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followed by a four−digit group giving the beginning
hour and ending hour of the time period during which
the thunderstorms or precipitation are expected.
NOTE−
Neither PROB30 nor PROB40 will be shown during the
first six hours of a forecast.
EXAMPLE−
PROB40 2102 1/2SM +TSRA . . . . “chance between
2100Z and 0200Z of
visibility one−half
statute mile in
thunderstorms and
heavy rain.”
PROB30 1014 1SM RASN . . . . . . “chance between
1000Z and 1400Z of
visibility one statute
mile in mixed rain
and snow.”
e. Forecast Change Indicators. The following
change indicators are used when either a rapid,
gradual, or temporary change is expected in some or
all of the forecast meteorological conditions. Each
change indicator marks a time group within the TAF
report.
1. From (FM) group. The FM group is used
when a rapid change, usually occurring in less than
one hour, in prevailing conditions is expected.
Typically, a rapid change of prevailing conditions to
more or less a completely new set of prevailing
conditions is associated with a synoptic feature
passing through the terminal area (cold or warm
frontal passage). Appended to the “FM” indicator is
the four−digit hour and minute the change is expected
to begin and continues until the next change group or
until the end of the current forecast.
A “FM” group will mark the beginning of a new line
in a TAF report (indented 5 spaces). Each “FM” group
contains all the required elements−wind, visibility,
weather, and sky condition. Weather will be omitted
in “FM” groups when it is not significant to aviation.
FM groups will not include the contraction NSW.
EXAMPLE−
FM0100 14010KT P6SM SKC − “after 0100Z, wind one
four zero at one zero, visibility more than six, sky clear.”
2. Becoming (BECMG) group. The BECMG
group is used when a gradual change in conditions is
expected over a longer time period, usually two
hours. The time period when the change is expected
is a four-digit group with the beginning hour and
ending hour of the change period which follows the
BECMG indicator. The gradual change will occur at
an unspecified time within this time period. Only the
changing forecast meteorological conditions are
included in BECMG groups. The omitted conditions
are carried over from the previous time group.
EXAMPLE−
OVC012 BECMG 1416 BKN020 − “ceiling one thousand
two hundred overcast. Then a gradual change to ceiling
two thousand broken between 1400Z and 1600Z.”
3. Temporary (TEMPO) group. The TEMPO
group is used for any conditions in wind, visibility,
weather, or sky condition which are expected to last
for generally less than an hour at a time (occasional),
and are expected to occur during less than half the
time period. The TEMPO indicator is followed by a
four-digit group giving the beginning hour and
ending hour of the time period during which the
temporary conditions are expected. Only the
changing forecast meteorological conditions are
included in TEMPO groups. The omitted conditions
are carried over from the previous time group.
2/17/05 AIM
Meteorology 7−1−69
EXAMPLE−
1. SCT030 TEMPO 1923 BKN030 − “three thousand
scattered with occasional ceilings three thousand broken
between 1900Z and 2300Z.”
2. 4SM HZ TEMPO 0006 2SM BR HZ − “visibility four in
haze with occasional visibility two in mist and haze
between 0000Z and 0600Z.”
2/19/04 AIM
Altimeter Setting Procedures 7−2−1
Section 2. Altimeter Setting Procedures
7−2−1. General
a. The accuracy of aircraft altimeters is subject to
the following factors:
1. Nonstandard temperatures of the atmosphere.
2. Nonstandard atmospheric pressure.
3. Aircraft static pressure systems (position
error); and
4. Instrument error.
b. EXTREME CAUTION SHOULD BE EXERCISED
WHEN FLYING IN PROXIMITY TO
OBSTRUCTIONS OR TERRAIN IN LOW TEMPERATURES
AND PRESSURES. This is especially
true in extremely cold temperatures that cause a large
differential between the Standard Day temperature
and actual temperature. This circumstance can cause
serious errors that result in the aircraft being
significantly lower than the indicated altitude.
NOTE−
Standard temperature at sea level is 15 degrees Celsius
(59 degrees Fahrenheit). The temperature gradient from
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