By Robert N. Rossier, EAA 472091
This piece originally ran in Robert’s Stick and Rudder column in the April 2025 issue of EAA Sport Aviation magazine.
It was one of those days. The automated surface observing system (ASOS) at virtually all airports in a 50-nm range were reporting clear skies, visibility better than 10, and temperature-dew point spreads of 3 degrees Celsius or more. The sky seemed just a bit hazy, but that’s no surprise in the summer in New England.
It was quite a surprise to lift off, climb through 300 feet AGL, and see the surrounding landscape seemingly shrouded in a layer of fog and mist. For a moment, I wondered what I had missed in my review of the current weather conditions. Didn’t the reports suggest good VFR conditions?
There had been one clue, one that I hadn’t quite picked up on. On the morning TV weather report, an image of the sunrise held that clue. It showed a bright orange sun with shades of red surrounding it — a clear indication of smoke from the wild fires in Canada and the west. Perhaps, if I had been more tuned in to what was happening, I would have picked up on that.
In retrospect, I should not have been surprised, but sometimes the reports we get from ASOS give us a false impression of what is really going on. The next day, the actual visibility was only about 3 miles when the reported conditions included 8-mile visibility. It’s with reflections on such events last summer that we face what might be another season of smoky surprises.
The capabilities of modern ASOS are truly amazing, especially when I think back to the tools we had just a few decades ago. In those days, we relied almost totally on reports made by certified weather observers, along with pilot reports and our own eyeball-to-sky observations. Now we can quickly pull up on our laptop or tablet a full-color map with color-coded dots depicting conditions across the region.
While that information is tremendously helpful in making decisions about and in flight, we still must recognize the limitations of the technology that provides the raw data from which those plots are created.
Smoke and Mirrors
A modern ASOS uses an array of sensors to continuously monitor and report a number of critical parameters, including wind speed and direction, temperature, dew point, visibility, cloud cover, cloud height, and the type of precipitation that may be occurring.
The instruments that make the observations are carefully calibrated to ensure that the visibility reports provided by the instrumentation correlate to those made by the human eye. However, there are limitations, particularly when we consider some specific elements that affect visibility like smoke, volcanic ash, and dust.
According to the ASOS User’s Guide, developed jointly by the National Oceanic and Atmospheric Administration, Department of Defense, Federal Aviation Administration, and United States Navy some 25 years ago, the physical limitations of the human eye and human subjectivity are greatly impacted by precipitation, day and night vision adaptation, contrast, physical obstructions, and perspective. These factors were all taken into consideration in the development of the automated visibility sensors used by ASOS.
The visibility sensors measure the attenuation of light due to scattering caused by particles in the air. Light from a pulsed xenon flash lamp in the blue portion of the visible spectrum is transmitted twice a second in a cone-shaped beam over a range of angles. The detector is set at a specified angle from the transmitter, allowing it to detect the light that has been scattered. The visibility sensors are designed to correlate well with atmospheric haze, snow, and rain events. Further adjustments are made for night visibility reporting due to how the human eye perceives visibility at night.
As we might anticipate, the specific nature of the particles in the atmosphere can affect the ability of the visibility sensors to accurately report visibility as a pilot will perceive it. It would seem that smoke particles scatter visible light differently than other particles we normally encounter in the atmosphere. My own personal experience finds that smoke from distant wildfires can easily cut the actual or pilot-observed visibility to half what an ASOS unit is reporting.
Volcanic ash consists of tiny, abrasive particles ejected during volcanic eruptions. These can be particularly difficult for pilots to detect visually, especially at night, yet they can represent some extremely hazardous conditions, effectively sandblasting cockpit windows, clogging filters, damaging instrumentation, abrading propellers, and rapidly causing serious engine damage. However, detecting the presence of volcanic ash in the atmosphere is beyond the capability of ASOS. Instead, it is detected by the nine Volcanic Ash Advisory Centers (VAACs) located around the world.
Likewise, dust particles vary widely in size and composition, and scatter light differently than the particles for which ASOS visibility sensors are optimized. ASOS sensors are well optimized for larger weather-related particles such as raindrops and snowflakes rather than fine dust, so we might expect some erroneous reports when conditions include blowing dust.
The bottom line is that when elements such as smoke, volcanic ash, or dust are in the atmosphere, we should anticipate visibility reports that don’t correspond as well to what we’ll perceive looking out the windscreen.
More Tricky Situations
Most pilots recognize that ASOS ceiling and visibility readings are averaged over time to give a more accurate representation of what is actually happening. Cloud height and ceiling information is averaged over a 30-minute period, while visibility is averaged over a 10-minute period, with wind also averaged over a 10-minute period. Thus, rapidly changing conditions in these parameters will not be immediately reported, which can occasionally put us a bit behind the eight ball.
Likewise, the instrumentation for cloud height, visibility, and cloud cover is limited by the location of the sensors, as well as the fact that they are extrapolating based on a limited range of observation. As described above, the visibility sensors measure light scattering in a relatively small volume of air near the sensor, and that data is extrapolated to an overall visibility measurement. An approaching fog bank, or a stationary one for that matter, cannot be observed by the instrument, which means it does not affect the measurement provided. Similarly, clouds not passing the field of view of the ceilometer are not sensed, and thus the ceiling report can occasionally be misleading.
In some cases, the report based on the upward view of the ceilometer may entirely miss what the human eye would see by simply gazing across a much broader swath of sky. Such micro-scale conditions can easily result in a false impression of the local conditions.
While we can all appreciate the astounding network of data provided by ASOS observations across the country, we also do well to recognize how the limitations of those automated systems can occasionally lead us astray. Especially when Mother Nature throws us a curveball such as fog banks, micro-scale weather conditions, or smoke in the atmosphere, we need to be prepared to see things that automated sensors might miss.
Robert N. Rossier, EAA 472091, has been flying for more than 40 years and has worked as a flight instructor, commercial pilot, chief pilot, and FAA flight check airman.