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Deciphering meteorological charts

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Weather maps can reveal a lot about the skies.


By Karsten Shein
Contributing Writer

Surface weather depiction chart for the coterminous US. Similar to surface analysis charts, but focused more to flight planning, weather depiction charts will show pressures and fronts across a wide area, along with simplified weather station models describing sky cover and present weather conditions, allowing pilots to visualize “big picture” weather factors along their proposed route.

With the proliferation of flight planning software and electronic flight bag (EFB) apps, getting graphical weather information ahead of and during a flight has never been easier – or more confusing.

Which map will provide the information you need? Can you decipher the codes used by meteorologists to describe weather conditions, or translate that information into an understanding of how the weather picture may develop over the next few hours? Weather maps are a simple way to distill a lot of meteorological information into a single graphic that, with a little training, can be used to unlock a clear understanding of the weather.

Weather maps have been around since before the Wright brothers’ first flight, and much of the information those early maps contained is still found in their contemporaries.

Although there are many varieties of weather map, there are a few standard versions from which pilots will get the most use. The surface analysis (SA) and weather depiction charts provide very similar information, and are intended to allow pilots to gain a complete picture of the general state of the weather over an area at a glance.

Surface analysis

The SA chart is generally the most comprehensive map for depicting current surface weather conditions. SA charts are updated every 3 hours and show synoptic features such as surface highs and lows, along with the fronts that drape between them. Depending on the provider, the SA map may also show pressure contours and a symbolic model of weather observations at various airports across the map. Because there are so many weather stations, a subset of larger airports may be displayed to avoid cluttering the map.

On interactive maps, such as the NWS Aviation Weather Center’s Observations map, zooming in will often reveal more stations across the zoomed area.

Pilots should be familiar with the standard weather station model, as this is how the weather at a location is presented. The model is a circle over the station. How much of the circle is filled in indicates sky cover. To the left of the circle, a symbol may be present, identifying the current weather (eg, rain, snow, fog, or thunderstorm). The absence of a symbol suggests no significant weather at that location.

Above and below the present weather symbol are temperature and dew point, while visibility is to the left of the symbol. Ceiling height in hundreds of feet is below the primary circle, and cloud type is above it. Altimeter information is to the right of the circle. Lastly, prevailing wind is indicated by a line with barbs on it extending from the circle in the direction the wind is coming from. The barbs illustrate the wind speed.

A weather depiction chart is a variation of the SA chart that is geared more toward flight planning. The weather station models provide less information than on the SA chart, normally omitting wind and pressure information. However, unlike the SA chart, the weather depiction chart will display areas of IFR, VFR, and MVFR surface weather.

Because there is no hard-and-fast standard for these maps, providers have substantial latitude in what they include. For example, one provider may include weather radar or satellite overlays to show cloud patterns or areas of rain, snow, or freezing rain, while another provider may not. Still others may serve interactive maps that reveal weather station information when the pilot taps on the station.

Whether looking at the SA or weather depiction chart, pilots should remember that, although they may contain information about IFR conditions and ceiling heights, these maps describe conditions at the surface. Naturally, large-scale surface conditions are indicative of conditions aloft. A surface high is more likely to support clear skies, and a cold front can be expected to bring clouds and precipitation.

If a pilot has the time, it is always worthwhile to examine the previous few surface maps in order to get a better understanding of how the weather patterns are developing. How fast are the fronts moving? Is the low deepening or filling in? Has the area where stations have been reporting thunderstorms grown or shrunk? These are a few of the questions that can be answered by looking at the past few hours of weather maps.

Significant weather charts

Mid-level (FL100–FL450) significant weather (SigWx) chart for the North Atlantic. SigWx charts provide general information about likely weather hazards over a broad area. These hazards include thunderstorms (red scalloped areas), icing, moderate or greater turbulence (yellow areas), volcanic eruptions, and tropical cyclones. The position and speed of the jet stream (green arrows) and tropopause height are also normally displayed.

A 3rd common weather map is the significant weather (SigWx) map. Issued 4 times per day, the SigWx map provides a graphical display of areas of hazardous weather conditions, such as icing, turbulence, or thunderstorms. These maps are issued for low level (surface to FL240) and high level (FL240 to FL630).

Mid-level (10,000 ft to FL450) SigWx maps are also produced for a few regions, such as  the North Atlantic. Significant weather charts will normally include all active airmets and sigmets, and, like SA charts, will include highs, lows, and fronts. Some may also include precipitation or satellite overlays, which can be very helpful in understanding the reason for the significant weather depictions.

Importantly for pilots, SigWx maps are also produced to display forecast conditions 12, 24, 36, and 48 hours ahead. These forecast maps are known as prognostic (prog) charts and will often display similar graphical information to current conditions maps, such as fronts and areas of hazardous weather.

Beyond these fundamental weather maps, pilots may also find it helpful to look at pressure level maps (constant pressure charts). Similar to surface maps, pressure level maps display information about conditions aloft, such as wind speed and direction, temperature, and humidity at specific flight levels.

The 500-hPa (500-mb) map, for example, shows the atmosphere at around 18,000 ft and is a commonly referenced chart that is useful for determining the dynamics of the lower atmosphere.

A 500-mb vorticity change, for example, is important to understanding the development and movement of highs and lows, and how energetic a frontal system may be. Depending on the time of year, the 300- or 250-hPa chart (around FL300 to FL340) is ideal for locating the jet stream and determining its speed.

In addition to specific information such as vorticity or humidity, most constant pressure charts will contain wind barbs or streamlines that show winds aloft and areas of potential windshear, where speed changes quickly over short distances, or the flow changes direction acutely.

Steep or tilted troughs in the upper air flow will normally indicate a highly dynamic flow in which severe to extreme turbulence may be found, particularly around the base of the trough, where a jet streak might be present. Referencing the upper air charts closest to your anticipated cruise altitude will provide a good visualization of your winds aloft and potential weather hazards. If conditions look unfavorable at that level, maps from other levels can help you find a better cruising altitude.

Conclusion

Every day, meteorologists collect millions of weather observations from surface stations, weather balloons, radars, satellites, ships, and aircraft, and these observations are used to create the weather maps on which we rely for flight planning. There are many ways of presenting this information graphically, and each weather service provider will have its own preference for designing its maps to deliver the information they deem most important to pilots.

Of course, it is wise to choose charts that provide similar information to those produced by national hydrometeorological agencies, such as the US National Weather Service (aviationweather.gov). If a weather-related incident occurs, investigators will evaluate whether the information you received was adequate. Nonstandard weather maps may be insufficient.

Also, while weather maps provide a solid foundation for interpreting a weather briefing and asking the
briefer informed questions, they are not a substitute for the weather briefing. And, even if your aircraft automatically transmits routine aireps via ADS-B, if you encounter adverse weather, submitting a pirep or special airep can improve the next round of weather maps.


SheinKarsten Shein is cofounder of 2DegreesC.org. He was director of the Midwestern Regional Climate Center at the University of Illinois, and a NOAA and NASA climatologist. Shein holds a comm-inst pilot license.