AWOS: what it cannot sense, code or report

AWOS (automated weather observation system) reports only what fixed sensors measure. Elements a human observer catches are missing, wrong-coded, or capped.

Thunderstorms: reported only via the lightning-detection network, tiered by strike distance. Outside network: no thunderstorm or lightning report.

Precipitation codes: AWOS reports weather phenomena only as these four (TC AIM MET 8.4.4 Table 8.3).

• Thunderstorms: TS + remarks on lightning location.
• Drizzle and freezing drizzle have no code:

• False freezing precipitation: sporadic at above 0°C and below +3°C, in wet snow, rain, drizzle, or fog.

Obstructions to vision:

• Reportable: haze HZ, mist BR, fog FG, freezing fog FZFG, blowing snow BLSN.
• Not reportable (need a human): smoke, volcanic ash, blowing dust, blowing sand.

Squalls and "in the vicinity":

• Squalls: never reported.
• Vicinity: only VCTS. Never VCFG or VCSH.

Sensing and range, AWOS vs human:

Cloud layers:

• Cannot report cloud type.
• Caps at four layers; can stack more than one overcast layer.
• Can mistake ice crystals, smoke aloft, or a strong temperature inversion aloft for a real layer.

Cloud and visibility vs human, by condition:

Remarks in AUTO reports:

• Never: human descriptive remarks on cloud type and obscuring phenomena.
• Limited set (TC AIM MET 8.4.4 Table 8.3):
  • VIS VRB + limits when visibility variable (e.g. VIS VRB 1-2)
  • ICE, ICE INTMT, or ICE PAST HR when icing detected
  • May also appear: precipitation amount, rapid pressure changes (PRESFR/PRESRR), lightning location

Sensor failure: malfunctioning or shut-down sensor drops that parameter from the report.

• Missing wind: five slashes in wind field (/////). Never zero, clear, or a stale value (TC AIM MET 8.4.4 Table 8.3 wind row and NOTE).

Report types and sampling (TC AIM MET 8.4.1):

• METAR AUTO: reported on the hour.
• SPECI AUTO: significant changes in ceiling, visibility and wind velocity; onset/cessation of thunderstorms, precipitation or icing.
• Samples atmosphere and builds a data message every minute, so normally produces more SPECIs than a human site.
• Treat single report with caution; summarize a series.

Other:

• Newer AWOS/LWIS ice-resistant anemometer essentially eliminates degradation from freezing precipitation, freezing fog, or snow contamination (TC AIM MET 8.4.3(b)).
• Altimeter setting, human or automated: fail-safe design, two or more independently operating pressure sensors agreeing within established tolerances (TC AIM MET 8.4.3).
• RVR: reported only at sites with RVR sensors installed (TC AIM MET 8.4.3(j)).

Trap: a "clean" AWOS report (no TS, good visibility) does not prove clean weather; the sensor may be unable to see or code that element.

Practice