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a variety of reports. The types of reports pilots are
likely to encounter include aviation routine weather
reports, radar weather reports, and pilot reports.
AVIATION ROUTINE WEATHER REPORT
An aviation routine weather report (METAR) is a
weather observer’s interpretation of weather conditions
at the time of the observation. This report is used
by the aviation community and the National Weather
Service to determine the flying category of the airport
where the observation is made. Based on the observed
weather conditions, this determination dictates
whether the pilots will operate under visual flight rules
(VFR), marginal visual flight rules (MVFR), or instrument
flight rules (IFR) in the vicinity of the airport.
Additionally, the METAR is used to produce an aviation
terminal forecast (TAF).
Although, the code that makes-up a METAR is used
worldwide, some variations of the code used in the
United States exist in other countries. In the United
States, temperature and dewpoint are reported in
degrees Celsius, using current units of measure for the
remainder of the report.
A METAR consists of a sequence of observed weather
conditions or elements, if an element is not occurring
or cannot be observed at the time of the observation,
the element is omitted from the report. The elements
of the report are separated by a space except temperature
and dewpoint, which are separated by a slash (/).
A non-routine METAR report, referred to as a SPECI
report, is issued any time the observed weather meets
the SPECI criteria. The SPECI criteria includes initial
volcanic eruptions and the beginning or ending of
thunderstorms as well as other hazardous weather
conditions.
The METAR for Los Angles in Figure 9-48 was given
on the 14th day of the month, at 0651 UTC. When a
METAR is derived from a totally automated weather
observation station, the modifier AUTO follows the
date/time element. The wind was reported to be 140°
at 21 knots with gusts to 29 knots. The reported surface
visibility is 1 statute mile. Runway visual range (RVR)
is based on the visual distance measured by a machine
looking down the runway. In the example above, the
runway visual range for runway 36 left is 4,500 feet
variable to 6,000 feet. The weather phenomena is rain
Figure 9-47. Average Relative Humidity Panel.
9-34
(RA) and mist (BR), note the minus sign preceding the
RAindicates light rain is falling. The sky condition element
indicates there are broken clouds at 3,000 feet
above the ground. Temperature and dewpoint are
reported in a two-digit format in whole degrees Celsius
separated by a slash. In the example, the temperature is
10°C and the dewpoint is 10°C. The altimeter setting is
reported in inches and hundredths of inches of mercury
using a four-digit format prefaced by an A and is
reported as 29.90 in. Hg. The Remarks element is
included in a METAR or SPECI when it is appropriate.
If the reporting station is automated, AO1 or AO2 will
be noted. Certain remarks are included to enhance or
explain weather conditions that are considered significant
to flight operations. The types of information that
may be included are wind data, variable visibility,
beginning and ending times of a particular weather
phenomenon, pressure information, and precise temperature/
dewpoint readings. Refer to Appendix A at
the end of this chapter for further explanation of TAF
and METAR codes and references.
PILOT REPORTS
Of all of the weather reports available to pilots,
PIREPs provide the most timely weather information
for a particular route of flight. The advantage for pilots
is significant because unforecast adverse weather conditions,
such as low in-flight visibility, icing conditions,
wind shear, and turbulence can be avoided along
a route of flight. When significant conditions are
reported or forecast, ATC facilities are required to
solicit PIREPs. When unexpected weather conditions
are encountered, pilots should not wait for ATC to
request a PIREP of conditions, but offer them to aid
other pilots. [Figure 9-49]
Another type of PIREP is an AIREP (ARP) or air
report. These reports are disseminated electronically
and are used almost exclusively by commercial airlines.
However, pilots may see AIREPs when accessing
weather information on the Internet.
RADAR WEATHER REPORTS
Radar weather reports (SDs), derived from selected
radar locations are an excellent source of information
about precipitation and thunderstorms. [Figure 9-50]
The report describes the type, intensity, intensity trend,
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