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
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.
AC 00-45F, CHG 2 Section 5: Graphical Observations and Derived Products Page 5-40
Aviation Weather Services, Advisory Circular 00-45F, Change 2 (March 2009) Table 5-12. K Index and Coverage of General Thunderstorms
K INDEX West of the Rockies
K INDEX East of the Rockies Coverage of General Thunderstorms less than 15 less than 20 None 15 to 20 20 to 25 Isolated thunderstorms 21 to 25 26 to 30 Widely scattered thunderstorms 26 to 30 31 to 35 Scattered thunderstorms Above 30 Above 35 Numerous thunderstorms Note: K value may not be representative of air mass if 850 mb level is near the surface.
The chart only uses radiosonde data in its K index analysis. This means only large synopticscale stability patterns can be determined. Smaller, mesoscale K index variations will be missed.
AC 00-45F, CHG 2 Section 5: Graphical Observations and Derived Products Page 5-41
Aviation Weather Services, Advisory Circular 00-45F, Change 2 (March 2009)
5.5 Weather Depiction Chart The Weather Depiction Chart (Figure 5-39) contains a plot of weather conditions at selected METAR stations and an analysis of weather flying category. It is designed primarily as a briefing tool to alert aviation interests to the location of critical or near-critical operational minimums at terminals in the conterminous US and surrounding land areas. The chart can be found at: http://weather.noaa.gov/pub/fax/QGUA00.TIF
Figure 5-39. Weather Depiction Chart Example
5.5.1 Issuance The Weather Depiction chart is issued eight times daily at the following times:
Table 5-13. Weather Depiction Charts Issuance Schedule Valid Time (UTC) 01 04 07 10 13 16 19 22
5.5.2 Station Plot Model METAR elements (Section 2.1) associated with weather flying category (visibility, present weather, sky cover, and ceiling) are plotted for each station on the chart (Figure 5-41). The station is located at the center of the sky cover symbol. Most stations are not plotted due to space limitations. However, all reporting stations are used in the weather flying category analysis.
AC 00-45F, CHG 2 Section 5: Graphical Observations and Derived Products Page 5-42
Aviation Weather Services, Advisory Circular 00-45F, Change 2 (March 2009)
Figure 5-40. Weather Depiction Chart Station Plot Model
5.5.2.1 Visibility When visibility is 5 miles or less, it is entered to the left of the station. Visibility is entered in statute miles and fractions of a mile.
5.5.2.2 Present Weather Present weather symbols are entered to the left of the station. If the present weather information is obtained by an automated system, the right bracket symbol ( ] ) is plotted to the right of the station.
When several types of weather and/or obstructions to visibility are reported, the most significant weather element is plotted. This is the first weather element coded in the METAR report (Section 2.1) and is usually the highest coded number in the Present Weather Symbols guide (Appendix I).
5.5.2.3 Sky Cover Sky cover represents the summation total of the sky condition element from the METAR report.
For example, if the METAR sky condition element was SCT030 BKN060 OVC090, the sky cover would be overcast. Sky cover symbols are listed in Figure 5-41.
5.5.2.4 Ceiling Ceiling is the height from the base of the lowest layer aloft covering more than one-half the sky to the ground. Additionally, vertical visibility into a total surface-based obscuration is defined as a ceiling. For a METAR report, the first broken (BKN) or overcast (OVC) layer is the ceiling.
For example, if the METAR sky condition element is SCT030 BKN060 OVC090, the ceiling is 6,000 feet.
For stations with broken to overcast layers, the ceiling height is plotted below the station.
Ceilings are reported as hundreds of feet above ground level (AGL).
AC 00-45F, CHG 2 Section 5: Graphical Observations and Derived Products Page 5-43
Aviation Weather Services, Advisory Circular 00-45F, Change 2 (March 2009) For a total surface-based obscuration, no ceiling is plotted and the METAR must be consulted.
Partial obscurations are not identified.
• For a partial obscuration with no layer above, the sky cover symbol will be plotted as missing (Figure 5-41).
• For a partial obscuration with a layer above, the sky cover and ceiling height will be plotted for the cloud layer only.
The METAR report should be consulted to identify the partial obscuration.
If the sky cover is clear, few, or scattered, no ceiling is plotted.
5.5.3 Weather Flying Category Analysis Instrument Flight Rules (IFR) indicated on the Weather Depiction Chart represents ceilings less than 1,000 feet and/or visibility less than 3 statute miles and IFR operations must be in place.
IFR areas are outlined on the chart with a solid line and are shaded. IFR areas are typically shaded red in colorized versions of the chart.
Marginal Visual Flight Rules (MVFR) indicated on the Weather Depiction Chart represents ceiling 1,000 to 3,000 feet and/or visibility 3 to 5 statute miles and VFR operations can take place. MVFR areas are outlined with a solid line, but the area is not shaded. MVFR areas are typically shaded blue in colorized versions of the chart.
Visual Flight Rules (VFR) indicated on the Weather Depiction Chart represents a ceiling greater than 3,000 feet or clear skies and visibility greater than 5 statute miles and VFR operations can take place. VFR conditions are not analyzed. This does not necessarily imply that the sky is clear.
5.5.4 Use The Weather Depiction Chart is an ideal place to begin flight planning or to prepare for a weather briefing. This chart provides an overview of weather flying categories and other adverse weather conditions for the chart valid time. The chart, though, may not completely represent the en route conditions because of terrain variations and the possibility of weather occurring between reporting stations. This chart should be used in addition to the current METAR reports, pilot weather reports, and radar and satellite imagery for a complete look at the latest flying conditions.
AC 00-45F, CHG 2 Section 5: Graphical Observations and Derived Products Page 5-44
Aviation Weather Services, Advisory Circular 00-45F, Change 2 (March 2009)
5.6 Alaska Weather Depiction Charts The Alaska Weather Depiction Charts (Figure 5-43) display color coded station plots which show: temperature, dew point, ceiling, visibility and wind direction/speed. A key to the station plots is found on each map.
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IFR MYFRAYFR 190 Figure 5-41. AAWU Alaska Weather Depiction Chart Example Thirteen charts cover Alaska (except for the Aleutians) and adjacent areas of Canada.
AC 00-45F, CHG 2 Section 5: Graphical Observations and Derived Products Page 5-45
Aviation Weather Services, Advisory Circular 00-45F, Change 2 (March 2009)
Table 5-14. AAWU Alaska Weather Depiction Charts Coverage Chart Coverage Scale Entire State of Alaska (1:12 million) All of Southeast Alaska (1:5 million) Southern Southeast Alaska (1:3 million) Northern Southeast Alaska (1:3 million) North Gulf Coast (1:5 million) South Central Alaska (1:5 million) Cook Inlet/Susitna Valley (1:2 million) Southwest Alaska (1:6 million) Western Interior (1:5 million) Central Interior (1:5 million) Northern Alaska (1:6 million) Southwest British Columbia (1:7 million) Yukon Territory/Northern British Columbia (1:8 million)
5.6.1 Issuance The charts are issued hourly and can be found on the Alaska Aviation Weather Unit (AAWU) web site at: http://aawu.arh.noaa.gov/Sigwx.php. The charts will first appear at about 10 minutes past the hour, with a second update at about 25 minutes past the hour.
5.6.2 Legends The Alaska Weather Depiction Charts depict numerous parameters including the flying category, sky cover and wind.
5.6.2.1 Flying Category Each station plot is color-coded according to the weather flying category reported (Table 5-16).
Red indicates instrument flight rules (IFR), blue indicates marginal visual flight rules (MVFR), and black is plotted for stations reporting visual flight rules (VFR).
Table 5-15 AAWU Alaska Weather Flying Categories and Criteria FLYING CATEGORY CEILING (feet) VISIBILITY (miles) VFR (black) Greater than 3,000 feet Greater than 5 miles MVFR (blue) 1,000 to 3,000 feet 3 to 5 miles IFR (red) Less than 1,000 feet Less than 3 miles
5.6.2.2 Station Plot METAR elements are plotted for each station on the chart (Figure 5-45). Some stations are not plotted due to space limitations, notably on the chart which covers the entire state of Alaska.
Figure 5-42. AAWU Alaska Weather Depiction Chart Station Plot Legend AC 00-45F, CHG 2 Section 5: Graphical Observations and Derived Products Page 5-46