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Pilot's Handbook of Aeronautical Knowledge
Aviation Weather Services

Aviation Forecasts

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Pilot's Handbook of Aeronautical Knowledge



Table of Contents

Chapter 1, Introduction To Flying
Chapter 2, Aircraft Structure
Chapter 3, Principles of Flight
Chapter 4, Aerodynamics of Flight
Chapter 5, Flight Controls
Chapter 6, Aircraft Systems
Chapter 7, Flight Instruments
Chapter 8, Flight Manuals and Other Documents
Chapter 9, Weight and Balance
Chapter 10, Aircraft Performance
Chapter 11, Weather Theory
Chapter 12, Aviation Weather Services
Chapter 13, Airport Operation
Chapter 14, Airspace
Chapter 15, Navigation
Chapter 16, Aeromedical Factors
Chapter 17, Aeronautical Decision Making




A typical RAREP will include:
• Location identifier and time of radar observation
• Echo pattern
1. Line (LN)—a line of precipitation echoes at least
30 miles long, at least four times as long as it is
wide, and at least 25 percent coverage within the
2. Area (AREA)—a group of echoes of similar type
and not classified as a line.
3. Single cell (CELL)—a single isolated convective
echo such as a rain shower.
• Area coverage in tenths
• Type and intensity of weather
• Azimuth, referenced to true north and range, in
nautical miles from the radar site of points defining
the echo pattern. For lines and areas, there will be two
azimuth and range sets that define the pattern. For
cells, there will be only one azimuth and range set.
• Dimension of echo pattern—given when the azimuth
and range define only the center line of the pattern.
• Cell movement—movement is coded only for cells;
it will not be coded for lines or areas.
• Maximum top of precipitation and location—
maximum tops may be coded with the symbols "MT"
or "MTS." If it is coded with "MTS," it means that
satellite data, as well as radar information was used
to measure the top of the precipitation.
• If the contraction "AUTO" appears in the report, it
means the report is automated from WSR-88D weather
radar data.
• The last section is primarily used to prepare radar
summary charts, but can be used during preflight
to determine the maximum precipitation intensity
within a specific grid box. The higher the number, the
greater the intensity. Two or more numbers appearing
after a grid box reference, such as PM34, indicates
precipitation in consecutive grid boxes.
TLX 1935 LN 8 TRW++ 86/40 199/115
20W C2425 MTS 570 AT 159/65 AUTO
^MO1 NO2 ON3 PM34 QM3 RL2=

The radar report gives the following information: The report
is automated from Oklahoma City and was made at 1935
UTC. The echo pattern for this radar report indicates a line of
echoes covering 810 of the area. Thunderstorms and very heavy
rain showers are indicated. The next set of numbers indicates
the azimuth that defines the echo (86° at 40 NM and 199° at
115 NM). The dimension of this echo is given as 20 NM wide
(10 NM on either side of the line defined by the azimuth and
range). The cells within the line are moving from 240° at 25
knots. The maximum top of the precipitation, as determined
by radar and satellite, is 57,000 feet and it is located on the
159° radial, 65 NM out. The last line indicates the intensity
of the precipitation, for example in grid QM the intensity is
3, or heavy precipitation. (1 is light and 6 is extreme.)

Aviation Forecasts

Observed weather condition reports are often used in the
creation of forecasts for the same area. A variety of different
forecast products are produced and designed to be used in
the preflight planning stage. The printed forecasts that pilots
need to be familiar with are the terminal aerodrome forecast
(TAF), aviation area forecast (FA), inflight weather advisories
(SIGMET, AIRMET), and the winds and temperatures aloft
forecast (FD).

Terminal Aerodrome Forecasts (TAF)
A TAF is a report established for the five statute mile radius
around an airport. TAF reports are usually given for larger
airports. Each TAF is valid for a 30-hour time period, and is
updated four times a day at 0000Z, 0600Z, 1200Z, and 1800Z.
The TAF utilizes the same descriptors and abbreviations as
used in the METAR report. The TAF includes the following
information in sequential order:
1. Type of report—a TAF can be either a routine forecast
(TAF) or an amended forecast (TAF AMD).
2. ICAO station identifier—the station identifier is the
same as that used in a METAR.
3. Date and time of origin—time and date of TAF
origination is given in the six-number code with the
first two being the date, the last four being the time.
Time is always given in UTC as denoted by the Z
following the number group.
4. "Valid period dates and times, either 24 hours or 30 hours.
The first two digits of EACH four digit number indicate the date
of the valid period; the final two digits indicate the time.