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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.

By Karsten Shein
Comm-Inst, Climate Scientist


Automated meteorological sensors such as these at a North Dakota airport provide thousands of the observations used to produce surface analysis charts for aviation. Surface weather maps are an essential tool for understanding the weather a pilot may face.

As your first cup of FBO coffee begins to battle the early-morning fog inside your head, the briefer explains that a line of storms, some severe, with tops between 30 and 40 thousand feet, can be expected after 18Z in a line from 20 miles southwest of CDH (Camden AR) north-northwest to 30 miles west of FAM (Farmington MO), and will be moving east at 40 kts.

You have a general idea that your route will take you through that area—but where exactly are CDH and FAM? What is driving those storms, and how far might you need to deviate to ensure you'll avoid the worst of the weather? For such information, a visual guide would be really helpful. Such situations are where having a copy of a current surface weather chart in front of you is essential.

Last month, we took a look at the 300-millibar (mb) chart and how conditions at that level (around 30,000 ft MSL) help to shape and drive the larger storm systems and air mass movements that occur in the lowest thermal layer of the atmosphere—the troposphere.

As we saw, 300 mb (300 hPa) occurs near the top of the troposphere. The surface weather chart in turn provides an overview of conditions at and near the surface—the bottom of the troposphere. Combined, these 2 weather charts can provide any pilot with a comprehensive understanding of large-scale weather patterns and what conditions they might expect to encounter at and between their departure and arrival airports.

Pilots have been using surface weather maps—known as surface analysis (SA)—for decades. If all a pilot uses a surface weather map for is to reference what a briefer has told them, or to locate the major weather-making fronts to avoid, the map is a very useful support tool. But this map contains a great deal more information that can help in making inform­ed decisions about upcoming flights.

To get the fullest use from a surface weather chart, one first needs to know what information is being presented, and what notation is used to present it. The basics of a surface weather chart are rooted in the fundamental station model—a meteorological shorthand that describes the primary conditions at the location of a particular weather station.

It helps that these weather stations are almost always located at airports, which can not only give an arriving or departing pilot knowledge of the conditions reported at that airport, but can also help them quickly choose an alternate based on conditions at nearby weather-station-equipped airports.

Station models

North American surface weather chart (also referred to as a surface analysis) produced by NOAA. Such maps are key to visualizing large-scale weather patterns that will affect flight weather. Although a great deal more information can be included, most surface charts will include basic information about sky cover, winds, pressure and temperature.

At the heart of the station model is a ring that represents the sky above the station. In the UK, if the reporting station is automated, this ring may instead be displayed as a triangle.

Sky cover is normally observed in eighths—in Metar terminology, 0/8 = CLR or SKC/NSC, 1–2/8 = FEW, 3– 4/8 = SCT, 5–7/8 = BKN, and 8/8 = OVC—so, if the ring's center is empty, the sky is clear (0/8). If there is a single vertical line through the ring, it is 1/8 cover.

A quarter coverage of the ring indicates clouds cover 2/8 of the sky. Three eighths coverage includes both the vertical line and quarter infill. Half sky coverage is denoted by half of the ring covered, with 5/8 further indicated by a horizontal line through the clear half. 6/8 sky cover means 3/4 of the ring is covered. At 7/8 coverage, the ring is completely filled in, but with a vertical line transecting the circle. A fully overcast sky (8/8) shows as a completely filled in ring.

If the sky is obscured, as with fog or haze, an "X" is placed inside the ring. Some weather maps however, will not display the vertical or horizontal lines, but rather limit the displays to the quarter fills. If this is the case, consider 1/4 fill to equal FEW, 1/2 fill to equal SCT, 3/4 fill to equal BKN, and full fill to be OVC.

Around the station model's central ring there can be information for up to 17 other weather elements. These include cloud types, height of the cloud base, 3-hr pressure tendency, and visibility.

24-hr prognostic chart of surface weather conditions for much of North America. These are often produced by plotting the output of computer weather models to estimate the location of fronts and pressure systems a day or two into the future.

However, except for weather maps that have been zoomed in on just a small area, or are very large in size, most of these elements are omitted in favor of being able to show clearly just a few critical ones—usually wind speed and direction, temperature, dew point and present weather conditions. A good description of the weather station model can be found on Wikipedia (en.wikipedia.org/wiki/ Station_model).

Winds at 10 m (33 ft) above the surface (the current standard height for measuring surface winds) are represented by a line (often referred to as a flagpole) resting on the rim of the station ring, pointing in the direction from which the winds are blowing. (Thus a flagpole atop the ring indicates that winds are blowing from the north.)

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