Telling Your ASOS From Your Elbow

The federal Automated Surface Observation System, or ASOS, is installed at more than 900 airports throughout the USA. The confusing thing is that ASOS sounds a lot and acts a lot like AWOS and ATIS – respectively Automated Weather Observation System and Automated Terminal Information System. And then there’s HIWAS – Here’s a lesson in Alphabet Soup:

ASOS Sensors measure cloud coverage, visibility, temperature, dewpoint and wind, and observe the weather, for example, whether it’s snowing or raining. These measurements in turn are used to make METARs (the traditional ICAO Meteorological Aerodrome Report you’re already used to) and SPECIs (Special observations). For as much as you’ll ever need to know about the technical mechanics of ASOS, a wonderful online resource is available free at the AOPA Air Safety Foundation,

ASOS can be flawed (say a cloud happens to be above the station but the rest of the sky is clear; ASOS would report a cloud ceiling when one doesn’t exist), and therefore the ASOS observations are always monitored and sometimes augmented or even replaced by human weather observers.

The Automated Terminal Information System is a looped recording that plays continuously over radio and, in many cases, available by telephone as well (telephone numbers are listed in the Airport/Facilities Directory – A/FDs). ATIS contains ceiling, visibility, obstructions to visibility, temperature, dew point, wind direction, wind speed, altimeter, and instrument approach(es) and runway(s) in use. It may also include information on Land and Hold Short Operations (see related story). It includes as well Remarks, which can include density altitude, variable visibility, variable wind direction, and the remarks of the weather observer making the recording, such as the type of precipitation or intensity of a storm.

Unlike the federally-funded ASOS, AWOS is almost entirely a state-funded program, and generally speaking somewhat less accurate, or at least less encompassing, than ASOS.

Hazardous In-Flight Weather Advisory Service. HIWAS is a looped, continuous recorded radio broadcast which contains in-flight advisories of severe weather over selected VORs. This includes any hazardous weather advisories received from the national Weather Service that are occurring or forecasted to occur within a 150 nautical mile radius of the selected VOR. They include AIRMETS, SIGMETs, convective SIGMETs, any urgent PIREPs, or any other information that qualifies in the judgment of the specialist placing the HIWAS.

The frequencies over which HIWAS is broadcast is listed on charts and in A/FDs. Any time a new HIWAS is recorded, the FSS will broadcast, on all radio frequencies except 121.5, an announcement telling pilots to listen to the HIWAS – “Attention all aircraft, monitor HIWAS or contact Flight Watch or Flight Service for a new urgent AIRMET information.” Do it. They sometimes will mention the frequency but again, the HIWAS frequency is listed on charts and the A/FD.