Elements of a weather briefing
As with other aviation tasks, such as preparing for takeoff, you should follow a checklist.
The checklist for a weather briefing is called a standard briefing. To see what's included, we will hit the highlights of such a briefing for a theoretical trip from Columbia, South Carolina, to Memphis, Tennessee. Each of the briefing elements is in boldface type.
We'll be flying a Cessna 172, and for planning purposes we'll assume our airspeed will be 100 knots, which means the 450-nautical-mile trip should take roughly four and one-half hours if we don't have headwinds. (If we were really going to fly this trip, we definitely would include a fuel stop.)
Takeoff time from Columbia is 1:30 p.m. Eastern Daylight Time, or 17:30 Zulu time. A four and one-half hour trip would put us in Memphis at 22:00 Zulu. We should be in the Huntsville, Alabama, area near the half-way point, around 19:30 to 20:00 Zulu.
We use Zulu time because that's what used for weather reports and air traffic control. Zulu time is the standard time at Greenwich, England, and used to be called Greenwich Mean Time. When the United States is on daylight saving time, as it is for our briefing, you add four hours to Eastern Time to obtain the Zulu time; add five hours for Central time, six for Mountain, and seven for Pacific. When the United States is on standard time, you increase the time difference by one hour.
Our briefing includes reports and forecasts available from Meteorlogix in the members-only section of AOPA Online and from the National Weather Service's Aviation Weather Center.
Here are the elements of the standard briefing:
Adverse conditions: The briefer checks for any sigmets (significant meteorological information), which advise of weather potentially hazardous to all aircraft such as extreme turbulence or icing; for convective sigmets, which are for dangers associated with thunderstorms, and airmets (airmen's meteorological information), which cover weather that's potentially hazardous to all aircraft but that does not meet sigmet criteria.
The only alert the morning of our flight is for turbulence along our planned route. The briefer tells us that moderate turbulence is expected along the entire route. The flight could be bumpy all of the way. We'll see what else is going on before deciding whether to go.
The synopsis is a general look at weather conditions using maps and text. Figure 1 from AOPA Online shows the general weather across the United States a couple of hours before our briefing. The fronts, rain (in green), and snow (in white) are north of our route, but close to it. We need to learn whether the rain or snow is forecast to move over our planned route.
Figure 2, which depicts the forecast for around the time you expect to arrive in Memphis, is also part of the synopsis. Comparing it with Figure 1 shows you how the weather should evolve during your flight. Together, the two maps show that the rain and snow are forecast to move to the east, staying to the north of your route. Figure 2 also shows an area of high atmospheric pressure building up over the south-central United States, which should be good news.
For current conditions we'll supplement Figure 1 with reports of the observed weather at Columbia and Memphis as well the halfway point at Huntsville. The text reports are in an international format called METARs, which is a French acronym for routine aviation weather observation. We will also look at pilot reports, or pireps, which refer to weather that pilots have reported.
The latest report of the observed weather at Huntsville is typical across the lower Southeast this morning: Temperature, 47 degrees Fahrenheit; dew point, 28 degrees; altimeter setting, 30.25 inches of mercury; winds from 310 degrees at 22 mph, gusting to 26 mph; visibility 10 miles or more; scattered clouds at 5,500 feet above ground level. As usual there are few pilot reports, but the crew of a Boeing 737 reported moderate turbulence on approach to Birmingham.
The current conditions tell you what's happening now, but you need to know what's expected to happen at the time of your flight. For that information we turn first to the en route forecast. Your best sources are the National Weather Service's area forecasts. Six of these cover the contiguous 48 states, and they are named for a major city in their area. For your trip you need to look at the Miami and the Dallas-Fort Worth area forecasts.
Area forecasts have a synopsis section, which would be used to supplement the maps during the synopsis part of the briefing. On this day it said that the front shown across the Ohio Valley would move to the Atlantic and Gulf coasts by the next day, but without widespread clouds or low visibility.
The part of the Miami area forecast for the Piedmont (the area between the mountains and the coastal plain) of South Carolina is for scattered clouds, covering from one-tenth to half of the sky between 6,000 and 8,000 feet above sea level, and winds from the west occasionally gusting to 25 knots. In northern Georgia the NWS expects clouds from 5,000 feet to 10,000 feet with a northeast wind occasionally gusting to 28 knots.
The parts of the Dallas-Fort Worth area forecast for northern Alabama and Mississippi, and western Tennessee are similar to those for Georgia and South Carolina, mostly scatted clouds and gusty winds.
The next part of the briefing is the destination forecast.
The terminal aerodrome forecast (TAF) for Memphis from a couple of hours before to a couple hours after your expected arrival calls for winds of 15 knots gusting to 24 knots, six miles' visibility, and clouds covering from six-tenths to nine-tenths of the sky (a ceiling).
The final weather section of a standard briefing includes the winds and temperatures aloft. So far on this day a flight from Columbus to Memphis doesn't appear to be dangerous, but with the winds and the forecast of moderate turbulence, it surely doesn't look like a comfortable trip.
For the rest of the day the winds from 3,000 feet and higher all along the route of flight are forecast to be 45 to 55 knots from the west, an almost direct headwind. This would cut your ground speed roughly in half and require at least one fuel stop.
Now, you're looking at a mostly bumpy flight that would probably take 10 hours, including a landing and takeoff in gusty winds, and a night arrival. Unless you are comfortable with turbulence and flying after dark, planning to go on another day is your best bet. At this stage, you could contact a Flight Service Station and ask for an outlook briefing for the next day. You can receive such a briefing for any flight more than six hours ahead. Then, you obtain a standard briefing shortly before you plan to take off.
As it turned out, the outlook for the following day was much better with generally light winds aloft and at the surface and mostly clear skies. It would have been a much better day to fly.
Our briefing is just a sample of the kinds of weather information you need to obtain and the kinds of decisions you need to make based on that information.
Even though you can follow the links from the NWS Aviation Weather Center Web site to make sure you obtain all of the data you need for a standard briefing, you should call a Flight Service Station for the briefing unless you are very comfortable with weather reports and forecasts.
You can also use the Direct User Access Terminal Service--available from two vendors, DTN and CSC--if you are a certificated pilot or a student pilot with a medical certificate. A free service to pilots, DUATS is a source of National Weather Service reports and forecasts; you can also use it to file flight plans.
When you call a Flight Service Station for a briefing, you should tell the briefer where you're flying from and where you're going, what kind of aircraft you'll be in, what time you plan to leave and expect to arrive, and what altitude you'll be flying at.
If you are comfortable with obtaining data from the Web or other computer sources, you should still call a Flight Service Station for an abbreviated briefing before making your go/no-go decision. You should tell the briefer what information sources you have used.
As with any aspect of flying, the best way to become skillful at at obtaining weather briefings is to practice. The AOPA, Aviation Weather Center, and DUATS Web sites enable you to plan imaginary flights and practice obtaining briefings.
Planning a dream flight and obtaining a weather briefing for it on the Web is a good way to pass the time--and improve your skills--when the weather or other circumstances don't allow you to go flying.
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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