February 2006Departments

The Weather Never Sleeps

Anatomy of a weather briefing

What to look at, and why

Good sense and the federal aviation regulations (FARs) demand that you learn as much as possible about the weather before flying away from an airport. Even if you're going to stay in the pattern practicing takeoffs and landings you should keep a close eye on the weather and plan to cut practice short at the first sign of danger--such as visibility starting to go downhill.

If you're planning to fly away from the "vicinity of an airport," FAR 91.103 requires that you "become familiar with" weather reports and forecasts. For most pilots, the best way to obtain a preflight weather briefing is to call the flight service station (FSS) that covers your area. You'll talk to a briefer whose profession is conveying weather information to pilots.

You can also use the FAA's Direct User Access Terminal (DUAT) system to obtain a briefing on your computer. But before doing this you should know which kinds of data to collect for even a short flight.

One way to become familiar with the kinds of weather information you need is to go to the National Weather Service's Aviation Weather Center Web site. You could also take a look at the Meteorlogix weather in the members-only section of AOPA Online; some of its maps are easier to read than those on the Aviation Weather Center site. You should use these only for practice or to get an idea of what's going on before calling flight service, however; the Web sites are not considered official sources of data.

Figure 1
Figure 1

When you call flight service, you should ask for a standard briefing. To see what this includes, look on the left side of the Aviation Weather Center Web page and click on "Standard briefing" near the bottom of the page, or consult section 7-1-4 of the Aeronautical Information Manual. Information on the Weather Center site, especially the maps, will help you to understand what the briefer is telling you.

Let's look at the kinds of information you would obtain during a preflight briefing for a flight from Jackson, Mississippi, to Norman, Oklahoma. You'll need to tell the briefer, among other things, your type of airplane, proposed flight route and altitude, what time you plan to leave, and how long you think the trip will take.

The air distance from Jackson to Norman is roughly 350 nautical miles. The Cessna 172 that you plan to fly will cruise at about 105 knots, which means the trip should take something less than three and one-half hours. You plan to take off at 1600 Zulu and arrive around 1930Z.

A standard briefing begins with adverse conditions that could affect your flight. You'll find this information in National Weather Service sigmets for weather that could affect any flight and airmets for weather that could be dangerous to small airplanes.

Figure 1 is an Aviation Weather Center map depicting the route of our Jackson-Norman flight. It shows airmets for poor visibility or low ceilings that are forecast to create instrument flight rules conditions. Jackson is on the northwest corner of such an area and the flight route clips another over Arkansas.

An FSS briefer, after looking at potential adverse conditions, might tell you that visual flight rules (VFR) flight isn't recommended. This doesn't mean you can't go; a briefer can't veto your flight. But unless you are an experienced pilot, you probably should take the briefer's advice.

In this case, the weather isn't all that bad and is likely to improve, which means we will continue to the synopsis. This is a brief statement describing the kinds of weather that could affect your flight and the current location and expected movement of this weather.

Figure 2 Figure 3 Figure 4
Figure 2 Figure 3 Figure 4

Figure 2 (p. 40) is the latest available weather depiction chart showing reported weather that could affect pilots. The blue areas show marginal VFR weather an hour before our planned departure time. The map also shows a cold front stretching across the proposed route. Since fronts often stir up dangerous weather, we'll have to see what this one is expected to do.

Figure 3 (p. 40) is part of a NWS Surface Prog Chart, which is a prognosis--a forecast--of ground-level weather features for 1800Z, which would be during our flight. We see that the cold front is expected to be east of Jackson by then and high air pressure is forecast over Oklahoma. So far, this looks like good weather for our trip.

The surface prog chart is one of the two maps in the Weather Service's "low-level SIGWX [significant weather] charts," which are produced four times a day with forecasts for 12 and 24 hours. Figure 4 is the other part, showing forecast weather of interest to pilots from the ground up to around 20,000 feet. It doesn't show anything of concern for a Jackson-Norman flight, but plenty of concerns to the north. The yellow dashed lines outline forecast turbulence. The blue lines outline areas of marginal VFR conditions with ceilings from 1,000 to 3,000 feet above the ground or visibility between 3 and 5 miles. Red lines show where even lower ceilings or visibility could create IFR conditions.

The next step in a standard briefing is a more detailed look at current weather conditions. The latest weather radar image is a good place to start, although it doesn't tell you everything you need to know about the current weather: weather radar detects precipitation--rain, snow, ice--but not clouds or fog.

To fill in what radar doesn't show check out the latest satellite imagery; and to visualize surface conditions at airports along the route, including your destination, look at METARs, the regular hourly weather reports. The Aviation Weather Center site offers you these as both coded reports and translations. Is the weather trending toward improved conditions or worsening weather? How do the METARs compare to what was forecast?

You should also check pilot reports, called pireps, to see what your fellow pilots have reported. The Aviation Weather Center site has links to maps displaying pireps. Not many were shown for the Jackson-Norman flight, but one from over northeastern Louisiana showed the air was smooth at 6,500 feet and another from over southeastern Oklahoma showed winds at 5,000 feet were from the northwest at 20 kt.

Once you take off you should plan on making regular pilot reports. Reports of good weather as well as bad are helpful to both your fellow pilots as well as forecasters.

While radar showed that no showers or thunderstorms were occurring along the planned Jackson-Norman route, or nearby where they could move onto the route, the METAR for El Dorado, Arkansas--along the route--showed clouds covering almost all of the sky only 1,600 feet above the ground. Before deciding whether to take off on a VFR flight, you'd want to see if this marginal VFR weather is forecast to improve. This is why the next step in a standard briefing is the enroute forecast.

The low-level SIGWX charts we looked at while getting the synopsis are the beginning of this, but as with current conditions we need to look in more detail--beginning with area forecasts. These are text products that give the general aviation weather picture for VFR clouds and weather conditions over areas of several states. They are issued three times a day, with amendments when needed. Each forecast covers 12 hours with a generalized "outlook" for the next six hours.

Since an area forecast gives the overall picture for a large area, you should check the terminal aerodrome forecasts, referred to as TAFs, for airports along and on either side of your planned route. For the Jackson-Norman flight, the TAF for El Dorado, Arkansas, showed that by 1600Z, the sky there should be clear and the visibility six miles. This sounds good; the low clouds should be gone by the time you fly over the area. But, to play it safe, you could check the forecasts to the north and south to see which might be the best way to divert if you begin to run into clouds.

If conditions have you thinking about diverting, or returning to your departure airport, call flight watch on 122.0 MHz and ask for a weather update. This would be a great time to give a pilot report.

The final steps in the weather part of your preflight briefing are the destination forecast and winds aloft forecast. With the forecast for the wind speeds and directions at the altitude you plan to use, you'll be able to plan your navigation.

You aren't finished yet. Before closing the briefing you should ensure you learn about any Notices to Airmen (notams) that apply to your flight. These can include invaluable information, such as temporary flight restrictions, navigation aids that are out of service, or the fact that the airport you're planning to fly to will be closed for an airshow when you expect to arrive.

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.