Automated Surface Observing System (ASOS) User’s Guide National Oceanic and Atmospheric Administration Department of Defense Federal Aviation Administration United States Navy March 1998 Foreward The 1990s have 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 21st 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 for a broad range of users. As of this writing (March 1998), there are about 500 commissioned ASOS’s nationwide. An additional 500 are coming on-line 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 Since the last Automated Surface Observing System (ASOS) User’s Guide was published in June 1992, numerous changes have occurred. These changes have, to the maximum practical extent, been incorporated into this updated version of the ASOS User’s Guide. These changes include the transition of observing code format from the Surface Aviation Observation (SAO) code to the Aviation Routine Weather Report (METAR) code in 1996; the implementation of new software loads into ASOS up to and including software Version 2.6; the incorporation of various sensor enhancements and improvements, including modification to the Heated Tipping Bucket precipitation accumulation gauge, the hygrothermometer, and anemometer; and incorporation of the Freezing Rain and Lightning Sensors into the ASOS sensor suite. Additional product improve- ment efforts are underway to further expand and improve the capabilities of ASOS. These efforts are also described in the ASOS User’s Guide. As of this writing (March 1998), there are about 500 commissioned ASOS’s nationwide. An additional 400 + are coming on-line in the next few years. This deployment fulfills the commitment 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 efforts of many people through- out the government and private industry working together as a team to conceive, plan, develop, test and evaluate, implement, commission, monitor, maintain and operate the 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 i Table of Contents CHAPTER ONE ..................................................................................................................................1 1.0 Introduction ................................................................................................................................ 1 1.1 Purpose and Scope .................................................................................................................... 1 1.2 Background ................................................................................................................................ 1 1.3 Total Surface Observation Concept........................................................................................... 2 1.4 Quality Control ........................................................................................................................... 2 1.5 General Conventions .................................................................................................................. 3 CHAPTER TWO .................................................................................................................................5 2.0 System Description .................................................................................................................... 5 2.1 System Components .................................................................................................................. 5 2.1.1 ASOS Sensor Groups ..................................................................................................... 5 2.1.2 Acquisition Control Unit .................................................................................................. 5 2.1.3 Operator Interface Device.............................................................................................. 9 2.2 ASOS Data Outlets ................................................................................................................... 9 2.3 ASOS Data Types ..................................................................................................................... 9 2.4 METAR Elements ..................................................................................................................... 9 2.5 Automated METAR vs. Manual METAR .............................................................................. 10 CHAPTER THREE .......................................................................................................................... 11 3.0 Automating the Objective Weather Elements.......................................................................... 11 3.1 Ambient and Dew Point Temperature ..................................................................................... 11 3.1.1 Ambient/Dew Point Temperature Sensor ..................................................................... 11 3.1.2 Ambient Temperature/Dew Point Temperature Algorithm .......................................... 12 3.1.3 Ambient Temperature/Dew Point Temperature Strengths and Limitations .................. 13 3.2 Wind ......................................................................................................................................... 14 3.2.1 Wind Sensor .................................................................................................................. 15 3.2.2 Wind Algorithm ............................................................................................................. 15 3.2.2.1 Wind Direction and Speed ................................................................................ 15 3.2.2.2 Wind Character ................................................................................................. 16 3.2.2.2a Gusts ................................................................................................. 16 3.2.2.2b Variable Wind ................................................................................... 16 3.2.2.2c Squalls............................................................................................... 17 3.2.2.3 Wind Remarks .................................................................................................. 17 3.2.2.3a Wind Shift ......................................................................................... 17 3.2.2.3b Peak Wind ........................................................................................ 18 3.2.3 Wind Strengths and Limitations ..................................................................................... 18 3.3 Pressure ................................................................................................................................... 18 3.3.1 Pressure Sensor ...........................................................................................................