CFI Instrument Practical Test Standards, FAA-S-8081-9B, June 2001

III. Preflight Preparation

A. Weather Information

1. Sources of weather
    a. AWOS, ASOS/AWSS, and ATIS reports
      1) Automated weather observing system (AWOS)
        a) Secondary U.S. weather observing system (predecessor to and being replaced by ASOS/AWSS)
        b) Provides continous data on conditions near the runway touchdown zone
        c) Some AWOS are augmented by certified observers when visiblity < 7 SM
          "Observer weather" including obstruction-to-vision information is placed in remarks
        d) Density altitude reported when exceed field elevation by > 1000 feet
        e) Visibility
          i) Sensor near runway touchdown point
          ii) Runway visibility value (RVV) using 10-minute harmonic average
          iii) Sensor calibrated against FAA transmissometer standards
        f) Sky condition/ceiling
          i) Ceilometer next to visibility sensor
          ii) Integrates last 30 minutes of data
        g) AWOS site information published in AFD and on the applicable IAP
        h) AWOS broadcasts
          i) Computer-generated +/- observer voice weather information message broadcast via telephone and/or VHF radio
          ii) May be discrete VHF frequency or via voice portion of local NAVAID receivable to maximum of
          • 25 NM from site
          • 10,000 feet altitude
          iii) Updated each minute
        i) AWOS information on FAA website
      2) Automated surface observing system (ASOS)/Automated weather sensor system (AWSS)
        a) ASOS/AWSS is the primary surface weather observing system of the U.S.
        b) Program to install ASOS/AWSS throughout the U.S. is a joint effort of the the NWS, FAA and DOD
        c) AWSS is follow-on program that provides data identical to that of ASOS
        d) Continuous, minute-by-minute observations provide sufficient data to generate METARs and other weather information
        e) Transmission of data
          i) Discrete VHF radio frequencies receivable to maximum of 25 NM and 10,000 feet altitude, or via voice portion of local NAVAID
          ii) Some also via telephone
          iii) Example: KLPR ASOS data available on discrete 121.425 and by telephone number 440-323-7088
        f) "Objective" elements data essentially the same as that taken by human observer (pressure, ambient temperature, dew point temperature, wind, precipitation accumulation)
        g) "Subjective" elements data is remarkably similar to that taken by human observer even though that automated systems use a fixed-location, time-averaging technic (sky condition, visibility and present weather)
        h) ASOS/AWSS components
          i) Weather sensors
          ii) Data collection and processing units
          iii) Peripherals and displays
        i) ASOS/AWSS sensors
          i) Cloud height indicator(s)
          ii) Visibility sensor(s)
          iii) Precipitation identification sensor
          iv) Freezing rain sensor (select sites)
          v) Pressure sensors (2 or 3)
          vi) Ambient temperature/dew point temperature sensors
          vii) Anemometer (wind direction and speed sensor)
          viii) Rainfall accumulation sensor
        j) ASOS/AWSS data outlets
          i) For onsite airport usage
          ii) National communication networks
          iii) Computer-generated voice (FAA radio broadcasts to pilots and dial-in telephone line)
        k) AUTO modifier indicates automatic only report with no human intervention
        l) Two types of automated stations
          i) AO1 - no precipitatin descriminator
          ii) AO2 - precipitation descriminator present (differentiates liquid from freezing/frozen precipitation; e.g. rain from snow)
        m) ASOS/AWSS information on FAA website
        n) Map of automatic weather observing sites
      3) Automatic terminal information service (ATIS) reports
        a) ATIS is the continuous broadcast of recorded essential but routine noncontrol information in high activity terminal areas in order to improve control effectiveness
        b) Broadcast continuously over discrete VHF radio frequencies receivable to maximum of 60 NM and up to 25,000 feet AGL, or via voice portion of local NAVAID
        c) ATIS information includes
          i) Time
          ii) Ceiling (may be omitted if > 5,000 feet)
          iii) Visibility and obstructions to visibility (may be omitted in > 5 SM)
          iv) Temperature and dew point temperature (if available)
          v) Wind direction (MAGNETIC) and velocity
          vi) Altimeter
          vii) Pertinent remarks
          viii) IAPs and runway(s) in use
        d) Listen to ATIS
          i) Preflight, pre-takeoff planning
          ii) En route preliminary to approach planning
        e) Notify ATC on initial contact that ATIS received by repeating the ATIS alphabetical code letter

Greg Gordon MD, CFII