Now the good news about ASOS. Its reports of wind speed and direction, altimeter setting, and temperature have proven to be very, very accurate and reliable. That's because these observations are made directly by proven instruments like the anemometer's wind vane, the barometer's expandable aneroid cells, and the trusty old thermometer. So when you tune in an ASOS in your airplane or look at an ASOS-inspired metar (aviation routine meteorological report), you can count on these direct observations.
Wind observations are updated at five-second intervals, then averaged over two minutes' worth of samplings. ASOS also looks for gusts by taking 10 minutes' worth of samplings and looking for changes of five knots or more. The equipment is designed so that the higher the gust speed, the faster the reporting response time. If it's gusting to 30 or 40 kt for five seconds or more, for example, the lag time between observation and report is only a few seconds.
Now the not-so-good news. Because of ASOS's use of time-based sampling logic and the limitations of its equipment, great discrepancies occur between human and ASOS observations. Trained weather observers routinely see faulty ASOS observations of cloud height, sky cover, visibility, and precipitation type.
ASOS uses a laser beam ceilometer to measure cloud heights. It's a narrow beam pointed straight up. Human observers take in the entire sky to make judgments about cloud heights, ceilings, and sky cover. They can also identify potentially dangerous cloud types. To ASOS, a cloud base is a cloud base, whether it's caused by a solitary fair-weather cumulus or a raging cumulonimbus.
The ceilometer also uses time averaging in its logic. It makes observations every 30 seconds and double-weights its reports based on the last 10 minutes' worth of cloud base readings. This means that it may take ASOS 10 minutes to report a sudden change in cloud height or sky cover - something to bear in mind if you're thinking of flying in marginal VFR weather or when conditions are changing rapidly. Before the 10-minute observation window has elapsed, the ceilometer may look up into a break in a broken layer of clouds and report a clear sky. ASOS reports sky cover in terms of clear, few, scattered, broken, or overcast conditions.
Another problem with the ceilometer is false low-ceiling reports when it's raining or snowing. Precipitation can act like chaff, making the laser bounce back off it instead of any cloud bases.
ASOS measures visibility with a forward-scatter meter. A Xenon flash lamp projects its beam to a receiving unit, which measures the amount of light that is scattered by any intervening moisture or other particles. In theory, it's a bit like measuring the headlights of an approaching automobile on a foggy day. That's forward scatter. A back-scatter measurement, on the other hand, would be like judging the visibility of the headlight beams of your own automobile as it moves through the fog. We're all familiar with the way this back-scattered light can ruin visibility and even temporarily blind a driver on a dark night.
One problem with ASOS visibility reports is that the sampling area is very small: The distance between the bulb and the receiver is less than a foot. This makes siting a real issue. An ASOS located in a low area that's susceptible to localized ground fog may be the only place on an airport with reduced visibility. There have been cases where runways and the sky overhead were clear, yet the ASOS reported visibilities of one-quarter mile because the ASOS site was surrounded by a lonely patch of ground fog.
Brightness can also affect the accuracy of visibility reports in certain reduced-visibility situations. That's because the scatter meter is designed to read forward scatter, but humans perceive visibility through back scatter. (Just remember that when driving in fog you can nearly always see an oncoming car's headlights farther away than you can see into the view ahead of your own headlights. )
Let's say that a thin fog or haze has reduced visibility to two miles, according to a trained human weather observer. Research has shown that if it's bright enough, ASOS can report visibility that is approximately twice what the human eye perceives. So you hear a four-mile visibility report on the ASOS frequency. This can trick a pilot into thinking that conditions are better than they really are and spring a potentially lethal trap.
One National Weather Service publication says that if it's bright enough to wear sunglasses and there's haze or thin fog, halve the ASOS visibility report. In dense fog, ASOS reports correlate well with human observations.
Visibility measurements are made every minute, then averaged over a 10-minute period. If visibility suddenly drops, say, from seven miles to one mile, it can take about three minutes for the 10-minute mean values to register a three-mile visibility and then make the ASOS transmit a special observation - marked by a SPECI prefix on metars. It will be 10 minutes, however, before the one-mile visibility is reported.
ASOS reports vertical visibilities in 100-foot increments whenever the following three events take place: cloud heights drop to or below 2,000 feet agl; the ceilometer beam is "smeared" by returns, presumably off moisture; and visibility measurements drop to one mile or lower. Vertical visibility reports are signified by the VV prefix, but ASOS can't tell you if the restrictions to visibility are the result of haze, fog, dust, or anything else.
ASOS's precipitation identification hardware uses a light-emitting diode to measure the fall rates of various types of precipitation. Snow falls slowly, this instrument's logic says; rain falls faster, and reports are generated accordingly.
This instrument has a hard time sensing light rain, light snow, and blowing snow. That's because its threshold is set at a precipitation fall rate of 0.01 inch per hour, and lighter precipitation won't hit this triggering level. So light precipitation and drizzle go unreported. Also, the precipitation identifier can report ice pellets or hail as rain because these phenomena have fall rates similar to those of rain. The National Weather Service has tried a variety of fixes for this - including installing microphones to detect the clatter of ice pellets-but no truly effective solution has been found.
There are other deficiencies in other ASOS components as well. Lightning detectors available at some ASOS sites act much like the equipment many of us use in our airplanes. They report nearby lightning activity but don't give its bearing from the site. Plans are under way to link the nation's ground-based lightning detection network into ASOS observations. Under this scheme, strike locations would be plotted according to latitude and longitude, then sent to the ASOS nearest to them, where ranges and bearings from the site could be provided to pilots and weather observers.
ASOS's freezing rain detectors are of the vibrating-probe type used in aircraft icing detectors. When freezing rain adheres to the probe, the vibration slows down and the sensor issues a freezing rain report. Unfortunately, anything adhering to the probe will have the same effect - frost, spider webs, bird droppings, and so on.
I know that all of this may sound like bad news. However, both the National Weather Service and the FAA are aware of the problems mentioned above - and more - and are working on fixes. Besides, there are real advantages to ASOS. These units work 24 hours a day. They don't take vacation, go on strike, or call in sick - unless their equipment is malfunctioning. As with AWOS, we have more data more often, and this helps to boost both our forecasting ability and the research community in the quest for better weather prediction. It's ASOS's time-based observation logic together with the sheer density of observations that tilts ASOS's accuracy over that of humans in the long run.
Observers say that ASOS is accurate when stable VFR weather prevails. But when the weather changes rapidly, as in frontal situations or when convective weather crops up, be prepared to get erroneous information from an ASOS.
The biggest single piece of advice for pilots approaching airports with ASOS: Listen to the broadcast for at least 10 minutes - or at 10-minute intervals - if the weather is poor, marginal, or changing. If the readings are all over the place, don't trust them. If they are consistent, then the time-averaging logic is working as advertised and you can expect the actual conditions to correspond roughly with those being reported. And always be ready to go around.
