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 1 Rising before a greater fall Rising before a lesser fall Steady Figure 5-27. AAWU Surface Analysis Chart Pressure Trends
5.1.6.3.9 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 5-28 shows the common cloud cover depictions:
Clear Scattered cloud:
(approximately 25% cloud cover (apply sinatly 50% cloud cover) (apst xinately 75% cloud cover) Overcast Sky Obscured Sky Cover Missing Figure 5-28. AAWU Surface Analysis Chart Sky Cover Symbols
5.1.6.3.10 Water Temperature Water temperature is plotted in whole degrees Fahrenheit.
AC 00-45F, CHG 2 Section 5: Graphical Observations and Derived Products Page 5-23
Aviation Weather Services, Advisory Circular 00-45F, Change 2 (March 2009)
5.1.6.3.11 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.
5.1.6.3.12 Wave Information The 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). Below are two examples:
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.
5.1.6.4 Analyses Isobars, pressure systems, and fronts are the most common analyses depicted on surface analysis charts.
5.1.6.4.1 Isobars An isobar is a line of equal or constant pressure commonly used to analyze 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 1032.0 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.
5.1.6.4.2 Pressure Systems On a Surface Analysis Chart, a High (H) is a maximum of atmosphere pressure, while a Low (L) is a minimum of atmospheric pressure. Central pressure is the atmospheric pressure at the center of a High or Low – the highest pressure in a High and the lowest pressure in a Low. The central pressure is denoted near each pressure center.
AC 00-45F, CHG 2 Section 5: Graphical Observations and Derived Products Page 5-24
Aviation Weather Services, Advisory Circular 00-45F, Change 2 (March 2009) A trough or an elongated area of relatively low atmospheric pressure is denoted by dashed lines and identified with the word TROF. A ridge or an elongated area of relatively high atmospheric pressure is denoted by saw-toothed symbols. Ridges are rarely denoted on charts produced by the AAWU.
5.1.6.4.3 Fronts The analysis shows positions and types of fronts by the symbols in Figure 5-22. 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.
AC 00-45F, CHG 2 Section 5: Graphical Observations and Derived Products Page 5-25
Aviation Weather Services, Advisory Circular 00-45F, Change 2 (March 2009)
5.2 Constant Pressure Charts Constant pressure charts are maps of selected conditions along specified constant pressure surfaces (pressure altitudes) and depict observed weather.
Constant pressure charts help to determine the three-dimensional aspect of depicted pressure systems. Each chart provides a plan-projection view at a specified pressure altitude.
Figure 5-29. Constant Pressure Chart Example
5.2.1 Issuance Constant pressure charts are issued twice per day from observed data obtained at 00Z and 12Z. Charts are available at the NWS Fax Chart web site at:
http://weather.noaa.gov/fax/barotrop.shtml.
5.2.2 Observational Data Observational data is plotted according to priority with some data deleted to prevent overlap.
The retention priority is:
AC 00-45F, CHG 2 Section 5: Graphical Observations and Derived Products Page 5-26
Aviation Weather Services, Advisory Circular 00-45F, Change 2 (March 2009) y Radiosonde observations (see Figure 5-30) y Weather reconnaissance aircraft observations y Aircraft observations on-time and on-level y Aircraft observations off-time or off-level y Satellite wind estimates
Many other data sources are used in the analysis but are not plotted. These include:
y Ships y Buoys y Tide gauges y Wind profilers y WSR-88D weather radar VAD wind profile s y Satellite sounder
Figure 5-30. U.S. Radiosonde Network
AC 00-45F, CHG 2 Section 5: Graphical Observations and Derived Products Page 5-27
Aviation Weather Services, Advisory Circular 00-45F, Change 2 (March 2009)
Table 5-6 Features of Constant Pressure Charts Height Plot Decoding Examples of Height Plotting Constant Pressure Chart Pressure Altitude (MSL) Isotachs Contour Interval (meters) Prefix Suffix Plotted Height (meters, MSL) 850MB 5,000 ft 1,500 m No 30 m 1 - 530 1,530 m 700MB 10,000 ft 3,000 m No 30 m 2 or 3* - 180 3,180 m 500MB 18,000 ft 5,500 m No 60 m - 0 582 5,820 m 300MB 30,000 ft 9,000 m Yes 120 m - 0 948 9,480 m 200MB 39,000 ft 12,000 m Yes 120 m 1 0 164 11,640 m Note: Pressure altitudes are rounded to the nearest 1,000 for feet and 500 for meters.
* Prefix a “2” or “3,” whichever brings the height closer to 3,000 meters.
Table 5-7. Constant Pressure Chart Plotting Models Radiosonde Reconnaissanc e Aircraft (RECCO) Aircraft Report (AIREP) Satellite Wind Estimate
SYMBOL MEANING
Wind plotted in standard notation. The stem points in the direction from which the wind is blowing plotted to 36 compass points, relative to true north.
Wind speed is denoted by a combination of flags (50 knots), barbs (10 knots), and half barbs (5 knots).
TT Temperature rounded to the nearest whole degree Celsius, with minus sign prefixed if negative.
hhh Height of the constant pressure surface in meters MSL. See the Table 5-8 to see how each chart abbreviates height.
PaPaPa Pressure altitude in hundreds of feet MSL.
DD Temperature-dew point spread (depression of the dew point temperature) rounded to the nearest whole degree Celsius.
When DD is less than or equal to 5 degrees Celsius, the station circle is darkened so a region of high moisture content will stand out. If DD is greater than 5 degrees Celsius, the station circle is not shaded. If the DD is too large to measure, an X is plotted. For RECCO reports, DD will be missing when the temperature is colder than -41oC.
R Reconnaissance aircraft type.
AC 00-45F, CHG 2 Section 5: Graphical Observations and Derived Products Page 5-28
Aviation Weather Services, Advisory Circular 00-45F, Change 2 (March 2009)
Table 5-8. Radiosonde Plotting Model Examples
Plotting Model 850MB 700MB 500MB 300MB 200MB
Wind* 190o/20 kt 190o/25 kt 270o/15 kt 240o/70 kt 250o/115 kt TT Temperature 20oC 8 oC -11 oC -36 oC -56 oC hhh Height 1,504 m 3,156 m 5,820 m 9,560 m 12,140 m DD Temperature -Dew Point Spread 5oC 6 oC Too dry to measure 21oC 7 oC hchc Height Change -10 m -20 m +10 m -50 m -20 m * Wind direction assumes that north is at the top of the page. Latitude and longitude lines, as well as other geographical references, must be used to determine actual compass direction.
5.2.3 Analyses All constant pressure charts contain analyses of height and temperature. Selected charts have an analysis of wind speed as well.
5.2.3.1 Height Heights are analyzed with contours. Contours are lines of constant height in MSL and are used to map height variations of constant pressure surfaces. They identify and characterize pressure systems on constant pressure charts.
Contours are drawn as solid lines labeled with 3-digit numbers in decameters. Intervals at which the contours are drawn at: 30 meters for the 850 mb and 700 mb charts, 60 meters for the 500-mb chart, and 120 meters for the 300-mb and 200-mb charts. The location of a High or Low is marked with a 9 symbol together with a larger H or L, and the central value in decameters printed under the center location.
Contour gradient is the amount of height change over a specified horizontal distance. Gradients identify slopes of constant pressure surfaces that fluctuate in altitude. Strong gradients are denoted by closely-spaced contours which identify steep slopes. Weak gradients are denoted by widely-spaced contours which identify shallow slopes.
Wind speeds are directly proportional to contour gradients. Faster wind speeds are associated with strong contour gradients and slower wind speeds are associated with weak contour gradients. In mountainous areas, winds are often variable on constant pressure charts with altitudes near terrain elevation due to friction.
5.2.3.2 Temperature Temperature is analyzed with isotherms which are lines of constant temperature. They are drawn as long dashed lines at intervals of 5° Celsius. They are given a two-digit label in whole degrees Celsius and are preceded with a + (positive) or – (negative) sign. The zero degree isotherm denotes the freezing level.
AC 00-45F, CHG 2 Section 5: Graphical Observations and Derived Products Page 5-29
Aviation Weather Services, Advisory Circular 00-45F, Change 2 (March 2009)
Temperature gradient is the amount of temperature change over a specified distance. Isotherm gradients identify the magnitude of temperature variations. Strong gradients are denoted by closely spaced isotherms and identify large temperature variations. Weak gradients are denoted by loosely spaced isotherms and identify small temperature variations.
5.2.3.3 Wind Speed Wind speed is analyzed with isotachs. Isotachs are lines of constant wind speed. They are drawn on the 300-mb and 200-mb charts with short-dashed lines at 20-knot intervals beginning with10 knots. They are labeled with a two- or three-digit number followed by a K for knots.
Regions of high wind speeds are highlighted by alternate bands of shading and no-shading at 40-knot intervals beginning at 70 knots. A jet stream axis is the axis of maximum wind speed in a jet stream. Jet axes are not explicitly indicated, but their positions can be inferred from the isotach pattern and plotted winds.
5.2.3.4 Use Constant pressure charts are used to provide an overview of selected observed weather conditions at specified pressure altitudes.
Pressure patterns cause and characterize much of the weather. Typically, lows and troughs are associated with bad weather, clouds and precipitation, while highs and ridges are associated with good weather.
Table 5-9. Reconnaissance Aircraft (RECCO) Plotting Model Examples
Plotting Model 850MB 700MB 500MB 300MB 200MB
Wind* 150o/90 kt 130o/35 kt 180o /60 kt 240o/30 kt 110o/30 kt TT Temperature 19oC 9 oC -5 oC -28 oC -53 oC hhh Height 1,366 m 3,146 m 5,800 m 9,660 m 12,420 m DD Temperature -Dew Point Spread 1oC 1 oC 2 oC 3 oC Missing R RECCO Type AA329A AA921A AA923A AA924A AA916A * Wind direction assumes that north is at the top of the page. Latitude and longitude lines, as well as other geographical references, must be used to determine actual compass direction.
AC 00-45F, CHG 2 Section 5: Graphical Observations and Derived Products Page 5-30
Aviation Weather Services, Advisory Circular 00-45F, Change 2 (March 2009)
Table 5-10. Aircraft Report (AIREP) Plotting Model Examples
Plotting Model 850MB 700MB 500MB 300MB 200MB
Wind* 20 o/10 kt Light and Variable 300 o/30 kt 190 o/5 kt 290o/50 kt TT Temperature -5oC 12 oC -10 oC -38 oC -54 oC PaPaPa Pressure Altitude (MSL) 6,000 ft 10,000 ft 18,000 ft 33,000 ft 36,000 ft * Wind direction assumes that north is at the top of the page. Latitude and longitude lines, as well as other geographical references, must be used to determine actual compass direction.
Table 5-11. Satellite Wind Estimate Plotting Model Examples
Plotting Model 850MB 700MB 500MB 300MB 200MB
Wind* 290o/30 kt 360o/20 kt 240o/10 kt 140o/165 kt 310o/60 kt PaPaPa Pressure Altitude (MSL) 7,000 ft 11,000 ft 17,000 ft 33,000 ft 36,000 ft * Wind direction assumes that north is at the top of the page. Latitude and longitude lines, as well as other geographical references, must be used to determine actual compass direction.
AC 00-45F, CHG 2 Section 5: Graphical Observations and Derived Products Page 5-31
Aviation Weather Services, Advisory Circular 00-45F, Change 2 (March 2009)
Figure 5-31. 850MB Analysis Chart Example
Figure 5-32. 700MB Analysis Chart Example AC 00-45F, CHG 2 Section 5: Graphical Observations and Derived Products Page 5-32
Aviation Weather Services, Advisory Circular 00-45F, Change 2 (March 2009)
Figure 5-33. 500MB Analysis Chart Example
Figure 5-34. 300MB Analysis Chart Example AC 00-45F, CHG 2 Section 5: Graphical Observations and Derived Products Page 5-33
Aviation Weather Services, Advisory Circular 00-45F, Change 2 (March 2009)
Figure 5-35. 200MB Analysis Chart Example AC 00-45F, CHG 2 Section 5: Graphical Observations and Derived Products Page 5-34