Aeronautical Information Manual (AIM), page 511

Index   510 -- Page 511 -- 512


sets, separated by a "/". The first four-digit set is a
two-digit date followed by the two-digit beginning
hour, and the second four-digit set is a two-digit date
followed by the two-digit ending hour. Although
most airports have a 24-hour TAF, a select number of
airports have a 30-hour TAF. In the case of an
amended forecast, or a forecast which is corrected or
delayed, the valid period may be for less than 24
hours. Where an airport or terminal operates on a
part-time basis (less than 24 hours/day), the TAFs
issued for those locations will have the abbreviated
statement "AMD NOT SKED" added to the end of
the forecasts. The time observations are scheduled to
end and/or resume will be indicated by expanding the
AMD NOT SKED statement. Expanded statements
will include:
(a) Observation ending time (AFT DDH-
Hmm; for example, AFT 120200)

(b) Scheduled observations resumption time
(TIL DDHHmm; for example, TIL 171200Z) or
(c) Period of observation unavailability
(DDHH/DDHH); for example, 2502/2512).

5. Forecast Meteorological Conditions. This
is the body of the TAF. The basic format is:
W I N D / V I S I B I L I T Y / W E AT H E R / S K Y

The wind, visibility, and sky condition elements are
always included in the initial time group of the
forecast. Weather is included only if significant to
aviation. If a significant, lasting change in any of the
elements is expected during the valid period, a new
time period with the changes is included. It should be
noted that with the exception of a "FM" group the
new time period will include only those elements

which are expected to change, i.e., if a lowering of the

visibility is expected but the wind is expected to
remain the same, the new time period reflecting the
lower visibility would not include a forecast wind.
The forecast wind would remain the same as in the
previous time period. Any temporary conditions
expected during a specific time period are included
with that time period. The following describes the
elements in the above format.

(a) Wind. This five (or six) digit group
includes the expected wind direction (first 3 digits)
and speed (last 2 digits or 3 digits if 100 knots or
greater). The contraction "KT" follows to denote the

units of wind speed. Wind gusts are noted by the letter
"G" appended to the wind speed followed by the
highest expected gust. A variable wind direction is
noted by "VRB" where the three digit direction
usually appears. A calm wind (3 knots or less) is
forecast as "00000KT."
18010KT . . . . . wind one eight zero at one zero (wind is
blowing from 180).
35012G20KT . . wind three five zero at one two gust two
(b) Visibility. The expected prevailing visi-
bility up to and including 6 miles is forecast in statute
miles, including fractions of miles, followed by "SM"
to note the units of measure. Expected visibilities
greater than 6 miles are forecast as P6SM (plus
six statute miles).
1/ SM - visibility one-half
4SM - visibility four
P6SM - visibility more than six
(c) Weather Phenomena. The expected
weather phenomena is coded in TAF reports using the
same format, qualifiers, and phenomena contractions
as METAR reports (except UP). Obscurations to
vision will be forecast whenever the prevailing
visibility is forecast to be 6 statute miles or less. If no
significant weather is expected to occur during a
specific time period in the forecast, the weather
phenomena group is omitted for that time period. If,
after a time period in which significant weather
phenomena has been forecast, a change to a forecast
of no significant weather phenomena occurs, the
contraction NSW (No Significant Weather) will
appear as the weather group in the new time period.
(NSW is included only in TEMPO groups).


It is very important that pilots understand that NSW only
refers to weather phenomena, i.e., rain, snow, drizzle, etc.
Omitted conditions, such as sky conditions, visibility,
winds, etc., are carried over from the previous time group.

(d) Sky Condition. TAF sky condition
forecasts use the METAR format described in the
METAR section. Cumulonimbus clouds (CB) are the
only cloud type forecast in TAFs. When clear skies
are forecast, the contraction "SKC" will always be
used. The contraction "CLR" is never used in the
TAF. When the sky is obscured due to a
surface-based phenomenon, vertical visibility (VV)
into the obscuration is forecast. The format for

Meteorology 7-1-67

Page 511 of the Aeronautical Information Manual (AIM.pdf)
AIM: Official Guide to Basic Flight Information and ATC Procedures

Index   510 -- Page 511 -- 512