National Oceanic and Atmospheric Administration Department of Defense Federal Aviation Administration United States Navy March 1998

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Automated Surface Observing System
User’s Guide
National Oceanic and Atmospheric Administration
Department of Defense
Federal Aviation Administration
United States Navy
March 1998

The shave witnessed a carefully planned and executed modernization of the nation’s weather services. The Automated Surface Observing System (ASOS) is the first system to be operationally deployed as part of theis modernization. ASOS is therefore in the forefront of system deployments and associated service improvements that will require most of this decade to complete. In this sense, ASOS is the harbinger of st century weather services.
In the end state, ASOS will be operational at about 1,000 airports across the United States. This system is the primary surface weather observing system in the United States, which supports the essential aviation observation programs of the National Weather Service (NWS), the Federal
Aviation Administration (FAA, and the Department of Defense (DOD).
The implementation of ASOS brings with it many opportunities and challenges. The opportunities include the unprecedented availability of timely, continuous and objective observations from many more locations. The challenges generally related to institutional learning needed to fully understand and adjust operation to take the greatest advantage of this new technological resource. The potential applications of the ASOS data go beyond that of providing basic weather information for aviation and forecasting ASOS also will provide enhanced support to vital national programs such as public safety, hydrology, climatology, agriculture, and environmental protection, just to name a few. The ASOS User’s Guide is intended as basic reference and introduction to ASOS fora broad range of users.
As of this writing (March 1998), there are about 500 commissioned ASOS’s nationwide. An additional 500 are coming online in the next few years. This deployment fulfills the commitment of the Government made over a decade ago to provide the nation a highly cost-effective, capable and reliable automated weather observing system for safe, efficient aviation operations and other applications. This achievement is made possible by the dedicated effort of many people throughout the government and private industry working together as a team to conceive, plan, develop, test and evaluate, implement, commission, monitor, maintain and operate a system.
This ASOS User’s Guide is gratefully dedicated to all who have worked so hard to make
ASOS a reality. Special thanks are extended to Dr. Jim Bradley for mentoring this program from the very beginning. Finally I wish to thank Dave Mannarano for coordinating the writing and production of this ASOS User’s Guide.
Vickie L. Nadolski
ASOS Program Manager


Executive summary
Table of contents
Purpose and scope
Total surface observation
Quality control
Level at the site
Level 3—national
General conventions
Figure 1.asos quality control concept
Asos sensor groups
Acquisition control unit
Figure 2.asos combined sensor group
Asos data types
Metar elements
Automated metar vs.
Automating the objective
Ambient/dew point
Chapter three
Ambient temperature/dew point
Ambient temperature/
Figure 6.asos anemometer
Wind direction and speed
Parameter range
Wind character
Wind strengths and limitations
Pressure sensor
Pressure algorithm
Pressure strengths
Figure 7.asos pressure sensor
Heated tipping bucket (htb)
Precipitation accumulation
Metar hourly message, “prrrr” remark:
Metar 3- and hourly report, “6rrrr” precipi-
Shef minute precipitation criteria message
Shef hourly routine precipitation message
Daily and monthly cumulative precipitation totals
Automating the subjective
Automating sky condition
Cloud height indicator sensor
Chapter fourfigure 9.
Sky condition algorithm
Figure 10. example of cloud hit at 4,500 feet
Meteorological discontinuity
Table 3.cover asos cloud amount
Sky condition strengths
Criteria for reporting a meteorological discontinuity ceiling remark
Automating surface
Figure 12. example of packing effect
Figure 13. forward scatter visibility sensor
Visibility algorithm
Forward scatter sensor
Figure 14. visibility sensor—top view
Visibility strengths and
Automating present
Single site lightning sensor
Single site lightning
Automated lightning detection
Asos lightning sensor
Precipitation identification
Figure 16. the precipitation
Pi strengths and limitations
Freezing rain sensor
Fzra algorithm
Zt = (40,000 - fn) * 0.000152
Figure 17. freezing rain sensor
Obscuration algorithm
Table 6.present weather reporting
One-minute observations
Asos aviation routine
Backup and augmentation
Missing vs. non-event data
Estimated data
Examples of metar/speci reports
Example 2: full report
Example 3: surface visibility vs. tower visibility
Example 4: meteorological discontinuity sensor data
Example 5: obstructions
Example 6: obstructions vs visibility
Example 7: obstructions vs. surface visibility
Example 8: variable wind
Example 9: surface visibility and rvr, plus remarks
Example 10: surface visibility and rvr
Example 12: augmentation for severe present weather
Erupted 241255 large ash cloud extending
Standard hydrometeorological
Example i.shef minute precipitation criteria message
Shef minute precipitation
Shef hourly routine
Daily and monthly
Daily summary message
Table 9.asos high-resolution data
Chapter six6.0
Interactive video screen
Non-interactive screen
Computer-generated voice
Figure 18. asos network data flow–—initial configuration (1997)
Planned product
Chapter seven
Ice-free wind sensor
Dew point sensor
Ceilometer/sky condition
All-weather precipitation
Enhanced precipitation

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