A surface analysis chart is one of the most information-dense weather products available to pilots and forecasters. Issued by the Weather Prediction Center (WPC) eight times daily at 00, 03, 06, 09, 12, 15, 18, and 21 UTC, it portrays sea-level pressure patterns through isobars, marks the positions of highs, lows, ridges, troughs, fronts, drylines, outflow boundaries, and sea-breeze fronts, and overlays hundreds of individual surface observations in the form of station plot models. Understanding how to read those plots is an essential skill for any pilot who wants to extract real meteorological meaning from a weather chart rather than simply note where a cold front is drawn.
Station models are plotted for land stations, ships, buoys, and Coastal-Marine Automated Network (C-MAN) platforms. Space limitations on the chart may prevent every reporting station from being plotted visually, but every observation is still used in the underlying analysis. The result is that each small cluster of numbers and symbols around a central circle tells a complete, standardized story about current conditions at that exact point on the Earth's surface.
Decoding the Station Identifier
Before reading any data values, it helps to know which type of platform made the observation, because the identifier format varies by platform type:
- Land stations always use a three-character identifier, making them instantly distinguishable from maritime platforms.
- Ships use four or five characters; if five characters are present, the fifth is typically a digit.
- Buoys (drifting or stationary) use a five-digit identifier whose first digit is always 4.
- C-MAN platforms are usually near coastal areas and look like five-character ship identifiers, but the fourth character identifies the state off whose coast the platform is located.
How the Station Plot Model Works
Think of the station plot model as a clock face with a filled circle at the center. Each data element occupies a specific, standardized position around that circle. Once you memorize the layout, reading any station plot becomes systematic and fast.
Sky Cover
The central circle itself conveys cloud coverage. The fraction of the circle that is filled (shaded) represents the approximate fraction of the sky covered by clouds. A completely empty circle means clear skies; a fully filled circle means overcast or sky obscured. Intermediate fills represent scattered and broken layers. This one symbol alone tells you at a glance whether the sky is open or closed.
Wind
A line called the stem extends from the central circle in the direction from which the wind is blowing, referenced to true north. Attached to the outer end of the stem are barbs and flags that encode wind speed in increments of 5 knots:
- A full barb (a long tick mark) = 10 knots.
- A half barb (a short tick mark) = 5 knots.
- A pennant or flag (a filled triangle) = 50 knots.
To read the speed, simply add up the values. A stem with one pennant, one full barb, and one half barb indicates 50 + 10 + 5 = 65 knots. If the wind is calm, no stem is drawn; instead, a second circle is drawn around the station circle. This calm-wind depiction is an important distinction—do not confuse a calm station with a missing observation.
Temperature and Dewpoint
Air temperature is plotted in whole degrees Fahrenheit in the upper-left position next to the station circle. Dewpoint temperature is plotted in whole degrees Fahrenheit in the lower-left position. The spread between temperature and dewpoint is a quick proxy for relative humidity and fog potential: when the two values are close together (within 2–4 °F), the air is near saturation and low visibility or fog is a real concern.
Pressure and Pressure Trend
Sea-level pressure appears in the upper-right position, encoded in a compact three-digit format that requires a simple decoding step. The leading digits—generally 9 or 10—are dropped, and only the last three digits plus a decimal place are shown. To reconstruct the full pressure:
- If the three-digit value is 500 or greater, prepend 9 and add a decimal before the last digit. Example: 872 → 987.2 mb.
- If the three-digit value is less than 500, prepend 10 and add a decimal before the last digit. Example: 103 → 1,010.3 mb; 410 → 1,041.0 mb.
The standard reference is 1,013 mb = 29.92 inHg. Directly below the pressure value is the pressure trend, which has two parts: a number representing the change in tenths of millibars over the past three hours, and a symbol showing the character of that change—steadily rising, steadily falling, rising then falling, falling then rising, and so on. A rapid three-hour pressure fall of 6 mb or more is a classic indicator of an approaching intense low or frontal system and is a significant preflight red flag.
Ceiling
Ceiling height is plotted in hundreds of feet AGL. A plotted value of 15 means a 1,500-foot ceiling. This is especially critical for instrument flight rules (IFR) planning—compare the ceiling value to IFR minimums for the destination and alternate airports.
Visibility
Surface visibility is plotted in whole statute miles. A value of 3 means 3 statute miles visibility. Values below 3 statute miles are particularly significant for IFR operations and should always be cross-checked against applicable weather symbols on the same plot.
Weather Symbols
A weather symbol appears on the plot only when precipitation is occurring at the time of observation or when a condition reducing visibility exists. Common symbols include rain, drizzle, snow, fog, and thunderstorms. No symbol means the sky is not producing precipitation or obscuration at observation time—it does not necessarily mean the weather is benign overall.
Ship and Buoy Additions: Water Temperature, Swells, and Waves
Maritime station plots include additional data elements not present on land plots. Water temperature is plotted in whole degrees Fahrenheit. Swell information appears as a six-digit code: the first two digits give swell direction (in tens of degrees from true north), the middle two digits give swell period in seconds, and the final two digits give swell height in half-meters. For example, 090703 means swells from 090° (due east), 7-second period, 3 half-meters (1.5 m) high. Wave information uses a five-digit code starting with the digit 1 (a group identifier), followed by two digits for wave period in seconds and two digits for wave height in half-meters.
Aviation-Specific Station Plot Model
The WPC produces a surface analysis chart tailored specifically to the aviation community. Its station plot model retains the core elements—wind, sky cover, temperature, dewpoint, pressure—and prominently features ceiling and visibility, the two parameters most directly relevant to flight safety under instrument conditions. Pilots using the aviation-specific chart will find the layout slightly different from the standard NWS model, so it is worth reviewing Figure 25-12 in FAA-H-8083-28B to avoid misreading a data field.
Why It Matters Operationally
Reading a surface chart armed with station plot knowledge lets a pilot do far more than locate frontal boundaries. By scanning several station plots across a region, you can personally verify the analysis: do winds on either side of a cold front actually shift direction as expected? Is the dewpoint rising ahead of a warm front? Is pressure falling faster at one station than another, suggesting an accelerating low? These are the kinds of cross-checks that build genuine situational awareness rather than passive acceptance of a computer-drawn product. The surface analysis chart is also one of the oldest and most validated weather products in aviation; it synthesizes raw observations with numerical analysis to give you the complete surface picture at a single glance.
Key Numbers and Rules
- WPC issuance: 8 times daily, valid at 00, 03, 06, 09, 12, 15, 18, 21 UTC.
- Wind barb increments: half barb = 5 kt, full barb = 10 kt, pennant = 50 kt.
- Wind direction: referenced to true north; stem points FROM the wind.
- Temperature and dewpoint: whole degrees Fahrenheit.
- Ceiling: hundreds of feet AGL.
- Visibility: whole statute miles.
- Pressure: tenths of millibars, leading 9 or 10 omitted; decode by prepending 9 (≥500) or 10 (<500).
- Pressure trend: change in tenths of mb over the past 3 hours.
- Land station identifiers: always 3 characters; buoy identifiers: always 5 digits starting with 4.
- Calm wind: depicted as a second circle around the station circle—no stem.
Common Test Traps
- Pressure decoding direction: Students often prepend 10 to all three-digit pressure codes. Remember that a three-digit value of 500 or greater means prepend 9, not 10. 872 is 987.2 mb, not 1,087.2 mb.
- Wind direction convention: The stem shows the direction the wind is blowing from, not toward. A stem pointing to the south means winds from the south—a southerly wind.
- Calm wind symbol: A second circle around the station circle means calm—not missing data or a special sky cover symbol.
- Ceiling units vs. visibility units: Ceiling is in hundreds of feet AGL; visibility is in whole statute miles. Mixing the two (e.g., reading a ceiling of 15 as 15 feet rather than 1,500 feet) is a common mistake.
- Weather symbol absence: No weather symbol does not mean good weather—it only means no precipitation or obscuration was occurring at observation time. Clouds, wind, and low pressure may still be significant.