Introduction
In aviation, weather service is a combined effort of the
National Weather Service (NWS), Federal Aviation
Administration (FAA), Department of Defense (DOD), other
aviation groups, and individuals. Because of the increasing
need for worldwide weather services, foreign weather
organizations also provide vital input.
While weather forecasts are not 100 percent accurate,
meteorologists, through careful scientific study and computer
modeling, have the ability to predict weather patterns, trends,
and characteristics with increasing accuracy. Through a
complex system of weather services, government agencies,
and independent weather observers, pilots and other aviation
professionals receive the benefit of this vast knowledge base
in the form of up-to-date weather reports and forecasts.
These reports and forecasts enable pilots to make informed
decisions regarding weather and flight safety before and
during a flight.
Aviation Weather Services
Chapter 13
Observations
The data gathered from surface and upper altitude
observations form the basis of all weather forecasts,
advisories, and briefings. There are four types of weather
observations: surface, upper air, radar, and satellite.
Surface Aviation Weather Observations
Surface aviation weather observations (METARs) are a
compilation of elements of the current weather at individual
ground stations across the United States. The network is
made up of government and privately contracted facilities
that provide continuous up-to-date weather information.
Automated weather sources, such as the Automated Weather
Observing Systems (AWOS), Automated Surface Observing
Systems (ASOS), as well as other automated facilities, also
play a major role in the gathering of surface observations.
Surface observations provide local weather conditions
and other relevant information for a specific airport. This
information includes the type of report, station identifier,
date and time, modifier (as required), wind, visibility,
runway visual range (RVR), weather phenomena, sky
condition, temperature/dew point, altimeter reading, and
applicable remarks. The information gathered for the surface
observation may be from a person, an automated station, or
an automated station that is updated or enhanced by a weather
observer. In any form, the surface observation provides
valuable information about individual airports around the
country. Although the reports cover only a small radius, the
pilot can generate a good picture of the weather over a wide
area when many reporting stations are viewed together.
Air Route Traffic Control Center (ARTCC)
The Air Route Traffic Control Center (ARTCC) facilities
are responsible for maintaining separation between flights
conducted under instrument flight rules (IFR) in the en
route structure. Center radars (Air Route Surveillance Radar
(ARSR)) acquire and track transponder returns using the same
basic technology as terminal radars. Earlier center radars
displayed weather as an area of slashes (light precipitation)
and Hs (moderate rainfall). Because the controller could not
detect higher levels of precipitation, pilots had to be wary
of areas showing moderate rainfall. Newer radar displays
show weather as three shades of blue. Controllers can select
the level of weather to be displayed. Weather displays of
higher levels of intensity make it difficult for controllers to
see aircraft data blocks, so pilots should not expect air traffic
control (ATC) to keep weather displayed continuously.
Upper Air Observations
Observations of upper air weather are more challenging
than surface observations. There are several methods by
which upper air weather phenomena can be observed:
radiosonde observations, pilot weather reports (PIREPs),
Aircraft Meteorological Data Relay (AMDAR) and the
Meteorological Data Collection and Reporting System
(MDCRS). A radiosonde is a small cubic instrumentation
package that is suspended below a six foot hydrogen- or
helium-filled balloon. Once released, the balloon rises at a rate
of approximately 1,000 feet per minute (fpm). As it ascends,
the instrumentation gathers various pieces of data, such as air
temperature, moisture, and pressure, as well as wind speed
and direction. Once the information is gathered, it is relayed
to ground stations via a 300 milliwatt radio transmitter.
The balloon flight can last as long as 2 hours or more and
can ascend to altitudes as high as 115,000 feet and drift as
far as 125 miles. The temperatures and pressures experienced
during the flight can be as low as -130 °F and pressures as
low as a few thousandths of what is experienced at sea level.
Since the pressure decreases as the balloon rises in the
atmosphere, the balloon expands until it reaches the limits
of its elasticity. This point is reached when the diameter has
increased to over 20 feet. At this point, the balloon pops and
the radiosonde falls back to Earth. The descent is slowed by
means of a parachute. The parachute aids in protecting people
and objects on the ground. Each year over 75,000 balloons
are launched. Of that number, 20 percent are recovered and
returned for reconditioning. Return instructions are printed
on the side of each radiosonde.
Pilots also provide vital information regarding upper air
weather observations and remain the only real-time source
of information regarding turbulence, icing, and cloud
heights. This information is gathered and filed by pilots
in flight. Together, PIREPs and radiosonde observations
provide information on upper air conditions important for
flight planning. Many domestic and international airlines
have equipped their aircraft with instrumentation that
automatically transmits in flight weather observations
through the DataLink system.
The Aircraft Meteorological Data Relay (AMDAR) is
an international program utilizing commercial aircraft to
provide automated weather observations. The AMDAR
program provides approximately 220,000-230,000 aircraft
observations per day on a worldwide basis utilizing aircraft
onboard sensors and probes that measure wind, temperature,
humidity/water vapor, turbulence and icing data. AMDAR
vertical profiles and en route observations provide significant
benefits to the aviation community by enhancing aircraft
safety and operating efficiency through improved weather
analysis and forecasting. The AMDAR program also
contributes to improved short and medium term numerical
weather forecasts for a wide range of services including
Weather Radar Echo Intensity
Light
Moderate
Heavy
Extreme
Reflectivity (dBZ) Ranges
<30 dBZ
30–40 dBZ
>40–50
50+ dBZ
Figure 13-3. WSR-88D Weather Radar Precipitation Intensity
Terminology.
Figure 13-1. Example of a weather radar scope.
Figure 13-2. WSR-88D Weather Radar Echo Intensity Legend.
severe weather, defense, marine, public weather and
environmental monitoring. The information is down linked
either via Very High Frequency (VHF) communications
through the Aircraft Communications Addressing and
Reporting System (ACARS) or via satellite link through the
Aircraft to Satellite Data Acquisition and Relay (ASDAR).
The Meteorological Data Collection and Reporting System
(MDCRS) is an automated airborne weather observation
program that is used in the U.S. This program collects and
disseminates real-time upper-air weather observations from
participating airlines. The weather elements are down linked
via ACARS and are managed by Aeronautical Radio, Inc.
(ARINC) who then forwards them in Binary Universal Form
for the Representation of Meteorological Data (BUFR)
format to the NWS and in raw data form to the Earth Science
Research Laboratory (ESRL) and the participating airline.
More than 1,500 aircraft report wind and temperature data
with some of these same aircraft also providing turbulence
and humidity/water vapor information. In conjunction with
avionics manufacturers, each participating airline programs
their equipment to provide certain levels of meteorological
data. The monitoring and collection of climb, en route, and
descent data is accomplished through the aircraft’s Flight Data
Acquisition and Monitoring System (FDAMS) and is then
transmitted via ACARS. When aircraft are out of ACARS
range, reports can be relayed through ASDAR. However, in
most cases, the reports are buffered until the aircraft comes
within ACARS range, at which point they are downloaded.
Radar Observations
There are four types of radars which provide information
about precipitation and wind.
1. The WSR-88D NEXRAD radar, commonly called
Doppler radar, provides in-depth observations that
inform surrounding communities of impending
weather. Doppler radar has two operational modes:
clear air and precipitation. In clear air mode, the radar
is in its most sensitive operational mode because a
slow antenna rotation allows the radar to sample the
atmosphere longer. Images are updated about every
10 minutes in this mode.
Precipitation targets provide stronger return signals;
therefore, the radar is operated in the Precipitation
mode when precipitation is present. A faster antenna
rotation in this mode allows images to update at
a faster rate, approximately every 4 to 6 minutes.
Intensity values in both modes are measured in
dBZ (decibels of Z) and are depicted in color on the
radar image. [Figure 13-1] Intensities are correlated
to intensity terminology (phraseology) for ATC
purposes. [Figures 13-2 and 13-3]
Symbol indicates HIWAS
Figure 13-4. HIWAS availability is shown on sectional chart.
2. FAA terminal Doppler weather radar (TDWR),
installed at some major airports around the country,
also aids in providing severe weather alerts and
warnings to ATC. Terminal radar ensures pilots
are aware of wind shear, gust fronts, and heavy
precipitation, all of which are dangerous to arriving
and departing aircraft.
3. The third type of radar commonly used in the detection
of precipitation is the FAA airport surveillance radar.
This radar is used primarily to detect aircraft, but it
also detects the location and intensity of precipitation,
which is used to route aircraft traffic around severe
weather in an airport environment.
4. Airborne radar is equipment carried by aircraft to
locate weather disturbances. The airborne radars
generally operate in the C or X bands (around 6
GHz or around 10 GHz, respectively) permitting
both penetration of heavy precipitation, required for
determining the extent of thunderstorms, and sufficient
reflection from less intense precipitation.
Satellite
Advancement in satellite technologies has recently allowed
for commercial use to include weather uplinks. Through the
use of satellite subscription services, individuals are now able
to receive satellite transmitted signals that provide near real-
time weather information for the North American continent.
Service Outlets
Service outlets are government, government contract, or
private facilities that provide aviation weather services. Several
different government agencies, including the FAA, National
Oceanic and Atmospheric Administration (NOAA), and the
NWS work in conjunction with private aviation companies
to provide different means of accessing weather information.
Flight Service Station (FSS)
The FSS is the primary source for preflight weather
information. A preflight weather briefing from an FSS can be
obtained 24 hours a day by calling 1-800-WX BRIEF from
anywhere in the United States and Puerto Rico. Telephone
numbers for FSS can be found in the Chart Supplement U.S.
(formerly Airport/Facility Directory) or in the United States
Government section of the telephone book.
The FSS also provides inflight weather briefing services
and weather advisories to flights within the FSS area of
responsibility.
Telephone Information Briefing Service (TIBS)
The Telephone Information Briefing Service (TIBS),
provided by FSS, is a system of automated telephone
recordings of meteorological and aeronautical information.
TIBS provides area and route briefings, airspace procedures,
and special announcements. The recordings are automatically
updated as changes occur. It is designed to be a preliminary
briefing tool and is not intended to replace a standard briefing
from a FSS specialist. The TIBS service can only be accessed
by a touchtone phone. The phone numbers for the TIBS
service are listed in the Chart Supplement U.S. (formerly
Airport/Facility Directory).
Hazardous Inflight Weather Advisory Service
(HIWAS)
Hazardous Inflight Weather Advisory Service (HIWAS),
available in the 48 conterminous states, is an automated
continuous broadcast of hazardous weather information
over selected VOR navigational aids (NAVAIDs). The
broadcasts include advisories such as AIRMETS, SIGMETS,
convective SIGMETS, and urgent PIREPs. The broadcasts
are automatically updated as changes occur. Pilots should
contact a FSS or EFAS for additional information. VORs that
have HIWAS capability are depicted on aeronautical charts
with an “H” in the upper right corner of the identification
box. [Figure 13-4]
Transcribed Weather Broadcast (TWEB) (Alaska
Only)
A continuous automated broadcast of meteorological and
aeronautical data over selected low or medium frequency (L/
MF) and very high frequency (VHF) omnidirectional range
(VOR) NAVAID facilities. The broadcasts are automatically
updated as changes occur. The broadcast contains adverse
conditions, surface weather observations, PIREPS, and
a density altitude statement (if applicable). Recordings
may also include a synopsis, winds aloft forecast, en route
and terminal forecast data, and radar reports. At selected
locations, telephone access to the TWEB has been provided
(TEL-TWEB). Telephone numbers for this service are found
in the Alaska Chart Supplement U.S. (formerly Airport/
Facility Directory). These broadcasts are made available
primarily for preflight and inflight planning, and as such,
should not be considered as a substitute for specialist-
provided preflight briefings.
Weather Briefings
Prior to every flight, pilots should gather all information
vital to the nature of the flight. This includes an appropriate
weather briefing obtained from a specialist at a FSS.
For weather specialists to provide an appropriate weather
briefing, they need to know which of the three types of
briefings is needed—standard, abbreviated, or outlook. Other
helpful information is whether the flight is visual flight rules
(VFR) or IFR, aircraft identification and type, departure
point, estimated time of departure (ETD), flight altitude, route
of flight, destination, and estimated time en route (ETE).
This information is recorded in the flight plan system and a
note is made regarding the type of weather briefing provided.
If necessary, it can be referenced later to file or amend a
flight plan. It is also used when an aircraft is overdue or is
reported missing.
Standard Briefing
A standard briefing provides the most complete information
and a more complete weather picture. This type of briefing
should be obtained prior to the departure of any flight and
should be used during flight planning. A standard briefing
provides the following information in sequential order if it
is applicable to the route of flight.
1. Adverse conditions—this includes information about
adverse conditions that may influence a decision to
cancel or alter the route of flight. Adverse conditions
include significant weather, such as thunderstorms or
aircraft icing, or other important items such as airport
closings.
2. VFR flight not recommended—if the weather for
the route of flight is below VFR minimums, or if
it is doubtful the flight could be made under VFR
conditions due to the forecast weather, the briefer may
state “VFR flight not recommended.” It is the pilot’s
decision whether or not to continue the flight under
VFR, but this advisory should be weighed carefully.
3. Synopsis—an overview of the larger weather picture.
Fronts and major weather systems that affect the
general area are provided.
4. Current conditions—the current ceilings, visibility,
winds, and temperatures. If the departure time is more
than 2 hours away, current conditions are not included
in the briefing.
5. En route forecast—a summary of the weather forecast
for the proposed route of flight.
6. Destination forecast—a summary of the expected
weather for the destination airport at the estimated
time of arrival (ETA).
7. Forecast winds and temperatures aloft—a forecast of
the winds at specific altitudes for the route of flight.
The forecast temperature information aloft is provided
only upon request.
8. Notices to Airmen (NOTAM)—information pertinent
to the route of flight that has not been published in the
NOTAM publication. Published NOTAM information
is provided during the briefing only when requested.
9. ATC delays—an advisory of any known ATC delays
that may affect the flight.
10. Other information—at the end of the standard briefing,
the FSS specialist provides the radio frequencies
needed to open a flight plan and to contact EFAS. Any
additional information requested is also provided at
this time.
Abbreviated Briefing
An abbreviated briefing is a shortened version of the standard
briefing. It should be requested when a departure has been
delayed or when weather information is needed to update
the previous briefing. When this is the case, the weather
specialist needs to know the time and source of the previous
briefing so the necessary weather information is not omitted
inadvertently. It is always a good idea for the pilot to update
the weather information whenever he/she has additional time.
Outlook Briefing
An outlook briefing should be requested when a planned
departure is 6 hours or more away. It provides initial forecast
information that is limited in scope due to the time frame
of the planned flight. This type of briefing is a good source
of flight planning information that can influence decisions
regarding route of flight, altitude, and ultimately the go/no-go
decision. A prudent pilot requests a follow-up briefing prior
to departure since an outlook briefing generally only contains
information based on weather trends and existing weather in
geographical areas at or near the departure airport. A standard
briefing near the time of departure ensures that the pilot has
the latest information available prior to his/her flight.
Aviation Weather Reports
Aviation weather reports are designed to give accurate
depictions of current weather conditions. Each report
provides current information that is updated at different times.
Some typical reports are METARs and PIREPs.