Closer Look - ASOS - Behind the Scenes

Private Pilot License (PPL) Notes

In this section, we explore the Automated Surface Observing System (ASOS) and Automated Weather Observing System (AWOS), their functions, limitations, and importance for pilots, especially new student pilots.

Understanding Automated Weather Systems:

1. Importance of Weather Observations:
   1. Crucial for flight safety and planning.
   2. Pilots rely on accurate weather data.
2. Manual vs. Automated Observations:
   1. Some airports still use manual observations.
   2. Most observations come from ASOS or AWOS.
3. Similarities between ASOS and AWOS:
   1. Both are collections of weather instruments near runways.
   2. Provide essential weather data to pilots.

The ASOS Components:

1. Temperature and Dew Point:
   • Measured by a thermometer and hygrothermometer.
   • Provide information on air temperature and humidity.
2. Precipitation Measurement:
   1. Heated tipping bucket gauge measures precipitation amount.
   2. Precipitation identification sensor determines rain or snow.
3. Wind Observations:
   1. Wind vane measures wind direction.
   2. Rotating cup anemometer measures wind speed.
4. Atmospheric Pressure:
   1. Sampled by a barometer.
5. Ceiling Measurements:
   1. Ceilometer uses a laser to measure cloud bases.
   2. Only detects clouds directly overhead up to 12,000 feet AGL.
6. Lightning Detection:
   1. Sensors report lightning strikes and their direction.
   2. Distinguishes between cloud-to-cloud and cloud-to-ground strikes.
7. Visibility Sensor:
   1. Uses light projectors and detectors to measure atmospheric clarity.
   2. Estimates visibility up to 10 miles based on sampled air clarity.
8. Freezing Rain Sensor:
   1. Determines if rain will freeze upon contact with surfaces.
9. Runway Visual Range (RVR) Sensors:
   1. Measure visibility near runways.
   2. Help pilots assess the ease of seeing surface markings and centerlines.

Data Processing and Reporting:

1. ASOS Algorithms:
   1. Process sensor data to determine current weather conditions.
   2. Identify phenomena like rain, snow, fog, and freezing fog.
2. Generating METAR Reports:
   1. Standardized weather report format for aviation.
   2. Reports are made available through ATIS, ASOS frequency broadcasts, phone calls, or mobile apps.
3. Update Frequency:
   1. ASOS frequency broadcasts update every minute.
   2. ATIS and apps typically update hourly or with significant changes.

Limitations of ASOS:

1. Un-reportable Phenomena:
   1. Cannot detect tornadoes, smoke, snowfall accumulations, or virga.
2. Delayed Reporting:
   2. Significant wind shifts may not be reported immediately.
   3. ASOS may take 10-15 minutes to report such changes.
3. Erroneous Precipitation Reports:
   1. Dew, frost, or dense fog can trigger false precipitation readings.
   2. Insects, birds, or blowing snow can cause incorrect reports.
4. Ceilometer Limitations:
   1. Only measures clouds directly overhead.
   2. May not report significant changes in cloud cover promptly.
5. Visibility Sensor Limitations:
   1. Does not measure horizontal visibility.
   2. Fog may develop away from the sensor and not be reported.
6. Precipitation Identification Challenges:
   1. Difficulty distinguishing between sleet, rain, and hail.
   2. Often struggles to detect light drizzle or flurries.

AWOS Specifics:

1. Differences from ASOS:
   1. May have fewer weather instruments.
   2. Prone to more frequent erroneous snow reports.
2. Clock Drift Issues:
   1. AWOS clocks can drift from the actual time.
   2. If observations seem incorrect, contact the airport manager.

Augmented METARs and Final Considerations:

1. Human Augmented METARs:
   1. FAA observers at major airports can adjust reports.
   2. Include significant or hazardous weather observed manually.
   3. Such augmentations may not be available on ATIS broadcasts.
2. Importance of Up-to-Date Information:
   1. Always check the date and time of weather observations.
   2. Ensure you have the most current information for decision-making.
3. Pilot Responsibility:
   1. Weather reports are tools for guiding flight decisions.
   2. Pilots must make the right decisions based on available data.

Remember, understanding the capabilities and limitations of ASOS and AWOS systems is essential for flight safety. Always use the most current weather information and exercise good judgment when planning and conducting flights.