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strength and direction. Figure 9-32(B) illustrates a
smaller-scale convergence line flowing around a single
hill or peak and forming a line of lift stretching
downwind from the peak.
Convergence also can form along the top of a ridgeline
or mountain range. In Figure 9-33, drier synoptic-scale
winds flow up the left side of the mountain, while a
more moist valley breeze flows up the right side of the
slope. The two flows meet at the mountain top and
form lift along the entire range. If cloud is present, the
air from the moist side condenses first often forming
one cloud with a well-defined “step,” marking the
convergence zone.
As a final example, toward evening in mountainous
terrain as heating daytime abates, a cool katabatic or
drainage wind flows down the slopes. The flow down
the slope converges with air in the adjacent valley to
form an area of weak lift. Sometimes the convergence
is not strong enough for general lifting, but acts as a
trigger for the last thermal of the day. In narrow valleys,
flow down the slope from both sides of the valley
can converge and cause weak lift. [Figure 9-34]
Figure 9-31. Sea-breeze front.
9-25
Many local sites in either flat or mountainous terrain
have lines or zones of lift that are likely caused or
enhanced by convergence. Chapter 10–Soaring
Techniques covers locating and using convergence.
OBTAINING WEATHER INFORMATION
One of the most important aspects of flight planning is
obtaining reliable weather information. Fortunately,
pilots have several outlets to receive reliable weather
reports and forecasts to help them determine if a proposed
flight can be completed safely. For VFR flights,
federal regulations only require pilots to gather
weather reports and forecasts if they plan to depart the
airport vicinity. Nevertheless, it is always a good idea
to be familiar with the current and expected weather
anytime a flight is planned. Preflight weather information
sources include Automated Flight Service Stations
(AFSS) and National Weather Service (NWS) telephone
briefers, the Direct User Access Terminal
System (DUATS), and the Internet. In addition, a multitude
of commercial venders provide custom services.
The following pages give a comprehensive synopsis of
available weather services and products. For complete
details, refer to the current version of AC 00-45,
Aviation Weather Services.
AUTOMATED FLIGHT SERVICE STATIONS
Automated flight service stations (AFSS) are a primary
source of preflight weather information. Abriefing can
be obtained from an AFSS, 24 hours a day by calling
the toll free number, 1-800-WX BRIEF. The National
Weather Service may also provide pilot weather
briefings. Telephone numbers for NWS facilities and
additional numbers for AFSSs can be found in the
Airport/Facility Directory (A/FD) or the U.S.
Government section of the telephone directory under
Department of Transportation, Federal Aviation
Administration, or Department of Commerce,
National Weather Service.
PREFLIGHT WEATHER BRIEFING
To obtain a briefing, certain background information
must be supplied to the weather specialist: type of
flight planned (VFR or IFR), aircraft number or pilot’s
name, aircraft type, departure airport, route of flight,
destination, flight altitude(s), estimated time of departure
(ETD), and estimated time enroute (ETE). At
many gliderports the operator or dispatcher will obtain
the weather reports and forecasts from the AFSS or
NWS at various times throughout the day and post
them on a bulletin board for easy reference.
A
B
Convergence Line
Convergence Line
Figure 9-32. Convergence induced by flow around topography.
Figure 9-33. Mountain-top convergence.
Slow Lift
Figure 9-34. Convergence induced by flow around topography.
9-26
Weather briefers do not actually predict the weather,
they simply translate and interpret weather reports and
forecasts within the vicinity of the airport, route of
flight, or the destination airport, if the flight is a crosscountry.
Apilot may request one of three types of briefings
standard, abbreviated, or outlook.
Astandard briefing is the most complete weather briefing
and should be requested when a preliminary briefing
has not been obtained or when the proposed flight
is a cross-country. When a standard briefing is
requested the following information will be provided
by the briefer.
ADVERSE CONDITIONS—This includes the type of
information that might influence the go, no-go
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