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.
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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.
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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
Aviation Weather Services, Advisory Circular 00-45F, Change 2 (March 2009) temperature is higher than 0°C. These regions are located in areas where adjacent pixels change by more than one color when compared against the color scale.
Case 4 (Figure 5-36, Case 4) is relatively simple and represents the condition where the temperature at the surface is above freezing and the air generally cools with height crossing the 0°C line once.
5.3.4 Use Freezing level graphics are used to assess the lowest freezing level heights and their values relative to flight paths. Clear, rime and mixed icing are found in layers with below-freezing (negative) temperatures and super-cooled water droplets. Users should be aware that official forecast freezing level information is specified within the AIRMET Zulu Bulletins (Contiguous U.S. and Hawaii) and the AIRMET “ICE AND FZLVL” information embedded within the Area Forecasts (Alaska only)
Figure 5-37. ADDS Freezing Level Graphic Example AC 00-45F, CHG 2 Section 5: Graphical Observations and Derived Products Page 5-38
Aviation Weather Services, Advisory Circular 00-45F, Change 2 (March 2009)
5.4 Lifted Index Analysis Chart The Lifted Index Analysis Chart (Figure 5-38) provides a data plot of observed lifted index (LI) and K index values for radiosonde sites and an analysis of LI for the contiguous U.S., southern Canada and northern Mexico.
Figure 5-38. Lifted Index Analysis Chart Example
5.4.1 Issuance The Lifted Index Analysis Chart is issued twice daily by the NWS and based solely on radiosonde observations from 00Z and 12Z. It is available at the NWS Fax Chart web site at:
http://weather.noaa.gov/pub/fax/QXUA00.TIF.
5.4.2 Lifted Index (LI) The lifted index (LI) is a common measure of atmospheric stability. The Lifted Index Analysis Chart depicts a number associated with the stability of a surface parcel of air lifted to 500 mb.
For more complete information on the computation, refer to the Aviation Weather manual (AC 00-6A).
Lifted index values range from positive to negative. A positive lifted index indicates stable air.
Larger positive numbers imply greater stability. A negative lifted index indicates unstable air.
AC 00-45F, CHG 2 Section 5: Graphical Observations and Derived Products Page 5-39
Aviation Weather Services, Advisory Circular 00-45F, Change 2 (March 2009) Larger negative numbers imply greater instability. A zero lifted index indicates neutrally stable air.
5.4.2.1 Data Plot Values of lifted index are plotted above the station circle for each available radiosonde station.
Missing values are denoted with M. Station circles are blackened for LI values of zero or less.
5.4.2.2 Analysis Isopleths, or lines of equal value, of lifted index are drawn for intervals of 4 units for index values of +4 and lower.
5.4.2.3 Use of Lifted Index LI values on the Lifted Index Analysis Chart must be used with caution. The chart is only issued twice per day and significant changes can occur between chart times. LI values are typically lowest (least stable) during the afternoon due to daytime heating and highest (most stable) at sunrise due to nighttime cooling.
LI values can change rapidly due to moving fronts, drylines, outflow boundaries, and other boundaries which change surface airmass characteristics. LI (and thus stability) is particularly sensitive to changes of surface dew point. Temperature changes at 500 mb also affect LI, but these changes are usually much less dramatic than those which occur with temperature and dew point near the surface.
The Lifted Index Analysis chart only uses radiosonde data in its LI analysis. This means only large synoptic-scale stability patterns can be determined. Smaller, mesoscale LI variations will be missed.
An unstable air mass (denoted by negative LI values) only implies the potential for thunderstorms. A lifting mechanism such as a front, dryline, upslope flow, outflow boundary from prior storms, or frictional convergence around lows and troughs is still necessary to initiate a thunderstorm.
5.4.3 K Index The K index (Figure 5-38) is a measure of thunderstorm potential based on vertical temperature lapse rate, moisture content of the lower atmosphere, and vertical extent of the moist layer. For more complete information on the computation, refer to the Aviation Weather (AC 00-6A).
5.4.3.1 Data Plot Values of K will be plotted below the station circle for each available radiosonde station.
Missing values are denoted with M. No analysis of the K-index is made on the Lifted Index Chart.
5.4.3.2 Use of K Index With the K index, the higher the positive number, the likelihood of thunderstorm development is greater. The computation of the K-Index biases it in favor of general thunderstorms and it works better for non-severe convection. The K-index is also an index for forecasting heavy rain.
Although K-index values can be correlated to a probability of thunderstorm occurrence, these values will vary with seasons, locations, and synoptic settings. The values listed in Table 5-12 were empirically-derived and should be used with caution.
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