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WIND/ V I S I B I L I T Y /WEATHER/ SKY
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 visibility
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
sixstatute miles).
EXAMPLE-
1/2SM - 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 BECMG or TEMPO
groups).
NOTEIt
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 vertical
visibility is “VV” followed by a three-digit height in
hundreds of feet.
NOTEAs
in METAR, ceiling layers are not designated in the TAF
code. For aviation purposes, the ceiling is the lowest
broken or overcast layer or vertical visibility into a
complete obscuration.
SKC . . . . . . . . . . . . . . “sky clear”
SCT005 BKN025CB “five hundred scattered,
ceiling two thousand
five hundred broken
cumulonimbus clouds”
VV008 . . . . . . . . . . . . “indefinite ceiling
eight hundred”
AIM 2/14/08
7-1-68 Meteorology
(e) Optional Data (Wind Shear). Wind
shear is the forecast of nonconvective low level winds
(up to 2,000 feet). The forecast includes the letters
“WS” followed by the height of the wind shear, the
wind direction and wind speed at the indicated height
and the ending letters “KT” (knots). Height is given
in hundreds of feet (AGL) up to and including
2,000feet. Wind shear is encoded with the
contraction “WS,” followed by a three-digit height,
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