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
Aviation Weather Services, Advisory Circular 00-45F, Change 2 (March 2009)
5.3 Freezing-level Graphics The freezing level is the lowest altitude in the atmosphere over a given location at which the air temperature reaches 0ºC. This altitude is also known as the height of the 0ºC constanttemperature surface. A freezing level chart shows the height of the 0ºC constant-temperature surface.
The concept of freezing level becomes slightly more complicated when more than one altitude is determined to be at a temperature of 0ºC. These “multiple freezing layers” occur when a temperature inversion at altitudes above the defined freezing level are present. For example, if the first freezing level is at 3000 ft MSL and the second is at 7000 ft MSL, a temperature inversion is between these two altitudes. This would indicate temperatures rising above freezing above 3000 ft MSL and then back below freezing at 7000 ft MSL.
The Aviation Weather Center (AWC) provides freezing level graphics available on the Aviation Digital Data Service (ADDS) web site at:
http://adds.aviationweather.noaa.gov/icing/frzg_nav.php
The ADDS Freezing Level graphics provide an initial analysis and forecasts at 3-, 6-, 9-, and 12- hours into the future. The forecasts are based on output from the National Weather Service’s (NWS) Rapid Update Cycle (RUC) numerical forecast model.
5.3.1 Issuance The initial analysis and 3-hour forecast graphics are updated hourly. The 6-, 9-, and 12-hour forecast graphics are updated every three hours.
5.3.2 Observational Data The RUC forecast model incorporates all of the latest weather observations in order to produce the best available analysis and forecast. These observations include:
• Commercial aircraft
• Profiler related:
o Wind profilers (404 and boundary layer 915 MHz)
o VAD (Velocity Azimuth Display) winds from WSR-88D radars
o RASS (Radio Acoustic Sounding System)
• Rawinsondes and special dropwinsondes
• Surface:
o GPS total precipitable water estimates
o GOES cloud-top data (pressure and temperature)
o GOES total precipitable water estimates AC 00-45F, CHG 2 Section 5: Graphical Observations and Derived Products Page 5-35
Aviation Weather Services, Advisory Circular 00-45F, Change 2 (March 2009)
o SSM/I total precipitable water estimates
o GOES high-density visible and infrared (IR) cloud drift winds
• Experimental:
o Radar reflectivity (3-d)
o Lightning
o Regional aircraft data with moisture (TAMDAR)
5.3.3 Format The colors represent the height in hundreds of feet above mean sea level (MSL) of the lowest freezing level.
• Regions with white indicate the surface and the entire depth of the atmosphere are below freezing.
• Hatched regions represent areas where the surface temperature is below freezing with multiple freezing levels aloft.
• Areas where the surface temperature is above freezing with multiple freezing levels aloft are in regions where adjacent pixels change by more than one color when compared against the color scale (e.g., orange to dark blue).
The following cases illustrate the interpretation of the graphic.
AC 00-45F, CHG 2 Section 5: Graphical Observations and Derived Products Page 5-36
Aviation Weather Services, Advisory Circular 00-45F, Change 2 (March 2009)
Figure 5-36. RUC 00-hour Freezing Level Graphic Example
Case 1 (Figure 5-36, Case 1) represents the condition where temperature is below freezing at the surface and all levels above the surface (represented in the graphic above by white-colored pixels).
Case 2 (Figure 5-36, Case 2) represents the condition where the temperature goes above and below freezing two or more times vertically through the atmosphere while the surface temperature is less than 0°C. These regions are hatched with white. The underlying color represents the lowest height where the temperature crosses the 0°C line as shown by the blue arrow on the vertical temperature graphic.
Case 3 (Figure 5-36, Case 3) represents the condition where the temperature goes above and below freezing three or more times vertically through the atmosphere while the surface AC 00-45F, CHG 2 Section 5: Graphical Observations and Derived Products Page 5-37