Content on this page requires a newer version of Adobe Flash Player.

Get Adobe Flash player

WX BRIEF

Surface weather charts aid pilots in predicting
storm movements

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




Surface analysis depictions of sky cover. Each included weather station is shown on a surface map as a ring, with none, part or all of the ring shaded in relation to the fraction of sky covered by cloud.

This is because stationary fronts are simply boundaries that have not moved significantly in the preceding 3 hrs. Occluded fronts—those where a cold front has overtaken the warm front, closing the central low—are often indicated in purple or, if using alternating blue and red, by having both teeth and humps pointing in the same direction.

Occasionally, you may see a dashed frontal line on a surface weather map. This is normally a blue or gray line and is a common way to indicate a weaker trough in the pressure pattern. Such troughs are not sufficient to be classified as a front, but nevertheless may produce frontal-type weather.

If a prefrontal squall line has sufficient strength, it may also appear on a surface map as a dashed trough line. A dry line may also be indicated on a surface weather chart as a dashed line.

A dry line is a front that differentiates dry air from moist air, as opposed to air of differing temperatures. These lines are common in places where moist air from a water body meets up with drier desert-sourced air—zones where supercell storms are frequently produced.

Reading the clues

Standard station model used in many surface weather charts. These graphics help meteorologists display a great deal of weather observation information in a small space on a weather map. Unfortunately, for a variety of reasons, most of these weather elements are omitted from many weather maps.

The basic meteorology that most pilots have obtained through ground school, aviation weather books and lifetimes of practical experience is enough to help use a current surface weather map to help determine what the conditions are likely to be over the next several hours.

Fronts tend to move counterclockwise around lows in the Northern Hemisphere, and clockwise in the Southern. So, in both hemispheres, a front to one's west in the middle latitudes normally signifies the approach of deteriorating conditions.

Even in the absence of station model wind barbs, we can figure out the rough cast of the wind by the airport's position relative to nearby highs and lows. At the surface, friction turns the counterclockwise airflow inward (by about 30°) around a Northern Hemisphere low, and clockwise outward from a Northern Hemisphere high.

Wind­speed can be deduced from the spacing of the isobars. The more closely spaced the isobars, the faster the wind. If a surface map shows only station models, you can quickly determine the approximate location of lows and highs by the wind barbs. They will always point toward a low pressure and away from a high pressure.

For example, a surface wind from the southeast will indicate a low roughly to the northwest of the station. However, in the vicinity of a stationary front, where there may be no nearby low, winds will generally blow parallel to the front.

Just as important is knowing where what types of clouds will occur and where precipitation will take place relative to the surface conditions. Convective clouds are most commonly found in relatively narrow bands along and just behind cold fronts, especially if the temperature or moisture (dew point) differences at stations on either side of the front are large.

The greater those temperature and moisture differences, the more unstable the air will be and the more likely the chance of thunderstorm activity. Also, plan on convection to parallel a strong cold front but as a squall line 50–200 miles ahead of the front itself. Warm frontal clouds and precipitation will be found ahead of the surface front, with the lowest clouds and heaviest precipitation occurring nearest to the front's surface position.

Stationary and occluded fronts often produce a narrower band of nimbostratus that may produce persistent drizzle—sometimes freezing—for several days. In the rear (poleward) quarter of a surface low pressure, expect the strongest winds, and in winter, blizzard conditions.

Evening surface maps can be useful for evaluating whether an airport might have a foggy morning. Espec­ially in regions where airports tend to be situated in valleys, look for nearby high pressures that generally mean clear skies and light winds, and airports that are reporting only a small difference between their temperature and dew point (generally 3° or less). Nocturnal cooling and cold air draining into the valleys can quickly saturate the surface air.

Past and future

The movement and evolution of surface weather patterns can be predicted by comparing the current surface weather map with those of the previous few hours. The differences in the positions of highs, lows and fronts will give you an idea of where those features will be in the next several hours.

Comparing the position of surface lows relative to upper air troughs will help you determine if the low is going to intensify or weaken—and therefore if the fronts will change speed or occlude. You can also look at the reported conditions at various airports over time, to determine how rapidly they may be changing in response to approaching or departing weather patterns. This information can be used to move up, reroute or delay your up­coming flight to avoid any adverse weather impacts.

Similarly, most weather services produce forecast surface weather maps, more commonly known as prognostic or "prog" charts, for 6, 12, 24, 36, 48 and, occasionally, even 54, 60 and 72 hrs in advance.

These prog charts use the output from computer forecast models, as well as input from human forecasters, to predict the future positions of the highs, lows, fronts and areas of possible adverse weather such as regions of IFR, icing, turbulence or convection. When available, prog charts will generally provide more comprehensive weather guidance than can be gained extrapolating from past weather maps.

At the least, you can use a current surface weather map as a handy reference when talking to a weather briefer. It can provide good visual clues about whether your proposed route of flight will encounter any rough weather, and can help you position any conditions noted by the briefer.

Having a prog chart for the time of your flight can be even better, since it will already contain information similar to what the briefer is using. However, as with any forecast, you should also refer to the current conditions or a current surface weather map to ensure that what conditions look like 6 or 12 hrs from now make sense given the existing conditions.

It is also a good idea to print a copy of the current weather chart (or download it onto a tablet) before taking off. This will provide a nice inflight reference for the big weather picture, especially if you should find you need to make a substantial deviation due to something unexpected. And if you do encounter some weather that was not indicated by the chart, a Pirep can help the weather forecasters to produce a better chart in the next update.

Karsten Shein is a climatologist with the National Climatic Data Center in Asheville NC. He formerly served as an assistant professor at Shippensburg University. Shein holds a commercial license with instrument rating.

3


1 | 2| 3 |