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Pilot's Handbook of Aeronautical Knowledge
Aviation Weather Services

Observations

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Pilot's Handbook of Aeronautical Knowledge

Preface

Acknowledgements

Table of Contents

Chapter 1, Introduction To Flying
Chapter 2, Aircraft Structure
Chapter 3, Principles of Flight
Chapter 4, Aerodynamics of Flight
Chapter 5, Flight Controls
Chapter 6, Aircraft Systems
Chapter 7, Flight Instruments
Chapter 8, Flight Manuals and Other Documents
Chapter 9, Weight and Balance
Chapter 10, Aircraft Performance
Chapter 11, Weather Theory
Chapter 12, Aviation Weather Services
Chapter 13, Airport Operation
Chapter 14, Airspace
Chapter 15, Navigation
Chapter 16, Aeromedical Factors
Chapter 17, Aeronautical Decision Making

Appendix

Glossary

Index

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), Air Route Traffic Control Center (ARTCC)
facilities, 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 radius of five miles of 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 looked at together.

Air Route Traffic Control Center (ARTCC)
The 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 only two methods
by which upper air weather phenomena can be observed:
radiosonde observations and pilot weather reports (PIREPs).
A radiosonde is a small cubic instrumentation package
which 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 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 .led 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 inflight weather observations through the Data Link
system to the airline dispatcher who disseminates the data to
appropriate weather forecasting authorities.

Radar Observations
Weather observers use four types of radar to provide
information about precipitation, wind, and weather systems.
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.

 

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