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Surface weather observations for reporting points across the United States are also depicted on this chart. Each of these reporting points is illustrated by a station model.
• Type of observation—a round model indicates an official weather observer made the observation. A square model indicates the observation is from an automated station. Stations located offshore give data from ships, buoys, or offshore platforms.
• Sky cover—the station model depicts total sky cover and will be shown as clear, scattered, broken, overcast, or obscured/partially obscured.
• Clouds—cloud types are represented by specific symbols. Low cloud symbols are placed beneath the station model, while middle and high cloud symbols are placed directly above the station model. Typically, only one type of cloud will be depicted with the station model.
• Sea level pressure—sea level pressure given in three digits to the nearest tenth of a mb. For 1,000 mb or greater, prefix a ten to the three digits. For less than 1,000 mb, prefix a nine to the three digits.
• Pressure change/tendency—pressure change in tenths of mb over the past 3 hours. This is depicted directly below the sea level pressure.
• Precipitation—a record of the precipitation that has fallen over the last 6 hours to the nearest hundredth of an inch.
• Dew point—dew point is given in degrees Fahrenheit.
• Present weather—over 100 different weather symbols are used to describe the current weather.
• Temperature—temperature is given in degrees Fahrenheit.
• Wind—true direction of wind is given by the wind pointer line, indicating the direction from which the wind is coming. A short barb is equal to five knots of wind, a long barb is equal to ten knots of wind, and a pennant is equal to 50 knots.
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Figure 4-28. Example of the information found on the Helicopter Emergency Medical Services (HEMS) weather display site.
Figure 4-29. Example of a section of a surface analysis chart with station model legend (inset).
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Figure 4-30. Weather depiction chart. Weather Depiction Chart
A weather depiction chart details surface conditions as derived from METAR and other surface observations. It is prepared and transmitted by computer every 3 hours beginning at 0100 Zulu time (0100Z), and is valid at the time of the plotted data. Designed to be used for flight planning, it gives an overall picture of the weather across the United States. [Figure 4-30]
The weather depiction typically displays major fronts or areas of high and low pressure. It also provides a graphic display of Instrument Flight Rules (IFR), VFR, and marginal VFR (MVFR) weather. Areas of IFR conditions (ceilings less than 1,000 feet and visibility less than 3 miles) are shown by a hatched area outlined by a smooth line. MVFR regions (ceilings 1,000 to 3,000 feet, visibility 3 to 5 miles) are shown by a non-hatched area outlined by a smooth line. Areas of VFR (no ceiling or ceiling greater than 3,000 feet and visibility greater than 5 miles) are not outlined. Weather depiction charts show a modified station model that provides sky conditions in the form of total sky cover, cloud height or ceiling, weather, and obstructions to visibility, but does not include winds or pressure readings like the surface analysis chart. A bracket ( ] ) symbol to the right of the station indicates the observation was made by an automated station. A detailed explanation of a station model is depicted in the previous discussion of surface analysis charts.Radar Summary Chart (SD)
A radar summary chart [Figure 4-31] is a computer-generated graphical display of a collection of automated radar weather reports (SDs). The chart is published hourly, 35 minutes past the hour. It displays areas of precipitation as well as information regarding the characteristics of the precipitation. An SD chart includes:
• No information—if information is not reported, the chart will read “NA.” If no echoes are detected, the chart will read “NE.”
• Precipitation intensity contours—intensity can be described as one of six levels and is shown on the chart by three contour intervals.
• Height of tops—the heights of the echo tops are given in hundreds of feet MSL.
• Movement of cells—individual cell movement is indicated by an arrow pointing in the direction of movement. The speed of movement in knots is the number at the top of the arrow head. “LM” indicates little movement.
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Figure 4-31. Radar summary chart.
• Type of precipitation—the type of precipitation is marked on the chart using specific symbols.
• Echo configuration—echoes are shown as areas, cells, or lines.
• Weather watches—severe weather watch areas for tornadoes and severe thunderstorms are depicted by boxes outlined with heavy dashed lines.
　
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