AIM

4/3/14

7−1−70

Meteorology

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

CONDITION/OPTIONAL DATA (WIND SHEAR)

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.”

EXAMPLE

−

18010KT wind 

one 

eight zero at one zero (wind is

. . . . .

blowing from 180).

35012G20KT

wind three five zero at one two gust two

. .

zero.

(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).

EXAMPLE

−

1

/

2

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).

NOTE

−

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