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WX BRIEF

Surface weather charts aid pilots in predicting
storm movements

Specialized maps provide essential information for navigating the big weather picture.




Surface analysis (or current surface weather) chart from adds.aviationweather.gov. Not all surface weather maps display current weather observations—some, such as this, display only current pressure patterns and major fronts that are generating the large-scale weather conditions that may affect flight.

On the flagpole, the windspeed will be indicated by a series of lines and pennants. If there are no lines on the flagpole, the wind is less than 5 kts. A short line indicates a 5-kt windspeed, a longer line is 10 kts, and a pennant is 50 kts.

The lines and pennants are additive with the largest indicators at the top of the flag pole—so a 75-kt wind would be indicated by a pennant, 2 long lines and 1 short line. If the wind is calm, the flagpole is replaced by a 2nd ring around the 1st.

To the left of the station ring are 2 numbers. The top one is the air temperature and the lower one is the dew point. Worldwide, these numbers are given in degrees Celsius, with the exception of the US, where surface maps depict these temperatures in degrees Fahrenheit. For pilots this information is invaluable for determining whether fog or precipitation might be likely at an airport, and whether any precipitation might fall as snow or freezing rain.

The last primary weather element found on most surface map station models is the present weather. This is a symbol normally found between the air temperature and dew point values.

Some common symbols indicate rain (2–4 dots depending on intensity), snow (2–4 asterisks), thunderstorms (an "R"-shaped symbol with an arrow terminating its right leg), rain or snow showers (a dot or asterisk above an inverted triangle), drizzle (2 commas), freezing rain or drizzle (a sideways "S" containing a dot or comma), fog (2 or 3 horizontal lines) or haze (a sideways "8").

There are 100 present weather symbols in use, although perhaps a dozen or so are most commonly indicated. The full 100 can be referenced at the National Weather Service website—srh.noaa.gov/jetstream/synoptic/ww_symbols.htm.

Surface pressure

Each morning, the US National Weather Service produces a prognostic surface chart of conditions expected to prevail during the day across much of North America. This chart includes the major fronts and pressure cells, as well as including regions and types of precipitation, convection, and other possible hazards such as fog.

Like nearly all weather charts, the surface weather chart contains information about barometric pressure at the surface. This pressure is barometric pressure corrected to sea level, and corresponds to what would be input as an altimeter setting.

On more detailed charts, sea-level pressure is displayed on the upper right side of each station model, using mb (or hPa) to 10ths, with the first 9 or 10 omitted. (Thus, 1031.2 is displayed as 312.) If the number is less than 500, the leading value was a 10—otherwise it was a 9.

To convert mb or hPa into inches of mercury, divide the value by 33.86, although the current altimeter setting should always be obtained from ATIS or other reliable source rather than from a "current" weather map that may be up to an hour old.

More commonly, sea-level pressure is depicted on surface weather maps as a series of isobars (lines of equal pressure). This method of display is advantageous, because on most weather maps you are looking at a large area, and the pressure patterns are important to understanding the wide-area weather conditions.

These patterns are more easily understood by looking at isobars than at hundreds of individual pressure observations.

In addition to the isobars, most surface weather charts will indicate the location of local pressure maxima and minima. These are your traditional high and low-pressure centers—the centers of anticyclones and cyclones.

This can be somewhat confusing, since a computer-generated map may simply show all local maxima/minima, making it appear that there are dozens of highs or lows over a region. A quick identification of the strongest ones, or those sporting warm and cold fronts, will tell you which to pay the most attention to, and which are more benign. Most charts also will indicate the boundaries between warm and cold air masses as fronts.

Unfortunately, as with highs and lows, many automated weather map generators will add fronts anywhere there is a trough in the pressure pattern, resulting in a confusing, and occasionally impossible, display of fronts headed off in all directions. In such cases, look for the fronts that make sense relative to their associated lows.

Also, compare the surface map with an upper air map to determine whether any of the lows are residing beneath an upper air trough. Those lows will probably be the dominant ones that will produce your adverse flying weather. In addition, because a front is defined by its movement, on any given chart, a warm front that has temporarily retreated might be displayed as a cold front, or vice versa.

The display of fronts varies from mapmaker to mapmaker, but there are certain similarities that will make it easy to figure out what sort of front you see. Cold fronts are almost always provided as solid blue lines that may or may not sport triangular "teeth" pointed in the direction of frontal movement.

Similarly, warm fronts are usually solid red lines, sometimes possessing rounded teeth, also oriented to the direction of movement.

Stationary fronts are normally shown as an alternating combination of the warm and cold front symbology used by the map maker—for example, an alternating red and blue line with blue triangles pointing one direction, and red humps pointing the other.

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