Pilots need more than you'll see on TV
| Figure 1 |
| Figure 2 |
| Figure 3 |
| Figure 4 |
| Figure 5 |
Still, what you see on television is a good place to begin. If you've been a regular television weather viewer, especially if you spend time watching The Weather Channel, you probably have a pretty good idea of what most of the symbols on the Figure 1 map mean. It's much like those that you see on television.
For instance, television meteorologists regularly point out that the blue line with the triangles designates a "cold front." If you saw this map in Little Rock, Arkansas; Nashville, Tennessee; or Scranton, Pennsylvania, the morning it came out, you weren't surprised by the colder weather.
A cold front is the boundary between warm and colder air with the cold air pushing under the warm air to bring cooler temperatures to the surface. Such weather action along a front can cause clouds, showers, and thunderstorms. At other times--Figures 1 and 2 are examples--little happens along fronts. Green areas on both maps show where rain is falling or forecast to fall, and the white indicates where snow is falling or forecast.
If the Figure 1 map were on television, the forecaster would put it in motion and you would see the blue line move from the position in Figure 1 to its location--farther to the south and east--in Figure 2. Figure 1 is what meteorologists call a surface analysis that shows the actual weather at the Earth's surface at some time in the last one to three or so hours. Figure 2 is a forecast, in this case for 12 hours after Figure 1. Meteorologists call forecast maps and word forecasts progs, for a prognosis.
If you've been paying attention to television weather, you probably realize that the red "L" over Lake Huron in Figure 1 is the center of an area of low atmospheric pressure and that winds will be blowing counterclockwise around it.
The alternating blue and red lines from Oklahoma north and west across Montana, and across the Florida Keys, in Figure 1 are stationary fronts, which are moving slowly if at all--the one in Florida is forecast to move north over the 12 hours between Figures 1 and 2.
A red line with half circles would show a warm front, where warm air is replacing colder air--neither Figure 1 nor 2 has any warm fronts.
Areas of low pressure often form on stationary fronts and move along them. We see such an area over the Texas and Oklahoma panhandles in Figure 1, but Figure 2 shows forecasters expect it to fade. But they expect a new low to form over Montana. Low pressure moving along stationary fronts can bring clouds and precipitation, but little of that has happened or is forecast to happen in Figures 1 and 2.
You don't always see the thin gray lines in Figures 1 and 2 on television maps. These are isobars, which are lines of equal surface atmospheric pressure. For example, the "24" next to Florida's East Coast in Figure 1 shows that the pressure along the line crossing Florida is 1,024 millibars--the metric atmospheric pressure measurement. If you follow the "24" line north to where it crosses the cold front you see that the pressures decrease as you go northeast along the front, first to 1,020 millibars, then 1,016 millibars, and so on.
Isobars show you at a glance where the winds are strongest; the closer the lines, the stronger the winds. In Figure 1 winds are faster over New England than over the central part of the nation. Figures 1 and 2 are from the Meteorlogix page on AOPA Online (www.aopa.org/members/wx).
While such maps will tell a casual viewer of television weather all he or she needs, a pilot needs more information. When you begin gathering the weather information you need for a flight, you begin by finding out what's going on now.
Figure 3 is part of the National Weather Service's Weather Depiction Chart, which adds details to the Figure 1 surface chart that pilots need. Each of the circles is an airport and the amount of shading shows cloud coverage. For instance, Watertown, New York (number 1 on the map) is overcast, and the two tiny "x" marks show snow is falling steadily. The dashed line (number 2) labeled "TROF" is a trough, an elongated area of weak low pressure. (If it were a strong trough, the isobars would show it.) The open circles over most of the East show that the sky is mostly clear, but shaded or partly shaded circles near the trough shows it has generated at least some clouds.
The shaded area over Watertown shows that the area is suitable only for instrument flight rules (IFR) flying because ceilings are less than 1,000 feet or visibility less than three miles. The circle surrounding the area with no shading shows "marginal" visual flight rules (VFR) conditions with ceilings between 1,000 and 3,000 feet or visibility from three to five miles. All of the symbols used on this and other charts can be found in FAA Advisory Circular AC 00-45C, Aviation Weather Services.
Once you know what the weather is doing now, you need to turn to forecasts of what it's expected to be doing at the time of your flight.
Figure 2, the forecast surface chart, gives you a general idea of what to expect. For example, it shows that unless you are an experienced pilot with an instrument rating, you probably shouldn't think of flying in the region from Lake Michigan across western and central New York and Pennsylvania, and southwest into West Virginia and Ohio.
As with the Figure 1, the NWS produces versions of the surface forecast shown in Figure 2 with added aviation information. This is a set of maps known as the Low Level Significant Weather Prog. It comes in two parts: a surface prog showing weather at the surface--Figure 4 is an example--and a map showing the expected weather up to approximately 24,000 feet, as in Figure 5. Similar charts are available for higher altitudes.
Figure 4 is much like the ordinary surface prognosis in Figure 2. Instead of the areas of green and white showing where rain or snow are forecast, it outlines such areas in green with light blue symbols for the type of precipitation--these are hard to see on the color charts found on the NWS Aviation Weather Center's Web site.
The other part, Figure 5, shows more information that pilots need. Here are the highlights of Figure 5:
" Dashed yellow lines: Turbulence forecast. The white numbers next to a white line. For example, the 240/180 shows turbulence in the areas where the white arrows' point will be from 18,000 to 24,000 feet. The symbol above the numbers is for moderate turbulence. " Wavy blue line across the country: Freezing level at the surface. " Dashed green lines: Altitude of the freezing level. For example, the white numbers on the lines off the coasts of Florida, Georgia, and South Carolina shows it's at 12,000 feet across Florida, 8,000 feet across Georgia and 4,000 feet across South Carolina and on across the country. " Solid red lines around areas near the Great Lakes: IFR conditions. " Light blue, scalloped lines: Marginal VFR.
Looking over the various maps is only the beginning of obtaining the weather data you need for a flight. The maps give you a good overview of what's going on, but to fill in those details you need to consult the various text products covered in a standard, preflight weather briefing. If you compare Figure 2 and Figure 4 you'll see they don't quite agree. For example, Figure 4 doesn't show the low-pressure area over Montana we see in Figure 2. But, Figure 5 does show an area of marginal VFR conditions in roughly the area where Figure 2 shows the low.
Such differences represent the fact that forecasts are done by computers with needed modifications by meteorologists. The weather's inherent chaos means that any weather forecast always has a degree of uncertainty, with the uncertainty growing with time. A forecast for two hours from new will almost always be better than one for 12 hours ahead.
If you were thinking of flying in the Billings, Montana, area, where Figure 2 shows the low pressure on a stationary front, and Figure 5 shows marginal VFR, you have an even stronger reason to obtain more information. As it turned out, the terminal aerodrome forecast (TAF) for Billings at the time shown in Figures 2, 4, and 5 called for light winds--seven knots--six miles' visibility, and a ceiling 9,000 feet above the airport.
Figure 2 has something else that could make you wonder what's going on. It shows both an "H" for the center of a high pressure area, and snow (a white area) forecast for western Kentucky. Precipitation usually doesn't fall in the center of a high-pressure area. The TAF for Paducah, in western Kentucky, predicted a ceiling 6,000 feet above the airport and scattered clouds at 3,000 feet, with six miles' visibility.
As you learn more about weather you will grow more likely to spot such apparent inconsistencies. At times, they can be an indication that the weather might not be too well behaved, and that you need to keep an eye on what it might do. Following what the weather maps show even on days when you aren't planning to fly is a good way to learn about weather.
Jack Williams is coordinator of public outreach for the American Meteorological Society. An instrument-rated private pilot, he is the author of The USA Today Weather Book and The Complete Idiot's Guide to the Arctic and Antarctic, and co-author with Bob Sheets of Hurricane Watch: Forecasting the Deadliest Storms on Earth.
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