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AIM CHG 2
7-1-30. International Civil Aviation
Organization (ICAO) Weather Formats
The U.S. uses the ICAO world standard for aviation
weather reporting and forecasting. The World
Meteorological Organization's (WMO) publication
No. 782 "Aerodrome Reports and Forecasts"
contains the base METAR and TAF code as adopted
by the WMO member countries.
a. Although the METAR code is adopted
worldwide, each country is allowed to make
modifications or exceptions to the code for use in
their particular country, e.g., the U.S. will continue
use statute miles for visibility, feet for RVR values,
knots for wind speed, and inches of mercury for
altimetry. However, temperature and dew point will
be reported in degrees Celsius. The U.S reports
prevailing visibility rather than lowest sector
visibility. The elements in the body of a METAR
report are separated with a space. The only exceptions
are RVR, temperature, and dew point which are
separated with a solidus (/). When an element does
not occur, or cannot be observed, the preceding space
and that element are omitted from that particular
report. A METAR report contains the following
sequence of elements in the following order:
1. Type of report.
2. ICAO Station Identifier.
3. Date and time of report.
4. Modifier (as required).
7. Runway Visual Range (RVR).
8. Weather phenomena.
9. Sky conditions.
10. Temperature/dew point group.
12. Remarks (RMK).
b. The following paragraphs describe the ele-
ments in a METAR report.
1. Type of report. There are two types of
(a) Aviation Routine Weather Report
(b) Nonroutine (Special) Aviation Weather
The type of report (METAR or SPECI) will always
appear as the lead element of the report.
2. ICAO Station Identifier. The METAR
code uses ICAO 4-letter station identifiers. In the
contiguous 48 States, the 3-letter domestic station
identifier is prefixed with a "K;" i.e., the domestic
identifier for Seattle is SEA while the ICAO identifier
is KSEA. Elsewhere, the first two letters of the ICAO
identifier indicate what region of the world and
to country (or state) the station is in. For Alaska, all
station identifiers start with "PA;" for Hawaii, all
station identifiers start with "PH." Canadian station
identifiers start with "CU," "CW," "CY," and "CZ."
Mexican station identifiers start with "MM." The
identifier for the western Caribbean is "M" followed
by the individual country's letter; i.e., Cuba is "MU;"
Dominican Republic "MD;" the Bahamas "MY." The
identifier for the eastern Caribbean is "T" followed
by the individual country's letter; i.e., Puerto Rico is
"TJ." For a complete worldwide listing see ICAO
Document 7910, Location Indicators.
3. Date and Time of Report. The date and
time the observation is taken are transmitted as a
six-digit date/time group appended with Z to denote
Coordinated Universal Time (UTC). The first two
digits are the date followed with two digits for hour
and two digits for minutes.
172345Z (the 17 th day of the month at 2345Z)
4. Modifier (As Required). "AUTO" identi-
fies a METAR/SPECI report as an automated weather
report with no human intervention. If "AUTO" is
shown in the body of the report, the type of sensor
equipment used at the station will be encoded in the
remarks section of the report. The absence of
"AUTO" indicates that a report was made manually
by an observer or that an automated report had human
augmentation/backup. The modifier "COR" indi-
cates a corrected report that is sent out to replace an
earlier report with an error.
There are two types of automated stations, AO1 for
automated weather reporting stations without a precipita-
tion discriminator, and AO2 for automated stations with a
precipitation discriminator. (A precipitation discriminator
can determine the difference between liquid and
frozen/freezing precipitation). This information appears in
the remarks section of an automated report.