Chapter 25, Analysis 25-14
Figure 25-14 includes some sample wind symbols.
Figure 25-14. NWS Surface Analysis Chart Sample Wind Symbols
25.2.3.4.6 Ceiling
Ceiling is plotted in hundreds of feet AGL.
25.2.3.4.7 Visibility
Surface visibility is plotted in whole statute miles.
25.2.3.4.8 Pressure
Sea level pressure is plotted in tenths of millibars, with the first two digits (generally 10 or 9) omitted. For
reference, 1,013 mb is equivalent to 29.92 inHg. Below are some sample conversions between plotted and
complete sea level pressure values.
410 1,041.0 mb
103 1,010.3 mb
987 998.7 mb
872 987.2 mb
Chapter 25, Analysis 25-15
25.2.3.4.9 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 three hours. The number provides the three-hour change in tenths of millibars,
while the symbol provides a graphic illustration of how this change occurred.
Figure 25-15 contains the meanings of the pressure trend symbols.
Figure 25-15. NWS Surface Analysis Chart Pressure Trends
25.2.3.4.10 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 25-16 contains the
common cloud cover depictions.
Figure 25-16. NWS Surface Analysis Chart Sky Cover Symbols
Chapter 25, Analysis 25-16
25.2.3.4.11 Water Temperature
Water temperature is plotted in whole degrees Fahrenheit.
25.2.3.4.12 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° (i.e., it is coming from due east).
• 07: The period of the swell is seven seconds.
• 03: The height of the swell is three half m.
271006:
• 27: The swell direction is from 270° (due west).
• 10: The period is 10 seconds.
• 06: The height of the swell is six half m.
25.2.3.4.13 Wave Information
Period and height of waves are represented by a five-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).
10603:
• 1: A group identifier. The first digit will always be 1.
• 06: The wave period is six seconds.
• 03: The wave height is three half m.
10515:
• 1: A group identifier.
• 05: The wave period is five seconds.
• 15: The wave height is 15 half m.
In some charts by the OPC, only the wave height (in feet) is plotted.
25.2.4 Unified Surface Analysis Chart
The NWS Unified Surface Analysis Chart is a surface analysis product produced collectively and
collaboratively by the NWS WPC, the OPC, the NHC, and WFO Honolulu. The chart contains an analysis
of isobars, pressure systems, and fronts.
This chart is available on the OPC’s website. Users can zoom in by clicking an area on the map to enlarge
(see Figure 25-17 and Figure 25-18) and show station plot models (see Section 25.2.3.4).
25.2.4.1 Issuance
The Unified Surface Analysis Chart is issued four times daily for valid times 00, 06, 12, and 18 UTC.
Chapter 25, Analysis 25-17
Figure 25-17. Unified Surface Analysis Chart Example
Figure 25-18. Unified Surface Analysis Chart Example (Enlarged Area)
25.2.4.2 Analysis Symbols
Unified Surface Analysis Charts use the symbols shown in Figure 25-6.
25.2.5 AAWU Surface Chart
The NWS Unified Surface Analysis Chart covers the Alaska area. The AAWU also provides a fixed area
image of the Unified Surface Analysis Chart centered over Alaska (see Figure 25-19). This chart is available
from the AAWU’s website.
Chapter 25, Analysis 25-18
Figure 25-19. Unified Surface Analysis Chart Example with Fixed Area Coverage over Alaska
25.3 Upper Air Analysis
An upper air chart (also known as a constant-pressure chart or an isobaric chart) is a weather map
representing conditions on a surface of equal atmospheric pressure. A constant-pressure surface is a surface
along which the atmospheric pressure is everywhere equal at a given instant. For instance, the 500 mb
constant-pressure surface has a pressure of 500 mb everywhere on it. For example, a 500 mb chart will
display conditions at the level of the atmosphere at which the atmospheric pressure is 500 mb.
Constant-pressure charts usually contain plotted data and analyses of the distribution of height of the surface
(contours), wind (isotachs), temperature (isotherms), and sometimes humidity (isohumes). The height
above sea level at which the pressure is that particular value may vary from one location to another at any
given time, and also varies with time at any one location, so it does not represent a surface of constant
altitude/height (i.e., the 500 mb level may be at a different height above sea level over Dallas than over
New York at a given time and may also be at a different height over Dallas from one day to the next). The
height (altitude) of a constant-pressure surface varies primarily due to temperature; these heights can be
measured by a rawinsonde.
Constant-pressure charts are most commonly known by their pressure value (see Table 25-3 for common
constant pressure levels). For example, the 1,000 mb chart (which closely corresponds to the surface chart),
the 850 mb chart, 700 mb chart, 500 mb chart, etc.
A contour analysis (see Figure 25-20 and Figure 25-21) can reveal highs, ridges, lows, and troughs aloft
just as the surface chart shows such systems at the surface. These systems of highs/ridges and lows/troughs
are called pressure waves. These pressure waves are similar to waves seen on bodies of water. Th ey have
crests (ridges) and valleys (troughs) and are in constant movement.
Chapter 25, Analysis 25-19
Figure 25-20. Schematic of 500 mb Constant-Pressure Chart
Table 25-3. Common Constant-Pressure Charts
Chart
Pressure Altitude (approximate)
Feet (ft) Meters (m)
100 mb 53,000 ft 16,000 m
150 mb 45,000 ft 13,500 m
200 mb 39,000 ft 12,000 m
250 mb 34,000 ft 10,500 m
300 mb 30,000 ft 9,000 m
500 mb 18,000 ft 5,500 m
700 mb 10,000 ft 3,000 m
850 mb 5,000 ft 1,500 m
925 mb 2,500 ft 750 m
25.3.1 Issuance
The NWS provides data to produce upper air analysis charts.