Aviation Weather Services, Advisory Circular 00-45F, Change 2 (March 2009) by the color scale used on the images. In general, regions displayed in shades of red are VERY dry in the upper atmosphere and MAY correlate to crisp blue skies from a ground perspective.
On the contrary, regions displayed in shades of blue or green are indicative of lots of high-level moisture and may also indicate cloudiness. This cloudiness could simply be high-level cirrus types or thunderstorms. That determination cannot be gained from this image by itself but could easily be determined when used in conjunction with corresponding visible and infrared satellite images.
Figure 4-27. Water Vapor Satellite Image – U.S. Example The scale is in degrees Celsius. Blue/green colors indicate moisture and/or clouds in the mid/upper troposphere, while dark gray/orange/red colors indicate dry air in the mid/upper troposphere.
AC 00-45F, CHG 2 Section 4: Radar and Satellite Imagery Page 4-23
Aviation Weather Services, Advisory Circular 00-45F, Change 2 (March 2009) AC 00-45F, CHG 2 Section 4: Radar and Satellite Imagery Page 4-24
Figure 4-28. Water Vapor Satellite Image – Regional-Scale Example The scale is in degrees Celsius. Blue/green colors indicate moisture and/or clouds in the mid/upper troposphere, while dark gray/orange/red colors indicate dry air in the mid/upper troposphere.
4.2.3.3.1 Water Vapor Image Data Legend The data legend (Figure 4-27) on a water vapor images is calibrated to temperature expressed in degrees Celsius. The actual data values on the water vapor images are not particularly useful. Interpretation of the patterns and how they change over time is more important. The legend may vary depending on the satellite image provider.
Figure 4-29. Water Vapor Satellite Image Data Legend.
The colors (values) represent temperature in degrees Celsius
Aviation Weather Services, Advisory Circular 00-45F, Change 2 (March 2009) 5 GRAPHICAL OBSERVATIONS AND DERIVED PRODUCTS
5.1 Surface Analysis Charts Surface Analysis charts are analyzed charts of surface weather observations. The chart depicts the distribution of several items including sea level pressure, the positions of highs, lows, ridges, and troughs, the location and character of fronts, and the various boundaries such as drylines, outflow boundaries, sea-breeze fronts, and convergence lines. Other symbols are often added depending upon the intended use of the chart. Pressure is referred to in mean sea level (MSL) on the surface analysis chart while all other elements are presented as they occur at the surface point of observation. A chart in this general form is commonly referred to as the weather map.
Figure 5-1. HPC Surface Analysis Chart Example
5.1.1 Issuance Five National Weather Service (NWS) offices issue surface analysis charts:
• The Hydrometeorological Prediction Center (HPC) in Camp Springs, Maryland is responsible for land areas of North America. The charts are available at o http://www.hpc.ncep.noaa.gov/html/sfc2.shtml o http://www.hpc.ncep.noaa.gov/html/avnsfc.shtml
AC 00-45F, CHG 2 Section 5: Graphical Observations and Derived Products Page 5-1
Aviation Weather Services, Advisory Circular 00-45F, Change 2 (March 2009)
• The Ocean Prediction Center (OPC) in Camp Spring, Maryland is responsible for the Atlantic and Pacific Oceans north of 30oN latitude. The charts are available at:
o http://www.opc.ncep.noaa.gov/
• The Tropical Prediction Center (TPC) in Miami, Florida is responsible for the tropical regions of the western hemisphere south of 30oN latitude and east of 160oE longitude.
The surface analysis charts are available at:
o http://www.nhc.noaa.gov/marine_forecasts.shtml
• The Alaskan Aviation Weather Unit (AAWU) in Anchorage, Alaska is responsible for the state of Alaska. The surface analysis chart is available at:
o http://aawu.arh.noaa.gov/surface.php
• The Weather Forecast Office in Honolulu, Hawaii (WFO HNL) is responsible for the tropical Pacific Ocean, south of 30oN latitude and west of 160oE longitude. The charts are available at:
o http://www.prh.noaa.gov/hnl/pages/analyses.php
Each office produces multiple versions of the surface analysis chart with varying formats.
5.1.2 HPC Surface Analysis Charts
• The Hydrometeorological Prediction Center (HPC) in Camp Springs, Maryland is responsible for land areas of North America. The charts are available at o http://www.hpc.ncep.noaa.gov/html/sfc2.shtml o http://www.hpc.ncep.noaa.gov/html/avnsfc.shtml
5.1.2.1 Issuance The Hydrometeorological Prediction Center (HPC) issues Surface Analysis Charts for North America eight times daily (Table 5-1).
Table 5-1. HPC Surface Analysis Charts Issuance Schedule Valid Time (UTC) 00 03 06 09 12 15 18 21
5.1.2.2 Analysis Symbols Figure 5-2 shows analysis symbols used on HPC surface analysis charts:
AC 00-45F, CHG 2 Section 5: Graphical Observations and Derived Products Page 5-2
Aviation Weather Services, Advisory Circular 00-45F, Change 2 (March 2009)
COLD FRONT - COLD FRONTOLYSIS WARM FRONT • WARM FRONTOLYSIS
STATIONARY FRONT STATIONARY
OCCLUDED FRONT FRONTOLYSIS
OCCLUDED FRONTOLYSIS
CHANGE OF FRONT TYPE
COLD FRONTOGENESIS HIGH PRESSURE CENTER
WARM FRONTOGENESIS LOW PRESSURE CENTER
STATIONARY TROPICAL (TRPL) WAVE
FRONTOGENESIS T.D.
TROUGH (TROF) OR XXX TROPICAL DEPRESSION
OUTFLOW BONDARY (OUTFLOW BNDRY) 6 TRPCL STORM TROPICAL STORM
DRYLINE
RIDGE HURCN HURRICANE
SQUALL LINE XXX Figure 5-2. HPC Surface Analysis Chart Symbols
5.1.2.3 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.
Figure 5-3 and 5-4 contain the most commonly used station plot models used in surface analysis charts:
Wind Sea-level pressure Temperature 57 107 Weather -→ • -6 56 Pressure trend (mb) Dew Point(F) Sky cover Figure 5-3. HPC Surface Analysis Chart Station Plot Model AC 00-45F, CHG 2 Section 5: Graphical Observations and Derived Products Page 5-3
Aviation Weather Services, Advisory Circular 00-45F, Change 2 (March 2009)
Figure 5-4. HPC Surface Analysis Chart Ship/Buoy Plot Model
HPC also produces surface analysis charts specifically for the aviation community. Figure 5-5 contains the station plot model for these charts:
Figure 5-5. HPC Surface Analysis Chart for Aviation Interests Station Plot Model
5.1.2.3.1 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.
5.1.2.3.2 Temperature The air temperature is plotted in whole degrees Fahrenheit.
AC 00-45F, CHG 2 Section 5: Graphical Observations and Derived Products Page 5-4
Aviation Weather Services, Advisory Circular 00-45F, Change 2 (March 2009)
5.1.2.3.3 Dew Point The dew point temperature is plotted in whole degrees Fahrenheit.
5.1.2.3.4 Weather A weather symbol is plotted if, at the time of observation, precipitation is either occurring or a condition exists causing reduced visibility.
Figure 5-6 contains a list of the most common weather symbols:
Figure 5-6. HPC Surface Analysis Chart Common Weather Symbols
A complete list of weather symbols can be found at in Appendix I.
5.1.2.3.5 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 (50kts), barbs (10kts), and half-barbs (5kts) found on the stem.
If the wind is calm at the time of observation, only a single circle over the station is depicted.
Figure 5-7 are some sample wind symbols:
AC 00-45F, CHG 2 Section 5: Graphical Observations and Derived Products Page 5-5
Aviation Weather Services, Advisory Circular 00-45F, Change 2 (March 2009)
Figure 5-7. HPC Surface Analysis Chart Wind Plotting Model
5.1.2.3.6 Ceiling Ceiling is plotted in hundreds of feet above ground level.
5.1.2.3.7 Visibility Surface visibility is plotted in whole statute miles.
5.1.2.3.8 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 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
5.1.2.3.9 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 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 5-8 contains the meanings of the pressure trend symbols:
AC 00-45F, CHG 2 Section 5: Graphical Observations and Derived Products Page 5-6
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 Rising before a greater fall Rising before a lesser fall Steady Figure 5-8. HPC Surface Analysis Chart Pressure Trends
5.1.2.3.10 Sky Cover The approximate amount of sky cover can be determined by the circle at the center of the station plot. The amount the circle is filled reflects the amount of sky covered by clouds. Figure 5-9 contains the common cloud cover depictions:
Clear catered cloud approximately 25% cloud cover (apply cinately 50% cloud cover) (apstYxinoely 75% cloud cover) Overcast Sky Obscured Sky Cover Missing Figure 5-9. HPC Surface Analysis Chart Sky Cover Symbols
5.1.2.3.11 Water Temperature Water temperature is plotted in whole degrees Fahrenheit.
AC 00-45F, CHG 2 Section 5: Graphical Observations and Derived Products Page 5-7
Aviation Weather Services, Advisory Circular 00-45F, Change 2 (March 2009)
5.1.2.3.12 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.
5.1.2.3.13 Wave Information 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).
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.
In some charts by the OPC, only the wave height (in feet) is plotted.
5.1.2.4 Analyses Isobars, pressure systems, and fronts are the most common analyses depicted on surface analysis charts.
5.1.2.4.1 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 millibars (mb). Each isobar is labeled. For example, 1032 signifies 1,032.0 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.
AC 00-45F, CHG 2 Section 5: Graphical Observations and Derived Products Page 5-8
Aviation Weather Services, Advisory Circular 00-45F, Change 2 (March 2009)
5.1.2.4.2 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 HPC.
Tropical storms, hurricanes, and typhoons (See Figure 5-2) are low-pressure systems with their names and central pressures denoted.
5.1.2.4.3 Fronts The analysis shows positions and types of fronts by the symbols in Figure 5-2. 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.
Figure 5-10. HPC Surface Analysis Chart – North America Example
AC 00-45F, CHG 2 Section 5: Graphical Observations and Derived Products Page 5-9
Aviation Weather Services, Advisory Circular 00-45F, Change 2 (March 2009)
Figure 5-11. HPC Surface Analysis Chart - Contiguous U.S. Example
Figure 5-12. HPC Surface Analysis Chart - Northeast U.S. Example
AC 00-45F, CHG 2 Section 5: Graphical Observations and Derived Products Page 5-10