aopa - asos

A little backgroundBack in the 1980s, the National Weather Service

(NWS), in cooperation with the Federal Aviation

Administration and the Department of Defense,

decided to make some major changes in the way

surface weather observations would be made in

the future. These observations were to be

automated, produced by sensors that would do

much of the work previously done by human

weather observers. These systems are called the

Automated Surface Observing System, or ASOS

for short. ASOS sensors sample the sky for cloud

coverage; take temperature, dewpoint, and wind

readings; determine visibility; and even detect the

present weather—if it is raining or snowing. ASOS

is installed at more than 900 airports across the

United States, where they make observations

intended for use by pilots and airport-based

weather personnel.

Pilots receive ASOS reports during preflight

briefings in the form of METARs and SPECIs, in

flight during contacts with Flight Watch, and

when approaching airports in the form of

automated terminal information service (ATIS)

broadcasts or automated messages on dedicated

ground-to-air ASOS radio frequencies.

S a f e P i l o t s . S a f e S k i e s .

S A F E T Y A D V I S O R

Technology No. 2

ASOSAutomated Surface Observing System

Designing automatedequipment to exactlymatch humanobservations isimpossible, but afteryears of development,ASOS correlatesquite closely withhuman observationsmost of the time.

Safety Advisor • Pg. 2

It’s only human to be suspicious of new technology,especially if it replaces a familiar and trustedservice. ASOS combines the new and the familiar.Human observations are made from horizon tohorizon. ASOS is designed to represent conditionsin a 5-mile radius of the installation. The majorchange in observations has to do with computerprocessing of all ASOS data and the automation ofsky condition, visibility, and present weather—observations that were made by humans in thepast. Human observers evaluate these elementsbased partly on their perception. Everyone seesdifferently and so it’s not unusual for reports ofidentical conditions to vary significantly fromobserver to observer. Designing automatedequipment to exactly match human observations is impossible, but after years of development, ASOScorrelates quite closely with human observationsmost of the time.

Automated systems provide consistency inobservations. Under identical conditions, allASOSs will report the same weather so pilots canexpect to see the same information from the samedata at any location. ASOS is superior to humanobservation in many ways, but it can be quitedifferent from what we have learned to expect. Inorder to fly safely, pilots need to learn as much asthey can about weather, and that includes anunderstanding of automated weather systems.

In the following pages, we’ll provide a briefexplanation of ASOS, its capabilities, andlimitations. At the end of this process, you’ll knowhow to include ASOS observations in your vitalpilot-in-command responsibility of assessingaviation weather.

ASOS FeaturesA. FAA Ground-to-

Air Radio

B. Wind Tower (Tilting)

C. Rain Sensor

D. Temp./Dewpoint Sensor

E. Precipitation Identification Sensor

F. Ceilometer

G. Freezing Rain Sensor

H. Visibility Sensor

A

B

C

D

EF

G

H

Maintenance

NWS (National WeatherService)

Local

Gravel Walkw

ay

Conduit

North

A change in the observations


ASOS componentsASOS’s sensors each detect a different weatherelement and issue updated data to a centralprocessing computer at the airport. Algorithms—defined computer processing steps—are used toprocess observation data and in self-test routinesthat keep track of the availability and accuracy ofASOS sensors. Whenever a sensor fails to meetinternal data quality checks or fails altogether, it istaken off-line, and a report is automaticallyforwarded to a national ASOS Operations andMonitoring Center (AOMC). This full-time facilitywill assign a technician to correct the problem.How long any ASOS element is unavailable isdetermined by the nature of the problem and theair traffic at the facility. Higher traffic installationswill receive repair priority.

ASOS sensors include the following:

• Wind speed and direction. Thisconsists of the familiar wind vaneand rotating anemometer cups. Thesesensors are very accurate, with windspeed reported to an accuracy of within 2knots or 5 percent of wind speed, whichever is greater, and wind direction reported totolerances of no more than 5 degrees of error.When ASOS reports the wind conditions, itbiases its observations toward those reported in the past 10 minutes.

ASOS also looks for gusts. ASOS continuallylooks for wind values that exceed the currentaverage speed by 5 knots and holds thosevalues for 10 minutes (the time required toevaluate wind gusts). If, at the end of the 10-minute period, the value exceeds the currentaverage wind by 3 knots, a gust is reported.

All wind sensors can freeze in certain snow/icingconditions. Frozen sensors will report zero windspeed and possibly a false wind direc-tion. An ultrasonic wind sensor is cur-rently available and can be added to theexisting ASOS wind tower with minormodification. This sensor has heatedprobes to prevent freeze-ups and containsno moving parts. It detects precise windspeed and direction and is the same

sensor that will be used in the low-level windshear alert system (LLWAS).

Because ASOS makes many more windobservations than a human observer, pilots willbenefit from more timely wind informationthan in the past.

• A laser beam ceilometer (LBC).This is used to measure cloudheight, vertical visibility, and skycoverage. A laser beam pointedvertically reflects off any cloud basesdirectly overhead, and the time for thebeam to return to the sensor is mea-sured. This is converted into a cloud basemeasurement, reported to tolerances of 100feet. The sampling rate is once every 30seconds, but a single sample is insufficient tocreate a report; 30 minutes of data arerequired to create a new observation everyminute. The observation is weighted towardthe most recent 10 minutes of data. Thisenables ASOS to report rapidly changing skyconditions.

The ceilometer is able to report no more thanthree different cloud height levels at a time.Current ceilometers measure up to 12,000 feetagl, so any clouds above that height won’t bereported. A new generation of LBCs will takereadings as high as 25,000 feet agl.

LBCs are very accurate, but the biggestdrawback is their narrow beams. If a singlecloud “parks” over the LBC for long enough, afalse cloud coverage report could begenerated. Moreover, LBCs cannot detect thepresence of any nearby hazardous cloudconditions such as cumulonimbus, funnelclouds, rotor clouds, or virga. Similarly, theycannot describe the nature of the clouds beingsensed. To an LBC, a cloud base is a cloudbase, whether it’s caused by fair-weathercumulus or a Level 6 thunderstorm.

Under certain conditions, LBCs may report alower than actual cloud height or ceiling. Thisis most likely to occur when obscurations orprecipitation is present. When the beambounces back from the falling precipitation,


such as snow, sleet, fog, or rain, instead ofcloud bases, ASOS can report cloud baseslower than they really are.

When visibility measurements drop to 1 mileor lower and, at the same time, the ceilometerdetects a smearing of the beam returns(presumably by moisture) and reports cloudheights of 2,000 feet agl or less, anotherformula kicks in that may cause ASOS to issuevertical visibility reports. Because the newMETAR format does not acknowledge the term“obscurations,” vertical visibility—abbreviated“VV”—is now used. ASOS reports verticalvisibilities in 100-foot increments but does notidentify the obscuring phenomena that reducevertical visibility.

The ceilometer lens can sometimes be affectedby dirt, dust, spider webs, bird droppings, andother kinds of contamination. Then theceilometer report on an ASOS broadcast will be“sky condition missing.”

Finally, the ASOS algorithm or formula used forthe ceilometer is predicated upon variousassumptions about the natural motions of theatmosphere. The 30-minute sample reflects theNWS’s belief that accurate, time-averaged LBCreadings—over the long run—are asrepresentative of true sky conditions as thelarge spatial samplings made hourly by humanweather observers.

Small numbers of cloud-base detectionsgenerate reports of cloud covers labeled “few”or “scattered.” “Broken” and “overcast” skiesare reported if more numerous cloud basereadings are made. ASOS sky-cover reports areweighted to favor an average of the past 10minutes’ worth of observations. In general, thishas resulted in accurate observations, but infrontal or convective weather, when cloud layersand breaks in clouds move rapidly and cloudheights quickly rise and fall, ASOS may differfrom human observations until conditionsstabilize.

How responsive is the ASOS ceilometer torapidly changing conditions? If an overcastsuddenly moves over an LBC, a report of FEW

clouds will be generated in two minutes andBKN will appear in 10 minutes. At airportsnear bodies of water or where unique weatherfrequently impacts airport operations, asecondary sensor may be installed to providean early alert to changing conditions.

An improved laser beam ceilometer capable of detecting cloud cover to 25,000 feet iscurrently in testing at Sterling, VA, by the NWSand at Charleston, SC, by the U.S. Navy. Thesesensors would be angled units that couldprovide wide area coverage and/or slant visualrange.

• Visibility. ASOS determines visibility with ascatter meter, a device that measures theamount of radiation scattered from a beamof light by particles in the air such as fog,rain, snow, or other airborne particulates.This yields a measurement of air clarity.The measurement is processed throughalgorithms designed to correlate the readingswith familiar visibility values. ASOS visibility isreported in varying increments from less than1/2 to 1 statute mile. ASOS reports thevisibility to 10 miles.

The visibility algorithms are designed torespond quickly to decreases in visibility andmore slowly to increases. For example, if asudden fog bank moves over an airfielddropping the visibility to near zero, ASOS willdrop below VFR (3 miles) to 2 miles in oneminute, 1 mile in two minutes, and to less than1/2 mile in three minutes. If conditions rapidlyimprove to above 3 miles, ASOS will go fromless than 1/2 mile to 2 miles in nine minutesand over 3 miles in 10 minutes.

Tests have shown that the ASOS scatter meter’saccuracy correlates well with the transmis-someters used to report runway visual ranges(RVRs), agreeing 80 percent of the time towithin plus or minus 1/2 mile, with RVRreadings up to 1 1/2 miles. This is good newsfor pilots who rely on RVR readings forapproaches in instrument conditions.

Under certain conditions, the scatter meter cangive erroneous readings, though. A localized


patch of ground fog around the sensor cancause ASOS to report low visibility when in factthe sky is clear, or there could be situationswhere a low layer of ground fog might exist,but visibility above the scatter meter isunlimited. For this reason, great care is takento site ASOS in a representative location on the airport.

Bright sunlight can also affect the representa-tiveness of visibility reports in certain reduced-visibility situations. One NWS publication saysthat if it’s bright enough to wear sunglasses andthere’s haze or thin fog with a clear sky above,halve the ASOS visibility report. In dense fog,ASOS reports correlate well with humanobservations.

Multiple sensors are being used in some areaswith varying weather conditions—such as onthe coast—to increase the accuracy ofobservations. For example, where fog typicallyaffects an airport that is located at a coastal site,an additional sensor group would enablecomparison of the weather on one side of theairport with weather on the other side todetermine if fog was rolling in. The differencewould then be reported in the remarks sectionof the weather report.

• Barometric pressure/altimeter setting.ASOS contains either two or three pressuretransducers for providing barometric pressure.At all sites, at least two sensors must agreewithin at least 0.04 inch of mercury to reportpressure. The lowest pressure reported by allthe working sensors is transmitted. ASOSreports ambient pressure, altimeter, densityaltitude, station pressure, pressure altitude, andpressure falling or rising rapidly. A newpressure value is computed every minute.

• Temperature. Temperature readings are made bya thermistor, a type of electronic thermometer.Readings are generally very accurate, and theupdate cycle is once per minute.

• Dew point. Dewpoint measurements aremade by a hygrothermometer. A fan drawsambient air into its housing and over a mirrorelectronically cooled to a lower than ambient

temperature. The cooling process continuesuntil the dewpoint temperature is reached, at which time a layer of dew forms on themirror. A laser beam and detector are used to detect dew formation. At that time, themirror temperature is read and recorded asthe dewpoint temperature.

Contamination of the mirror by dirt, dust,sand, corrosion, spider webs, and otherforeign material will cause erroneous readings.If conditions are right, the sensor may also“ice up” when dew points are around freezingand the cooling and heating of the mirrorcannot completely remove the ice from themirror. Newer, capacitance-typehygrothermometers are now being tested andmay be phased into ASOS sites in the future.

• Precipitation identifier. Also known as alight-emitting-diode weather identifier, orLEDWI, this device measures the passageof particles through a sensor beam. Muchof this instrument’s logic is tied intocalculations relating to fall velocitypatterns. Snow falls slowly and makesone type of pattern. Rain falls at a fasterrate, creating another signature.

The LEDWI was designed only to report theoccurrence of rain or snow at precipitation fallrates of .01 inch per hour or greater. Light rain,light snow, and blowing snow are frequentlyunreported because their fall rates are usuallybelow the threshold—although blowing snowhas been reported as rain or snow. Pilotsshould suspect snow or freezing rain whenprecipitation is reported and the temperatureis at or near freezing.

What happens when other phenomena passthrough the beam? Sleet may be reported asrain, hail may be reported as rain, andblowing snow may be reported as snow orrain, depending on the wind speed and flakesize. Insects or even spider webs bobbing inthe wind can sometimes trigger a light rainreport. The development of new algorithmsshould allow further refinement of thereported elements.


The algorithms for reporting blowingsnow and ice pellets have beenintegrated into an ASOS softwaremodification that is currently beinginstalled in all of the commissionedASOSs. An enhanced LEDWI sensorunderwent testing in winter, 1998.Modification to the existing LEDWIsensor includes an acoustic channel todetect ice pellets and a reduced aperture forinfrared signals that will allow detection ofdrizzle size droplets. Further development ofthis sensor is expected to enable the reportingof hail. Additional software modification willrelate the LEDWI and freezing rain sensors todetermine freezing drizzle. This enhancedsensor is expected to complete testing by 2000.It is anticipated that the NWS will implementthis modification by 2005.

• Freezing rain. ASOS detects freezing rain bymeans of a small vibrating probe. Nearly identicalto the ice detectors found on many newer, largergeneral aviation and airline airplanes, this probemeasures a change in its resonant frequencywhen an additional mass adheres to it.

The freezing rain sensor updates its informationat one-minute intervals. After the probe hasaccumulated one-tenth inch of ice, a signal issent that heats the probe and melts the ice.Then the probe is ready to sound the alarmonce again, should icing conditions persist orrecur.

Once ASOS’s freezing rain probe freezes up, itcontinues to report freezing rain until the layerof ice melts. The accuracy of subsequentreadings depends on the rate of the probe’scooling. Once the probe’s temperature dropsto ambient, a new sensing cycle begins.

• Lightning. Lightning detectiontechnology is being added to ASOS.There are currently 24 airports that have“fourth generation” Single PointLightning Sensors installed, tested andcertified by the National WeatherService. These are now an integralpart of the ASOS reporting process.These Single Point Sensors utilize

electric and magnetic field detection alongwith an optical channel. This type of sensor iscapable of detecting cloud-to-cloud and intra-cloud lightning, which comprise half of all lightning activity. At the present time, theSingle Point Sensors report lightning strikesdetected within 5-10 miles. Modification isexpected late 1999 to add azimuth reporting.Until it is modified to report azimuth of thelightning strike, it will not report any strikesbeyond 10 miles.

A second source of lightning detection forASOS is currently being tested. AutomaticLightning Detection and Reporting System(ALDARS) is anticipated to be employed at theFAA in the near future. This system gathersdata from a commercially available lightningdetection network of more than 140 sitesaround the country known as the NationalLightning Data Network (NLDN). The ASOSsites currently need computer softwareupgrades to enable collection of the NLDN.The FAA has indicated that the first ASOS unitto be modified to accept ALDARS will besometime in 1999.

ASOS’s strengthsASOS sensors—when all is well—are extremelyprecise, and the system’s deficiencies are wellknown and being addressed. At the largest, busiestairports—and some of the less congested ones—human observers augment ASOS observations.Augmentation consists of oversight of ASOSgeneral operation, as well as editing of anyunrepresentative observations.

ASOS works 24 hours a day, seven days a week (itdoes, however, call in sick once in a while). ASOSalso provides a more constant stream of data atmore locations than human observers. Thisbenefits the forecast and research communitiesand promotes more accurate forecasts of allkinds—not just those applying to aviation weather.ASOS sensors also perform well at night, a difficulttime for human observers to make accurateobservations.


Spreading the wordEach ASOS is equipped to transmit weatherinformation via voice or computer throughtelephone or on VHF radio frequencies. Theofficial hourly and special weather reports for thefield are automatically transmitted to NWS andFAA communication systems. These are the ASOSMETARs and SPECIs pilots receive from flightservice or DUATS. Pilots can also dial the ASOSphone number to hear a current weatherobservation, or they can tune the ASOS radiofrequency to hear the latest ASOS weather.

FAA towers or other facilities on the field can havea direct computer line from ASOS to a VisualDisplay Unit (VDU). The VDU will display currentweather and the most recent hourly METAR orSPECI. That’s the official weather for the field,and that’s what will be used to record the ATIS.Controllers or flight service personnel use acombination of ASOS and other sensors whenreporting weather conditions to pilots.

If the weather is changing but not changingenough to force a SPECI report, the ATISinformation you hear, although derived fromASOS, could be up to an hour old, and that cancause pilots to doubt ASOS’s veracity. At fieldswith no ATIS or FAA facility, you’ll hear the up-to-the-minute ASOS report.

Human observationWeather observers make their observationsaccording to rules and methods, some of whichwere established long ago. Observations areordinarily made every hour, with the observationprocess beginning at 45 to 50 minutes after thehour. Ten minutes is the usual time needed tomake the weather observations, record them, andpass them along to the NWS for distributionthroughout the rest of the weather-reportingnetwork—including the FAA. Specialobservations— SPECIs—are made and reportedwhenever certain weather conditions (e.g., anabrupt drop in visibility or ceiling) change rapidlyenough to warrant special mention.

Observers use their trained eyes, ears, and othersenses to ascertain the prevailing weather. Theyscan the sky and make a judgment about thenature of the sky cover: Is it scattered, broken, orovercast? They look for any menacing clouds, suchas a thunderstorm’s cumulonimbus or rotor clouds.They listen for thunder to confirm a thunder-storm’s presence. They look at objects situated at a known distance from the observation site todetermine visibility. They look at the sky andground to determine the type and nature of anyprecipitation. In short, they and ASOS follow theweather observer’s bible—Federal MeteorologicalHandbook Number 1, or FMH-1—to make theirreports to government-issue standards.

Human vs. ASOS observations


Unlike ASOS’s time-based sampling, humanobservers make judgments on a larger, spatialscale that take in more variables and are open tothe subjective inclinations of each individualobserver. Still, humans do share commonexperiences as they are trained as observers, thenrefine their skills by working as apprentices withmore-experienced observers. However, becauseeach human observation is a judgment call, anelement of error can creep in. There arelimitations to human observation—just as thereare limitations to ASOS. Studies have shown thatvisibility disagreements between human observersoccur as much as 40 percent of the time; with skycover, the disagreement is somewhat less, at 15percent. Often, there’s never complete agreementbetween human observers.

What ASOS doesn’t measureASOS makes pinpoint weather observations anduses complicated mathematical algorithms toextrapolate conditions around the ASOS location.The ceilometer only “sees” the sky directly aboveits beam. If the sky’s only cloud is parked abovethe LBC for 10 minutes or more, it will likelyreport a broken or overcast sky. Because it can’tsee the sky all around it, the ceilometer cannotmake judgments of sky cover.

The scatter meter measures a parcel of air about.75 cubic feet in volume, and it senses thediffusion of the light striking the particulates inthat parcel. It cannot make judgments aboutprevailing, sector, tower, or flight visibility, and itcan’t make RVR measurements.

At this writing, ASOS will not report blowing dust,blowing sand, tornadoes, funnel clouds, hail, icecrystals, drizzle, freezing drizzle, thunderstorms,smoke, amount of snowfall, snow depth, hourlysnow increase, or clouds above 12,000 feet.Many of these capabilities may be added over thenext several years as the system matures.

What ASOS does measureASOS does directly measure wind speed anddirection, barometric pressure, temperature, and

dew point. In sites equipped for lightningdetection, ASOS will tell if lightning is nearby butwill not issue bearing and distance information.

What veteran observers sayExperienced weather observers familiar with ASOSsay that it can, depending on the nature of theweather phenomenon, issue unrepresentativereports when IFR and low IFR conditions prevail.Whenever this happens, observers at ASOS sitesmust monitor the ASOS’s Operator InterfaceDevice (OID), compare the ASOS report therewith their own observation, edit unrepresentativereadings, and then enter the corrections on theOID using a keyboard.

When weather conditions are changing rapidly, asis so often the case with frontal or convectiveweather, the need to augment/correct ASOSreadings rises.

Pilots and weather observers agree that ASOS ismost accurate when stable, non-frontal weatherprevails. The more stable the cloud cover, thebetter. When it comes to visibility measurements,isolated fog conditions cause the most problems.Fog that extends above the visibility sensor canyield low visibility reports, while fog confined tothe runway will go undetected by ASOS.

Experienced weather observers who have spentmonths and years augmenting ASOS observationssay disagreement is most likely when moisture-laden fronts move quickly by an airport. They sayASOS can report confusing up-and-down cloudheights as layers of clouds at different altitudespass overhead and the ceilometer reads the heightof each passing cloud.

One reason for this disagreement may simply beASOS’s ability to make many more observationsthan human observers. Observers can make nomore than one observation each 10 minutes or amaximum of six observations in one hour. In thathour, ASOS will make up to 12 observations.When the weather is changing, ASOS will reportthose changes more frequently than a humanobserver would, and that can be confusing forpilots who are used to hearing only one or twoobservations per hour. The trick is for pilots to


monitor ASOS prior to arrival at the airport. Thatway, you can get a general picture of what theweather is doing, and if it’s changing rapidly, you’ll likely see some trends, as well.Observers also say that the precipitation reportscan be misleading when rain or snowshowersbegin. If showers are very local, it can be rainingat the runway and dry at the ASOS. Convectiveweather poses similar difficulties. Couple this withthe fact that ASOS can’t issue tactical thunder-storm information, and it becomes clear thatpilots must supplement ASOS observations whenthunderstorms are in the picture. Because ATCradar is not designed to detect weather, controllerscan’t be much help in finding ways betweenclosely packed thunderstorms. ATC can tell you,workload permitting, if there is significantprecipitation near the airport, how far away it is,and what direction it’s moving. They can alsoforward relevant pilot reports.

Levels of ASOS serviceThe FAA, NWS, and the aviation industry haveestablished four levels of ASOS service: A, B, C,and D.

Service Level D is provided by stand-alone ASOSunits located at more than 450 smaller, usuallynontowered airports with lower traffic counts.There is no human augmentation or backup ofmissing ASOS reports at Level D sites. Only thebasic weather elements of wind, visibility,precipitation, obscurations, sky conditions,temperature, dewpoint temperature, altimeter,and freezing rain are reported at these sites.

Service Level C is located at approximately 270full- and part-time towered airports. These ASOSswill offer basic Level D service, but when humanobservers are available (i.e., when the tower isopen), they will provide additional weather reportsas part of the ASOS transmission. These include

• Thunderstorms,• Tornadoes,• Hail,• Virga,• Volcanic ash, and• Tower visibility.

In the case of part-time towers, ASOSaugmentation stops when the controllers go home,then the airport reverts to Level D service.

Service Level B ASOSs are installed atapproximately 60 towered airports where contractweather observers are available 24 hours a day toprovide augmentation and backup. Towerpersonnel will provide tower visibility. Level Bsites have all the features of levels C and D, plusthe following additional observer-providedaugmentations:

• RVR (runway visual range),• Freezing drizzle versus freezing rain,• Ice pellets,• Snow depth and “snow increasing rapidly”

remarks,• Thunderstorm and lightning location

remarks, and• Any observed significant weather not at

the station.

Service Level A ASOSs are installed at 81 majorairports, some of them in or near Class B airspace.In addition to Level B service, these installationshave human observers who may report thefollowing phenomena:

• 10-minute RVR reports, or if no RVR is available, additional visibility increments of 1/8 mile, 1/16 mile, and 0 miles,

• Sector visibility,• Variable sky conditions,• Cloud layers above 12,000 feet

and cloud types,• Widespread dust, sand, and other

obscurations; and• Volcanic eruptions.


Ranking of ASOS service levelsThe decision regarding which airports receivewhich level of service is based on a rankingprocess. The airports chosen to receive higherservice levels are those that routinely experiencesignificant weather and higher traffic counts, arefarther from suitable alternate airports, and havecertain airport characteristics. Factors such as abusy tower or that an airport is a hub or that it islocated in mountainous terrain, for example, canbias an airport toward a higher service level.

Scores are assigned to each of these variables, andcomposite scores are developed. Those that comeout higher on the list receive higher service levels.For this reason, some airports with service levels Aor B might not always be large airports in busyterminal areas. Instead, they might be relativelyremote but known for their high number of dayswith thunderstorms or freezing precipitation.

ASOS is not AWOSThe acronyms may sound the same, and theymight perform some of the same functions, butAWOS (Automated Weather Observation System)is not the same as ASOS. ASOS is a moresophisticated observing system with a higher levelof computer processing and more quality controlchecks. ASOS is totally federally funded. AWOS isprimarily funded at the state or municipal level,although there are a handful of federally fundedAWOSs.

AWOS comes in four levels of service:• AWOS-A only reports altimeter setting.• AWOS-1 usually reports altimeter, wind data,

temperature, dew point, and density altitude.• AWOS-2 adds visibility reporting to the

AWOS-1 reporting features.• AWOS-3 reports everything AWOS-2 does,

plus cloud and ceiling data.

How to use ASOSPilots today face more challenges in obtaining andinterpreting weather information than ever before.The widespread availability and use of aviationweather data from private vendors means thatmountains of reports and images are there for theasking—all for just a few keystrokes on a computerkeyboard. Similarly, the FAA-endorsed Direct UserAccess Terminal System (DUATS) lets pilotsdownload reams of weather information, as wellas file flight plans and perform other route-planning chores via a number of computerprograms.

This is a double-edged sword. We have moreinformation, but we also have the responsibilityand need to interpret more raw data. In short,pilots have to train themselves to become morelike meteorologists in interpreting weather reportsand forecasts. Although the flight service station(FSS) network, with its telephone briefers, willprobably always be there to perform someinterpretive tasks for pilots, it’s no secret that thenumber of FSSs has been cut back. Consequently,it can be difficult to reach FSS briefers in a timelymanner when the weather is changeable andmore pilots want longer telephone briefings.


ASOS strategies andconsiderationsObtain a complete preflight weather briefingprior to departure. This goes without saying but isparticularly important when planning a trip to anairport with ASOS Service levels C and D. Forexample, if you are concerned about elementssuch as freezing drizzle, you must realize that thiselement won’t be reported at D and some C levelsites. By keeping close tabs on the forecastweather, you can compare ASOS reports as younear the destination airport and tune in the ASOSfrequency. If you don’t know whether the ASOS atyour destination or alternate is augmented byhuman observers, then it’s certainly worth findingout during the preflight briefing.

A good synoptic weather brief is essential. Know the big picture before takeoff. Thencompare this information with enroute weatherreports to verify accuracy of both forecasts andMETARs.

Look for an AUTO prefix to a METAR. Thismeans that the observation is fully automated andhas no augmentation by either weather observersor ATC personnel. Level D sites will always showthe AUTO prefix. When part-time towers arestaffed at Level C sites, the AUTO prefix isremoved when a controller signs on for duty.

Is IFR or MVFR weather forecast? If so, be waryof unaugmented ASOS reports, and be prepared—as always—for a missed approach and trip to analternate.

Contact Flight Watch (122.0 MHz) and obtainpireps en route. These are excellent tactics forupdating conditions all along the route of flight. Bycomparing near-real-time pilot and briefer reportswith forecast weather and ASOS reports, you’ll bebetter able to judge whether a forecast wasbotched, or if ASOS reports might be misleading.Pireps may also fill in those missing areas ofinformation along a route where weatherinformation is sparse.

Don’t depend too much on pireps, though.Pireps are most useful when weather is bad,

and that’s just the time when pilots and controllersare at their busiest. Many, if not most, pilots flyingIFR at the lower altitudes are flying withoutcopilots. You may be too busy to forward a pirepwhen it’s most needed, but you should try—aslong as safety will permit—to do so.

Check the latest METARs for missing orunavailable elements from ASOS sites.Especially at D level sites where observers are not available to back up the ASOS, it’s wise todetermine if all weather elements needed areavailable. Contact Flight Watch for the latestobservation and listen for missing information. If a sensor is out of service, information will beomitted from the observation. For some sensors,you’ll hear or see remarks such as:

RVRNO The RVR sensor is not operating.PWINO Present weather sensor is not

operating.FZRANO The freezing rain sensor is not

operating.TSNO Thunderstorm information is not

available (sensor may be broken or observer unavailable)

VISNO Second site visibility sensor is not operating. (VISNO RY12 indicates the runway location of the failed sensor).

CHINO Second site cloud height indicator is not operating. (CHINO RY24L indicates the runway location of thefailed sensor).

The missing sensor may be just the one you needto provide the information for a safe landing. Youwouldn’t want to fly into a site with missing skyconditions if you expect low ceilings andprecipitation.

Know what observation you’re using. All theASOS observations you get through DUATS, flight service, ATC, private vendors, or from FlightWatch are the latest “transmitted” METAR orSPECI. METARs are usually transmitted about 5minutes before the hour. A SPECI is transmittedevery time a major change occurs in weatherconditions that could impact the level of serviceat an airport.


The only time you’ll receive the one-minuteobservation, updated minute-by-minute, is whenyou’re accessing Service D sites or at Service LevelC sites when the tower is closed and no observeris available. At towered sites, the latestobservation will be the last transmitted METAR orSPECI, not the latest one-minute observation.Therefore, it’s important to note the time of theobservation. Even in the era of automatedweather, you may still be using data nearly anhour old.

Contact ATC radar facilities for thunderstorminformation. Although ATC radar is intended fordetecting aircraft, controllers, especially inapproach control facilities, can see some rain.They are also talking to pilots of radar-equippedairplanes who may be deviating aroundconvective weather. Air traffic controllers can,work load permitting, be very helpful in weather-avoidance tactics. They are understandablyreluctant to assist with weather penetration. Thatis best left for fully equipped airplanes flown byproficient instrument pilots.

Tune in early and often. Punch in the ASOSfrequency as far from the airport as the signal’sstrength permits, then listen for several minutes.This is especially important when MVFR or IFRconditions are expected or forecast. Update theASOS report at least twice en route and makenotes on the weather. This way, you can compareobservations and develop valuable trendinformation.

Tune other ASOS frequencies in the area.Because ASOS is available in many places wherethere was previously no weather information,there will usually be one or two in the neighbor-hood. Comparing their observations with those of the ASOS at your destination will give you amore complete weather picture. You’ll also beable to spot any inconsistencies in the observa-tions that may indicate a sensor problem.

Count on accurate wind, altimeter, andtemperature information. The instrumentsproviding this information have proven very

accurate and reliable. At Level D airports withinstrument approaches, this information alone maylet you descend to lower minimums—and mightmake the difference between successfullycompleting an approach or having to perform amissed approach. The wind speed and directioninformation lets you make a logical runwaychoice—always a critical decision, but even moreso when flying a circling approach in instrumentweather conditions. The value of this informationcan be, well, invaluable.

Ask ATC if missed approaches are beingperformed. If the ASOS reports above minimumsbut pilots are missing approaches, it’s a fair betthat ceilings and visibilities are lower than theASOS readings. Conversely, if ASOS reports belowminimums but pilots are landing without difficulty,the observation may be conservative.

Look for SPECI observations. A SPECIobservation is an unscheduled METAR, issuedwhen weather worsens or improves beyondvarious thresholds in specified periods of time. ASOS tends to transmit more SPECI observationsthan human observers and often leads humanobservers in reporting critical conditions—especially at night.

Finally, learn METARs and TAFs if you haven’talready. ASOS broadcasts are made in the METARformat, with wind direction first, followed byvisibility, present weather type, sky condition,temperature/dew point, and altimeter.

Report anomalies. Consistent unrepresentativeASOS reports can indicate siting problems andshould be reported to flight service or ATC. Self-testing equipment is supposed to report hardwareproblems automatically, but it doesn’t hurt toreport these, as well. The ASOS Operations andMonitoring Center (AOMC), which monitorsoutages and component failures, can be reachedat 800/242-8194, 24 hours a day.


ASOS UpgradesUpgrade Added Capability Benefits

Lightning sensors Augments cloud-to-ground Reduces workload of controllersstrikes with cloud-to-cloud and at Level C airports; reduces needin-cloud lightning reports. for contract weather observers

(CWOs) at Level B airports.

Replace dew cell sensor Provides direct measurement of Replaces sensor that isrelative humidity that can be responsible for a large percentageconverted to dew point reports. of maintenance calls.

Ultrasonic wind sensor Provides ice-free wind speed/ Eliminates freeze-ups anddirection sensor with no moving preventive maintenance calls.parts.

Additional ceiling and Improves representativeness of Reduces workload of controllersvisibility sensors ceiling/visibility observations. at Level C airports and reduces

need for CWOs at Level B airports.

Enhanced LEDWI Provides reports of hail, Enhances safety; reducesdrizzle, and ice pellets. workload of controllers at Level

C airports and reduces needfor CWOs at Level B airports.

25,000-feet ceilometer Allows reports of cloud cover Improves aviation communityto 25,000 feet. acceptance. Reduces work load

of controllers at Level C sites and reduces need for CWOs atLevel B airports.


TYPE

OF

REPO

RTST

ATIO

N

IDEN

TIFI

ER

DAT

E AN

D

TIM

E O

FRE

PORT

REPO

RT

MO

DIF

IER

WIN

DD

IREC

TIO

NAN

D S

PEED

VISI

BILI

TY

RUN

WAY

VIS

UA

L RA

NG

EW

EATH

ER

PHEN

OM

ENA

SKY

CO

ND

ITIO

NTE

MPE

RATU

RE/D

EW P

OIN

TA

LTIM

ETER

SE

TTIN

G

KEY

TO A

SOS

(Aut

omat

ed S

urfa

ce O

bser

ving

Sys

tem

) WEA

THER

OBS

ERVA

TIO

NS

MET

ARKH

TM23

1351

ZAU

TO34

009G

14KT

3/4S

MR3

2/50

00FT

- RA

FGO

VC00

523

/22

A299

2

TYPE

OF

REP

ORT

:M

ETA

R: H

ourl

y (s

ched

uled

) rep

ort.

SPEC

I:Sp

ecia

l (un

sche

dule

d) r

epor

t.ST

ATI

ON

ID

ENTI

FIER

:IC

AO

(Int

erna

tiona

l Civ

il A

viat

ion

Org

aniz

atio

n) lo

catio

n id

entif

ier

(usi

ng fo

ur a

lpha

betic

cha

ract

ers)

.D

ATE

AN

D T

IME

OF

REP

ORT

:A

ll da

tes

and

times

in C

oord

inat

ed U

nive

rsal

Tim

e (U

TC)

usin

g a

24-h

our

cloc

k; tw

o-di

git d

ate

and

four

-dig

it tim

e; le

tter

Z a

ppen

ded

to in

dica

te U

TC.

REP

ORT

MO

DIF

IER

:A

UTO

:Fu

lly a

utom

ated

rep

ort.

No

hum

an in

terv

entio

n.C

OR

:C

orre

ctio

n to

a p

revi

ousl

y di

ssem

inat

ed r

epor

t.W

IND

DIR

ECTI

ON

AN

D S

PEED

:W

ind

dire

ctio

n in

tens

of d

egre

es fr

om tr

ue n

orth

(thr

ee d

igits

);Sp

eed

in w

hole

kno

ts (t

wo

digi

ts);

Gus

ts (G

) fol

low

ed b

y m

axim

um o

bser

ved

spee

d; fo

llow

ed b

y K

T to

indi

cate

kno

ts;

0000

0KT

for

calm

. Var

iabl

e w

ind

repo

rted

whe

n di

rect

ion

vari

es b

y 60

deg

rees

or

mor

e w

hen

the

aver

age

win

d sp

eed

is

grea

ter

than

6 k

nots

. Var

iabl

e w

inds

less

than

or

equa

l to

6 kn

ots

are

show

n w

ithou

t deg

rees

: VR

B05

KT.

Exam

ple:

210

16G

24K

T 18

0V24

0 =

Win

ds fr

om 2

10

degr

ees

at 1

6 kn

ots

with

gus

ts to

24

knot

s. W

ind

dire

ctio

n va

ries

from

180

to 2

40 d

egre

es.

Not

e:W

ind

dire

ctio

n is

rep

orte

d fr

om m

agne

tic n

orth

in

grou

nd-t

o-ai

r br

oadc

ast a

nd te

leph

one

dial

-in

mes

sage

s.V

ISIB

ILIT

Y:V

isib

ility

in s

tatu

e m

iles

and

frac

tions

(spa

ce b

etw

een

who

le

mile

s an

d fr

actio

ns);

alw

ays

follo

wed

by

SM to

indi

cate

sta

tute

mile

s; v

alue

s le

ss th

an 1

/4 r

epor

ted

as M

1/4S

M. M

axim

um

repo

rted

vis

ibili

ty is

10S

M.

RU

NW

AY V

ISU

AL

RA

NG

E (R

VR

):Id

entif

ied

runw

ay te

n-m

inut

e RV

R v

alue

(in

hund

reds

of f

eet)

is r

epor

ted

if vi

sibi

lity

one

mile

or

less

or

RVR

600

0 fe

et o

r le

ss, f

ollo

wed

by

FT to

indi

cate

feet

. V

alue

pre

fixed

with

M

or P

to in

dica

te v

alue

is lo

wer

or

high

er th

an r

epor

tabl

e RV

R

valu

e.Ex

ampl

es:

P600

0FT

= g

reat

er th

an 6

000

feet

M10

00FT

= le

ss th

an 1

000

feet

.

WEA

THER

PH

ENO

MEN

A:

ASO

S re

port

s:R

ain

(RA

): liq

uid

prec

ipita

tion

that

doe

s no

t fre

eze

Snow

(SN

): fr

ozen

pre

cipi

tatio

n ot

her

than

hai

l Pr

ecip

itatio

n of

unk

now

n ty

pe (U

P)Pr

ecip

itatio

n in

tens

ity in

dica

ted

by a

pre

fix:

light

(-),

mod

erat

e (n

o si

gn),

heav

y (+

)Fo

g (F

G)

Mis

t (B

R)

Haz

e (H

Z)

Free

zing

Rai

n (F

ZR

A)

S

qual

l (SQ

) Fr

eezi

ng fo

g (F

RFG

): te

mpe

ratu

re b

elow

0∞

CW

hen

augm

ente

dby

obs

erve

r ASO

S m

ay r

epor

t:Fu

nnel

Clo

ud/T

orna

do/W

ater

spou

t (FC

) Thu

nder

stor

m

(TS)

VA

(vol

cani

c as

h)H

ail (

GR

)

Sm

all h

ail (

GS)

: hai

l <1/

4 in

ch)

Not

e:A

dditi

onal

wea

ther

phe

nom

ena

may

be

adde

d us

ing

pla

in la

ngua

ge o

r ab

brev

iatio

ns.

SKY

CO

ND

ITIO

N:

ASO

S w

ill n

ot r

epor

t clo

uds

abov

e 12

,000

feet

AG

L un

less

aug

men

ted

(add

ed) b

y a

wea

ther

obs

erve

r.C

loud

am

ount

(in

eigh

ths

of c

over

age)

and

hei

ght:

CLR

no c

loud

s de

tect

ed b

elow

12,

000

feet

)FE

Wfe

w: <

1/8

- 2/

8 sk

y co

vera

geSC

Tsc

atte

red:

3/8

- 4

/8 s

ky c

over

age

BK

Nbr

oken

: 5/8

- 7

/8 s

ky c

over

age

OV

Cov

erca

st: 8

/8 s

ky c

over

age

follo

wed

by

3-di

git h

eigh

t in

hund

reds

of f

eet.

Inde

finite

sky

co

nditi

on is

exp

ress

ed a

s ve

rtic

al v

isib

ility

: (V

V),

follo

wed

by

3-di

git h

eigh

t in

hund

reds

of f

eet.

TEM

PER

ATU

RE/

DEW

PO

INT:

Each

is r

epor

ted

in w

hole

deg

rees

Cel

sius

usi

ng tw

o di

gits

. Val

ues

are

sepa

rate

d by

a s

lash

(/).

Sub-

zero

va

lues

are

pre

fixed

with

an

M (m

inus

).Ex

ampl

e: 0

4/M

02: T

empe

ratu

re +

4 d

egre

es C

Dew

Poi

nt -

2 d

egre

es C

elsi

usA

LTIM

ETER

SET

TIN

G:

Alti

met

er s

ettin

g al

way

s pr

efix

ed w

ith A

indi

catin

g i

nche

s of

mer

cury

- r

epor

ted

in h

undr

edth

s of

an

inch

an

d us

ing

four

dig

its.

Exam

ple:

A29

92 =

29.

92 in

ches

REM

AR

KS

IDEN

TIFI

ER:

RM

K

REM

AR

KS:

Add

ition

al in

form

atio

n m

ay b

e ad

ded

to th

e w

eath

erob

serv

atio

n. A

utom

ated

rem

arks

are

gen

erat

ed b

y th

e sy

stem

. A

ugm

ente

dre

mar

ks a

re a

dded

by

a hu

man

obs

erve

r.

Rem

arks

may

incl

ude

(but

are

not

lim

ited

to):

TOR

NA

DO

, FU

NN

EL C

LOU

D, W

ATER

SPO

UT:

(Tor

nadi

c A

ctiv

ity);

AO

1:

(Aut

omat

ed s

tatio

n w

ithou

t pre

cipi

tatio

n se

nsor

); A

O2:

(Aut

om

ated

sta

tion

with

pre

cipi

tatio

n se

nsor

); PK

WN

D: (

Peak

W

ind)

; WSH

FT: (

Win

d Sh

ift);

FRO

PA: (

Fron

tal P

assa

ge);

TWR

V

IS: (

Tow

er v

isib

ility

); SF

C V

IS: (

Surf

ace

visi

bilit

y); V

IS [

min

V

max

]: (V

aria

ble

visi

bilit

y); V

IS [

loca

tion]

: (V

isib

ility

at a

sec

ond

loca

tion)

; LTG

[ty

pe]

[loc

atio

n]: (

Ligh

tnin

g); T

SB [

min

utes

] E

[min

utes

]: (t

ime

past

the

hour

Thu

nder

stor

m B

egan

, End

ed);

RA

B [

min

utes

] E

[min

utes

]: (t

ime

past

the

hour

Rai

n B

egan

, En

ded)

; VIR

GA

: (Pr

ecip

itatio

n no

t rea

chin

g th

e gr

ound

); C

IG

[min

V m

ax]:

(Var

iabl

e ce

iling

); C

IG [

loca

tion]

: (C

eilin

g he

ight

at s

econ

d lo

catio

n); P

RES

RR

: (pr

essu

re r

isin

g ra

pidl

y); P

RES

FR:

(pre

ssur

e fa

lling

rap

idly

: SLP

: (Se

a-le

vel p

ress

ure)

.SY

STEM

STA

TUS:

Sens

ors

not o

pera

ting

(NO

) are

indi

cate

d:TS

NO

: Thu

nder

stor

m In

form

atio

n; R

VR

NO

: Run

way

Vis

ual

Ran

ge; P

WIN

O: P

rese

nt W

eath

er Id

entif

ier;

SLP

NO

: Sea

Lev

el

Pres

sure

; PN

O: P

reci

pita

tion

Am

ount

; FZ

RA

NO

: Fre

ezin

g R

ain

ASO

S vo

ice

mes

sage

sco

ntai

n th

e ba

sic

wea

ther

obs

erva

tion

and

sele

cted

rem

arks

suc

h as

Den

sity

Alti

tude

and

NO

TAM

s.D

ECO

DED

OB

SERV

ATI

ON

:A

viat

ion

Rou

tine

Wea

ther

Rep

ort f

or H

omet

own

Mun

icip

al

Air

port

. Wea

ther

obs

erve

d on

the

23rd

day

of t

he m

onth

at

1351

UTC

(Z).

Aut

omat

ic o

bser

vatio

n (n

o hu

man

obs

erve

r).

Win

d 34

0 de

gree

s (tr

ue) a

t 9 g

ustin

g to

14

knot

s. V

isib

ility

3/4

st

atut

e m

ile. R

VR

for

runw

ay 3

2 is

500

0 fe

et. L

ight

rai

n. F

og.

Cei

ling

500

feet

ove

rcas

t. Te

mpe

ratu

re 2

3 de

gree

s C

elsi

us.

Dew

poi

nt 2

2 de

gree

s C

elsi

us. A

ltim

eter

29.

92 in

ches

. R

emar

ks: A

utom

ated

sta

tion

with

pre

cipi

tatio

n se

nsor

. Thu

nde

rsto

rm in

form

atio

n no

t ava

ilabl

e. T

ower

vis

ibili

ty 1

sta

tue

mile

. Sea

leve

l pre

ssur

e is

101

3.2

Hec

topa

scal

s.N

OTE

: Ref

er to

the

ASO

S G

uide

for

Pilo

tsan

d th

e A

eron

autic

al

Info

rmat

ion

Man

ual f

or m

ore

info

rmat

ion.

Ref

er to

the

Air

port

/Fac

ility

Dir

ecto

ry,a

eron

autic

al c

hart

s, a

nd r

elat

ed p

ubli

catio

ns fo

r br

oadc

ast,

tele

phon

e, a

nd lo

catio

n da

ta. C

heck

N

otic

es to

Air

men

for A

SOS

syst

em s

tatu

s.

Cop

yrig

ht ©

199

9, A

OPA

Air

Safe

ty F

ound

atio

n

S

afe

Pilo

ts. S

afe

Skie

s.

REM

ARK

S ID

ENTI

FIER

REM

ARK

SSY

STEM

STA

TUS

RMK

A02

TSN

O T

WR

VIS

1SLP

132

Safe Pilots. Safe Skies.

AOPA Air Safety FoundationChartered in 1950, the AOPA Air Safety Foundation is the nation’slargest nonprofit organization providing aviation safety education andprograms to the general aviation community.

The mission of the Foundation is to save lives and promote accidentprevention through pilot education. To serve the nation’s 622,000general aviation pilots, the Foundation:

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An annual report is readily available by writing or calling the Foundation at:

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800/638-3101Publisher: Bruce Landsberg

Editors: John Steuernagle, Kathy DondzilaConsultants: Tom Horne, Art Flior, Larry Smith

All pilots who contribute $50 or more each year will receive the Safety Advisor series on an annual basis.Contact ASF to take advantage of this latest opportunity in safety education and awareness.

Safe Pilots. Safe Skies.

SA09-7/99

For copies of these Safety Advisors sendyour check or money order to: AOPA ASF,421 Aviation Way, Frederick, MD 21701.

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ance tips for safer flying in and around nontowered airports.

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flight service can provide weather

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