P2PN Wx Brief


On a recent cross country with my best old ex-friend Larry, we fell from the sky at Gillette, WY. Per normal operating procedures I spooled up the Gillette ASOS prior to entering their airspace just to see what the weather was, even though I could reliably pick the airport out of the haze from 20 miles away. There he was, the same familiar voice I have come to know and love, confidently reading me the observation for the area. Seems everywhere I go, rain or shine, night or day, there he is. How does that work? Must be some old guy in comfortable shoes sipping coffee somewhere and reading the weather all day.

I marvel sometimes at how complacent we flyers may have become. I will certainly be the first to note that change on the part of the Government for the benefit of aviators appears to sometimes move at a snail’s pace. Still, I have to admit, between the changes to ATC, airman medicals, weather dissemination, and the great blossoming of technology, my time in the cockpit certainly has become much more rewarding and less intensive.

Now days that once lofty monolith the VOR is conspicuously absent from most of my flight planning. In fact the FAA is making them disappear altogether in many locations.  All I need to do any more is lay in a couple of way-points on ForeFlight and through the magic of technology, I get my time en-route all factored for winds aloft along with my estimated fuel burn. Weather is available nearly everywhere at my fingertips and even filing a flight plan it has become “technologized” and impersonal. The demise of the old regional Flight Service Station is nearly complete. We all fly a magenta line now and even approaches, procedure turns, and hold entries are graphically depicted with little or no expenditure of gray matter on our part.

I was ruminating on all we have to work with the other day when I realized that I had overlooked the obvious. AWOS and ASOS broadcasts abound at smaller airports throughout the country these days and have made obtaining an accurate description of weather in real time nearly commonplace. They have become so common that they are often taken for granted. Remember the days when you used called in on the Unicom frequency for a field advisory? Remember the days when no one answered your call?  Now a real-time, accurate, reliable broadcast of station name, time of observation, wind direction and speed, visibility, sky conditions, precipitation, obstruction, ceiling, temp/dew point, and altimeter setting is available at numerous airports throughout the U.S. and is delivered mostly by the same calm and friendly voice over a dedicated frequency. If where you are going doesn’t happen to have an ASOS or AWOS, the chances are there is an airport close by your destination that does. How do we pilots manage to get all this information so reliably? To find out I asked my buddy, all around “Radio Messiah” and State of Michigan employee, Tom Smith what he does to assure that I hear that friendly voice giving me a weather update nearly everywhere I go.

Tom is one of us; a pilot I mean. He’s a bit of a purist and flies an L-3 Aeronca and is restoring a super cub. His aviation career took a bit of a twist when he completed the avionics program at LCC. After his graduation in 1999, he took the common route and found a job as an avionics technician. Then a position at the Michigan Department of Transportation crossed his path in 2008 and he was soon taking training on how to massage various parts of an AWOS system into complete weather observations. He’s been with the MDOT ever since. MDOT is ultimately responsible for the installation and maintenance of all AWOS and ASOS stations in our state.

Tom is one of two technicians employed by the State of Michigan charged with dragging their road weary butts all over the state of Michigan in order to perform maintenance on the forty two ASOS and AWOS sites located throughout the state (nationwide the number is more like 1000!) Rambling around the state in a fully stocked van full of test equipment and spare parts, they are required to visit every site for routine maintenance and calibration three times a year. They also visit sites as needed if something goes awry. Typically they try to schedule their visits sometime other than the frigid dark months of the winter but, demand maintenance sometimes dictates that they take a trip to oh, say, Newberry in January when they have been notified of a failure or problem. Tom recounted how on one “demand maintenance” service call they discovered that the local farmer had plowed up the wiring between the array and the broadcasting equipment. Somehow, it was not working. Go figure.

An internet company called “anyAWOS” monitors the reporting stations around the state (and other states) and sends out an email alert when there is a problem with any of the sites. That’s how Tom gets word, for example that, an AWOS is completely off line or a component of the system is not working properly. And, just in case you believe that he is working on these weather sites in some nice cushy building with heating and cooling, think again. The weather stations are a compact collection of ingenious instruments located out in the free air where they can complete their task without interference from any type of altered atmosphere.

I met with Tom this past summer at the Fremont Airport. Upon arrival at the site the first order of business was the dispatch of a nasty hornets nest in the top of the laser driven ceiling sensor housing. Since this system has an open top for shooting a laser aloft, it is only natural that Mother Nature’s critters would find a way to make use of the housing. Birds too can be a problem and some portions of the equipment are fitted with anti-landing serrations or points, to eliminate the tendency of perching or nesting. I couldn’t help but smirk at the common sense approach designers had taken in assuring that this hardware portion of the system works in spite of what nature could throw at it. Think the rain off the housing will drip in front of the visibility sensor? Put a drip deflector on it. Think snow and ice will clog a sensor? Build a small heater into it. What if the power goes out? Put a battery back-up system on it. Think the birds will land there? Put a point on it.

Now days most of these “all in one” sites provide wind speed and direction not by means of an anemometer, but rather an ultrasonic (sound) sensor with no moving parts to freeze up. Probes are heated to avoid ice and snow build up, and the unit makes computer aided calculations for wind speed and direction based on wind disturbance of sound waves between the three probes.

Lasers are a common part of our lives these days and lasers in this case are used in measuring the cloud bases. A Ceilometer bounces a laser beam off the cloud bases and takes a reading every 30 seconds. Sixty readings are required to assure an observation is generated every minute. Observations are weighted toward the last 10 minutes in order to display a trend. The draw back to the ceilometer is its narrow field of view. If one cloud gets parked over the sensor, the height of that cloud is what gets reported even if all the surrounding area is clear and a million. Heavy rain, snow, fog or other obstruction may provide an inaccurate reading for a brief period of time. When the visibility sensor array detects visibility of 1 mile or less and the ceilometer also detects a disturbance in the laser beam returns to it, a new report is issued stating “vertical visibility.” Readings can also be skewed by spider webs, birds, droppings, or insect nests like the wasp nest that Tom dealt with on my visit. In these cases, the broadcast may state, “Sky conditions missing.” Ceilometers are typically accurate to within 100 feet and provide readings up to 12,000 feet; some newer ones up to 25,000 ft.

Visibility readings are taken by a device reading the amount of radiation scattered from a beam of light by particles in the atmosphere such as rain, fog, dust, snow, etc. This unit has a sending section and a receiving section. It reminds me of the safety light beam on a garage door opener! It reads the scatter of light through the atmosphere between the two sensors due to some kind of obstruction. The computer interprets the readings and broadcasts the visibility in increments of less than half a mile to a mile, and is good for visibilities up to 10 miles. To test this device a filter is placed between the sensors that provides a known visibility reduction value. The resulting reading is compared to the known result stored in the computer. As with all this equipment, adjustments can then be made if required.

Barometric pressure is provided by means of a, you guessed it, barometer. However, here too, the system is designed to rule out false readings by comparing the readings from several sources at a time. Either two or three barometers are employed and the computer is looking for “agreeing” values between at least two of the barometers that are within 0.04 inches of each other.

Temperature readings are taken by means of a “Thermistor” which is a resistor that demonstrates a reliable change in electrical resistance based on temperature. Again the computer interprets and reports its findings. (And you thought someone was reading a thermometer!)

Dew point is found by drawing ambient air across an electronically cooled mirror. The air in contact with the mirror is cooled until dew forms on the mirror at which time a laser and detector system note the formation of the dew and record the temperature of the mirror. Newer systems are becoming available and rely on “capacitance,” again the resistance of electrical current, to make a reading rather than dealing with occasionally false readings that may occur as a result of dust, sand, spider webs, etc. on the mirror system.

Precipitation is observed through the use of light emitting diodes. The computer interprets the passing of atmospheric particles through an LED sensor beam. Another garage door opener sensor! The computer has parameters for the rate of fall of rain or snow and applies them to the noted fall rate of the particles through the sensor. The device is calibrated to read light rain, rain, heavy rain, snow etc. Later models have an acoustic channel built into the system to aid in the identification of ice pellets and enhanced software now allows some units to identify drizzle, freezing drizzle and blowing snow. The system is ever evolving and AWOS stations are now up to incarnation IV or V. It all depends on just how much observation the system provides.

For readings concerning freezing rain, a vibrating probe is mounted on the sensing array and provides information on the formation of ice based on the change in resonant frequency when an additional mass (ice) is added to the probe. This is the same kind of system used on some large commercial aircraft to detect the formation of ice. Freezing rain is detected by the computer when this frequency change occurs. The unit does periodic heating cycles to rid itself of the ice load and will send another alert to the computer should the frequency change occur again.

Lightening detection is now provided as the result of an interface of the AWOS system with commercially available strike sources located around the US. The old system was dependent on both an electromagnetic and optical sensor located directly on the AWOS site and was good for observations from 5 to 10 miles of the airport. Current sources provide for a much broader picture of the occurring lightening pattern.

Signals from this gaggle of relatively low tech, Rube Goldberg-like equipment array are interpreted and sent to a base station located at the airport via underground cable, and of course broadcast in that familiar voice over the appropriate frequency at the airport as well. A company called Mackinaw Software provides for most of the computer based broadcasting capabilities here in the Great Lakes area. These weather broadcasts are also available via telephone and I find it very handy to have a number of these AWOS numbers stored in my smart phone for easy access to local weather on a timely basis.

You may have noted that other than the employment of lasers, a wide variety of somewhat “low tech” sensors provide the raw information which needs to be “massaged” into a completed observation. Much like the Scarecrow in the Wizard of Oz, this system would be nothing without the computer brain that interprets the data it is fed. The whole array is managed by a carefully and specifically designed computer that interprets the mass of information received and distills it into its now familiar form. Until recently a Finnish company, Viasala, provided all the software, hardware and know-how for building and integrating one of these systems. Lately, a new US based company appears to be making ASOS/AWOS equipment and software for the majority of stations, and Viasala appears to have now pulled out of the market. No matter who is making the systems, at an estimated cost of $150,000 to $200,000 per copy, it is no wonder that there are techs like Tom Smith running around the state making sure the system remains serviceable.

Finally, about that voice . . . It has always fascinated me (I know, some simple things elude me) how I could be getting weather in KOEB, fly to KMGY and hear that same cool commanding voice offering me at least some hope of the visual approach when I was just sure the weather would be below minimums upon arrival. “What a job” I thought. “This guy has it made; just sit around and read the weather all day!” Well, it turns out I was partially correct. While the phraseology is computer generated, the actual words in the vocabulary are said to have been generated and recorded by a Mr. James Hershey. He is believed to have been an employee of the Viasala Company when the system was being put together. He actually recorded phrases like, “Light rain” and “Gusts,”  “25,” and “Sky conditions” as well as all those other oh, so familiar phrases and words. The computer determines which phrases or words are used when it interprets the inbound flow of data and generates the proper audio stream. More information about Mr. Hershey proved unavailable for this article. I wonder if he has any idea how many of us listen for that friendly voice at all the stations we visit.

So, the next time you are flying into some far flung destination and dial up the local ASOS or AWOS to receive your airport advisory, say a little mental “thank you” to the maintenance technicians of the state operating the systems. Those folks keeping the systems up and running make it easy for airplane junkies like you and me to enjoy our arrivals at romantic destinations around the country. Oh, and you might say a silent “Thanks Jim” to Mr. Hershey too. His voice makes all of those destinations seem so friendly and welcoming.

Tom Speerstra

Author: Tom Speerstra

Tom Speerstra has had an enduring love affair with aviation for over 40 years. Countless adventures have been enjoyed flying students, people, paper, and parcels in everything from Champs to Citations. He has held positions as both Chief Pilot and Director of Operations for Part 135 carriers and holds an ATP, MEII, SES ratings and a Citation type rating. Tom makes his home in Michigan with his wife Elizabeth and the two dogs Hess and Pappy.