Chapter 25, Analysis 25-8
A surface chart (also called surface map or sea level pressure chart) is an analyzed chart of surface weather
observations. Essentially, a surface chart shows the distribution of sea level pressure (lines of equal pressure
are isobars ). Hence, the surface chart is an isobaric analysis showing identifiable, organized pressure
patterns. The chart also includes the positions of highs, lows, ridges, and troughs, and the location and
character of fronts and various boundaries, such as drylines, outflow bo undaries, and sea breeze fronts .
Although the pressure is referred to as MSL, all other elements on this chart are presented as they occur at
the surface point of observation. A chart in this general form is the one commonly referred to as the weather
map. See Figure 25-5 for a schematic example of a surface chart.
Figure 25-5. Schematic of Surface Chart Pressure Patterns
Figure 25-7 is one example of several surface analysis products available on the WPC’s website.
Some of the WPC’s surface analysis charts are combined with radar o r satellite imagery (see Figure 25-8
and Figure 25-9) as well as having different background features (e.g., terrain).
25.2.3.1 Issuance
The WPC issues surface analysis charts for North America eight times daily, valid at 00, 03, 06, 09, 12, 15,
18, and 21 UTC.
Chapter 25, Analysis 25-9
25.2.3.2 Analysis Symbols
Figure 25-6 shows analysis symbols used on NWS surface analysis charts.
Figure 25-6. NWS Surface Analysis Chart Symbols
25.2.3.3 Examples
Figure 25-7. Example of a Surface Chart with Surface Observations
Chapter 25, Analysis 25-10
Figure 25-8. Surface Analysis with Radar Composite Example
Figure 25-9. Surface Analysis with Satellite Composite Example
Chapter 25, Analysis 25-11
25.2.3.4 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.
Figure 25-10 and Figure 25-11 contain the most commonly used station plot models used in surface analysis
charts.
Figure 25-10. NWS Surface Analysis Chart Station Plot Model
Figure 25-11. NWS Surface Analysis Chart Ship/Buoy Plot Model
Chapter 25, Analysis 25-12
The WPC also produces surface analysis charts specifically for the aviation community. Figure 25-12
contains the station plot model for these charts.
Figure 25-12. NWS Surface Analysis Chart for Aviation Interests Station Plot Model
25.2.3.4.1 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.
25.2.3.4.2 Temperature
The air temperature is plotted in whole degrees Fahrenheit.
25.2.3.4.3 Dewpoint
The dewpoint temperature is plotted in whole degrees Fahrenheit.
25.2.3.4.4 Weather
A weather symbol is plotted if, at the time of observation, precipitation is either occurring or a condition
exists causing reduced visibility.
Chapter 25, Analysis 25-13
Figure 25-13 contains a list of the most common weather symbols.
Figure 25-13. NWS Surface Analysis Chart Common Weather Symbols
25.2.3.4.5 Wind
Wind is plotted in increments of 5 kt. 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 kt), barbs (10 kt), and half-barbs (5 kt) found on the stem.
If the wind is calm at the time of observation, only a single circle over the station is depicted.