office (WFO) Honolulu. The chart contains an analysis of isobars, pressure systems, and fronts.
This chart is available on the OPC Web site as well as the AWC. Users can zoom in by clicking an area on the map (see Figure 4-11, Unified Surface Analysis Chart Example) to enlarge (see Figure 4-12, Unified Surface Analysis Chart Example) and show station plot models (see paragraph 4.1.1.3).
4.1.2.1 Issuance.
The Unified Surface Analysis Chart is issued four times daily for valid times 00, 06, 12, and 18 UTC.
Figure 4-11. Unified Surface Analysis Chart Example
11/14/16 Aviation Weather Services AC 00-45H 4-12 Figure 4-12. Unified Surface Analysis Chart Example
4.1.2.2 Analysis Symbols.
Unified Surface Analysis Charts use the symbols shown in Figure 4-3.
4.1.3 AAWU Surface Chart.
The NWS’s Unified Surface Analysis chart covers the Alaskan area. The AAWU also provides a fixed area image of the Unified Surface Analysis Chart centered over Alaska (see Figure 4-13, Unified Surface Analysis Chart Example With Fixed Area Coverage Over Alaska). This chart is available from AAWU’s Web site.
11/14/16 Aviation Weather Services AC 00-45H 4-13 Figure 4-13. Unified Surface Analysis Chart Example With Fixed Area Coverage Over Alaska
4.2 Ceiling and Visibility.
4.2.1 Ceiling and Visibility Analysis (CVA).
The CVA product provides a real-time analysis of current observed and estimated ceiling and visibility conditions across the continental United States (CONUS). The product is primarily intended to help the general aviation pilot (particularly the Visual Flight Rules (VFR)-only pilot) avoid instrument flight rules (IFR) conditions. However, CVA’s overview of ceiling and visibility conditions can be useful to others involved in flight planning or weather briefing.
4.2.1.1 Issuance.
CVA is issued every 5 minutes and is available through http://www.aviationweather.gov.
4.2.1.2 Content.
CVA presents information via full-CONUS graphic and 18 regional graphics.
Each graphic is rendered on a horizontal grid of 5 km resolution and shows viewer-selectable representations of ceiling height (AGL), surface visibility in statute miles (sm) and flight category designation.
Each regional display includes an overlay of station plots showing the current ceiling and visibility observations reported at selected Aviation Routine Weather Report (METAR) stations. To avoid overcrowding, the overlay
11/14/16 Aviation Weather Services AC 00-45H 4-14 shows only a subset of the total number of stations available at the CONUS scale view. Additional stations are displayed when zooming in.
4.2.1.3 CVA Ceiling Analysis.
The CVA Ceiling Analysis uses observed and estimated ceiling heights as follows:
• At display points corresponding to METAR locations, CVA uses the ceiling values observed by the nearest METAR.
• At display points between METAR locations (where there are no direct observations), CVA uses estimated ceiling values derived by adjusting the ceiling observation from the nearest METAR to take into account intervening changes in terrain height. When corresponding Geostationary Operational Environmental Satellite (GOES) observations yield an unambiguous indication that no cloud is present, adjusted ceiling observations are reset to indicate clear conditions.
• At display points where observed or estimated ceiling height is less than 200 feet (ft) AGL, CVA reports “Possible Terrain Obstruction.”
The CVA Ceiling Analysis graphic is rendered as follows:
• Ceiling less than 1,000 ft AGL: pale yellow.
• Ceiling less than 200 ft. AGL and Possible Terrain Obstruction: orange.
Figure 4-14, CONUS Display of CVA Ceiling Analysis, shows a CONUS-scale CVA Ceiling Analysis. Figure 4-15, Regional Display of CVA Ceiling Analysis, shows a regional-scale Ceiling Analysis.
11/14/16 Aviation Weather Services AC 00-45H Figure 4-14. CONUS Display of CVA Ceiling Analysis Ceiling and Visibility Display at 1340 UTC 19 Jun 2015
52.118,-128.075 Map: Plot Options: Data Options: Overlays:
• Light • Hover 1340 UTC 19 Jun • Time 1 Highways
• Dark FitCat • Plottype Top Jetroutes
• Simple JARTCC/FIR Bounds FItCat Possible Terrain Obscuration JER FIt Cat: OVFR •MVFRO IFR OLIFR 4-15
11/14/16 Aviation Weather Services AC 00-45H Figure 4-15. Regional Display of CVA Ceiling Analysis Ceiling and Visibility Display at 1325 UTC 19 Jun 2015
41.794. -101.71 Map: Plot Options: Data Options: Overlays:
• Light • Hover 1325 UTC 19 Jun • Time • Highways © Dark Visib • Plottype • Top Jetroutes
• Simple • ARTCC/FIR Bounds VIS FIt Cat: O VFROMVFROIFROLIFR Less than 3 miles
4.2.1.4 CVA Visibility Analysis.
The CVA Visibility Analysis (shown as a regional view in Figure 4-16, Regional Display of CVA Visibility Analysis) reports the observed visibility at the nearest METAR. The graphic is rendered as follows:
• Surface visibility less than 3 sm: pale yellow.
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