Beginning
1 July 1996 at 0800 UTC , the United States undertook the most significant change
for observing, reporting, and coding surface weather observations and terminal
forecasts in the past forty years. Not since the early 1950s, when the present
airways code (commonly known as Surface Aviation Observation or SA code), and
Terminal Forecast (FT) codes were adopted, has there been such a major code change
for weather observations and forecasts. On 1 July 1996, the international standard
code for hourly and special surface weather observations, METAR/SPECI, took effect.
The METAR acronym roughly translates from the French as Aviation Routine Weather
Report . A special report, SPECI, is merely a METAR formatted report which is
issued on a non-routine basis as dictated by changing meteorological conditions.
The SPECI acronym roughly translates as Aviation Selected Special Weather Report
. Meanwhile, the international standard code format for terminal forecasts issued
for airports, TAF , also took effect. The acronym translates to Aerodrome Forecast
.
The Federal Aviation Administration (FAA), which determines aviation
requirements in the United States, has determined that the domestic transition
to the METAR/TAF code is vital to the standardization of these reports worldwide.
The National Weather Service (NWS) and Department of Defense (DOD) are complying
with this requirement. The benefits of having the U.S. standardize to these new
code formats are as follows. Hourly and special observations are used both as
stand alone data for the sites and as inputs to global weather models for both
analysis and forecasting. It is this global use of each small bit of information
which drives the need for standardization. Additionally, the increase in international
flights between the U.S. and other nations from more U.S. locations than ever
before lends itself to developing a more "seamless" international standard
for aviation. Moreover, standardization becomes vital for the general aviation
community for flights from the U.S. to Canada, the Caribbean Area, and Mexico.
The U.S. METAR code is described in Federal Meteorological Handbook (FMH) No.
1 "Surface Observations and Reports", while the U.S. TAF code procedures
used by the NWS are described in Weather Service Operations Manual Chapter D-31.
Both of these standards are tailored to reflect existing longstanding U.S. national
practices. For example, in order to lessen the burden on the U.S. aviation community,
a number of exceptions to metric reporting units have been filed by the U.S. Winds
will continue to be reported in knots (as opposed to meters per second), cloud
layer heights, and runway visual range (RVR) will continue to be reported in feet
(as opposed to meters), visibility will continue to be reported in statute miles
(as opposed to meters), and altimeter settings will continue to be reported in
inches of mercury (as opposed to hectoPascals). The only element that will be
converted to metric units is the temperature/dewpoint field which will be reported
in whole degrees Celsius. In order to allow for a better conversion between Celsius
and Fahrenheit, the hourly temperature/dewpoint will be in tenths of degrees Celsius
in the additive data remarks section of the METAR report from selected stations
in the U.S. This is, however, not an attempt by the U.S. to convert to metric
units in weather reporting. All other products that are issued by the NWS will
continue to be in the units (including temperature in Fahrenheit) that everyone
is used to. The biggest change in converting to METAR is the change in
the order of how elements are reported, for example the winds field (an important
aviation element) will be reported first rather than in the middle of the observation.
However, remarks and additive data will continue to be included and reported much
as they are today. The sea-level pressure that you were used to seeing in the
body of the observation will now be reported in the remarks section. While
the METAR code uses some non-English words for some present weather phenomena,
the U.S. standard for METAR was developed in a cooperative effort between the
NWS , FAA, DOD, and domestic and international aviation industry and organizations.
As in any standard developed by a multi-agency group, compromise is essential.
Some of the coding groups (e.g., GR for hail or FU for smoke) are based on French
words, but many English abbreviations have been adopted. For example, the international
abbreviations for Fog and Rain are FG and RA respectively. Essentially, the better
aspects of the international and North American codes were merged. Some
other significant changes from the currently used airways or SA code are as follows:
(1) individual elements shall not be reported if they are missing or indeterminable,
(2) METAR requires the 4 LETTER ICAO station identifier (e.g., KFTW, PANC, or
KMFR), (3) METAR has no explicit ceiling designator; the first broken or overcast
layer aloft is inferred to be the ceiling, (4) the reporting and evaluating units
for sky cover will be eighths or oktas rather than tenths, and (5) runway visual
range (RVR)¹ information shall be reported in the body of the METAR observation
rather than in remarks as it is reported in the SAO code. For more details, please
consult the FMH-1 standard.
A sample observation in the SAO code appeared
as follows:
IAD SA 1055 11 SCT E15 OVC 1/2S-F 045/33/29/2119G27/945/R04VR30
PK WND 1929/16 Whereas the same observation in the 1996 U.S. METAR
code appears as follows: METAR KIAD 081055Z AUTO/COR² 21019G27KT 1/2SM
R04R/3000FT -SN FG SCT011 OVC015 01/M02 A2945 RMK PK WND 19029/16 SLP045
T00081016 Despite all the high speed computers and communications that
we have today a weather code is still required but is nothing new. The current
SA code has been in place for over 40 years, and the conversion to METAR is a
follow-on which is not very different. As for having these products reported in
a plain language format, this is not feasible. Despite the advances in today's
technology, the communication circuits used for transmitting the large and diverse
suite of meteorological products (radar, upper air, climatological data, forecasts,
watches, warnings, outlooks, etc.) have a finite capacity. The conversion to a
plain language format for thousands of domestic and international observations
that are generated each hour of the day is impractical and would easily overwhelm
our meteorological communication circuits. However, having now standardized to
a considerable extent does allow computer programs to expand the "code"
into plain language. However, if you are getting plain language reports,
it is because the service you subscribe to (DUATS, WSI, Pan Am data, etc.) is
providing that for you. These services are well aware of the transition to METAR
and should continue providing the same service that they do today. You should
check with your weather provider and ask what their plans are for METAR. The
U.S. has been using METAR and TAF codes prior to this time. The upcoming domestic
conversion to METAR is really phase 2 of a two-phased project. Phase 1 was completed
in July 1993 when the U.S. began converting SA formatted products to METAR for
international dissemination. In addition the NWS has issued both the domestic
terminal forecast (FT) and TAF formatted forecasts for 90 of the larger international
airports in the U.S. There are over 500 additional airports where terminal forecasts
are currently produced in only the FT format. Some of the significant dates
in the changeover from SA to METAR, and from FT to TAF are as follows. On January
1, 1996, the NWS converted to the new international METAR format for international
dissemination and to the new international TAF format at 90 locations. On February
1, 1996, TAF formatted forecasts issued for an additional 12 U.S. airport locations
were instituted. The NWS completely converted to the new METAR/TAF code formats
for domestic dissemination beginning at 0800 hours UTC on July 1, 1996 . At that
time, SAs and FTs have been replaced with METARs and TAFs and SAs and FTs are
discontinued. There are also some commercially available METAR/TAF training
and education products on the market that you may wish to consider. For example,
the King Schools in San Diego, CA are now developing a METAR/TAF training video
that should be available commercially sometime this Spring. In addition, Jeppesen
Sanderson, Inc. in Englewood, CO has a chapter on the METAR/TAF code in its Private
Pilot Manual, and also has plans to produce a videotape with METAR/TAF training
information. This is by no means a complete list of such commercially available
products. We suggest that you consult aviation publications and associations for
listings of other such commercially available METAR/TAF training products. The
METAR conveys much the same information to the users as the SAO code does but
there are changes. Examples of changes are given in the following paragraphs.
The FMH-1 METAR will clearly define all observing standards under METAR and the
reader should refer to the FMH-1 METAR as is posted on this information page.
Note 1: Please note that for some interim period of time that augmented
RVR values from ASOS at level 5 airports will appear in the remarks section while
RVR from all manual sites will be in the body of the observation. Note 2: This
AUTO/COR field is new and indicates one of two things. If it is AUTO, it indicates
that the data is from an automated station with no human intervention; if it is
blank it indicates that it is either a manual station or an automated station
with an observer signed on. COR, as in the SA code, indicates a corrected observation.
|
weather.gov 
