Whether in the form of a
METAR
or by the ground-to-air radio broadcasts, we use surface observations
to make many routine operational
decisions during any particular
flight. As we listen to the broadcast prior to taxi, it
provides us with an altimeter setting and will likely determine the
runway we use for departure. When approaching an
airport, it will help us determine if we’ll be flying a visual approach
or need to execute a standard instrument approach procedure.
And when Mother Nature is at her worst, it will let us know when we
should skip the airport altogether and fly to our alternate
destination.
Surface observations are one of those
data points that pilots often
take for granted. The truth is that they play a monumental
role in many of our most routine decisions. They are not just
used by pilots; surface observations also provide air traffic
controllers and weather forecasters with a reasonable depiction of the
weather conditions at an airport. Even with something as
ubiquitous as a surface observation, there are some nuances you should
understand.
Pilots at all experience levels should
be familiar with the two primary
automated observing systems deployed at many airports throughout the
United States. This includes the Automated Surface Observing
System (ASOS) and the Automated Weather Observation System
(AWOS). Both of these automated systems consist of a
collection of electronic sensors that measure the environment, and then
process the data to create an observation once every minute.
It is all about
sampling the atmosphere
While many high-impact airports throughout the
U.S. still rely on a trained weather observer to construct the routine
or special observation (SPECI), automated systems supply them with
uniform and objective data for the observation. However,
automated systems measure only the weather that passes directly through
the sensor array so it is not able to report what’s happening outside
the airport’s runway complex. Weather observers can certainly
augment the observation to add these details.
At airports without a trained observer,
pilots must completely rely on the “raw” automated
observation. This observation, however, isn’t as raw as you
might think. In order to provide a representative
observation, the automated hardware must continuously collect the
sensor’s real-time data over a period of time. The automated system
applies an algorithm, to the collected data to extrapolate the weather
to cover a wider area.
When the weather is sampled over a
specified period it will tend to “smooth out” the conditions, but also
will account for the normal meteorological variations that we see in
the weather. Each of the various weather elements shown in
the table below identifies the required sample times for its algorithms
and provides a summary of where the data are considered valid.
PARAMETER
PROCESSING RADIUS
INTERVAL VALIDITY
(MINUTES) (MILES)
SKY
CONDITIONS
30
3-5
VISIBILITY
10
2-3
PRECIPITATION
10
1-2
FREEZING
RAIN
15
2-3
TEMP/DEWPOINT
5
5
WIND
2
1-2
PRESSURE
1
5
For example, 30 minutes of data
provides a fairly reasonable description of sky conditions.
This means that the system will detect and process all the clouds (if
any) passing over the sensor in the past 30 minutes. To
account for the latest sky conditions, the result is biased by double
weighting (counted twice) the last 10 minutes of data. Using
the last 30 minutes of data in this way will allow the system to
determine the height and sky cover included in the surface observation
and becomes a reasonable estimate of the sky conditions over a three to
five statute mile radius around the airport.
 Beware of rapidly changing weather
Even though an ASOS creates a completely new observation
every minute, automated systems must have adequate sensor samples to
develop an accurate observation. Therefore, in rapidly changing
conditions, pilots should expect that most of the weather elements from
the automated observations to trend slightly behind the actual
weather. For example, if skies are clear and a sudden broken
sky appears on the sensors, ASOS will take only two minutes to report a
scattered deck of clouds even though a trained observer may report a
broken sky cover. It’ll take a total of 10 minutes before the
observation system will catch up and indicate a broken layer.
This may or may not trigger a
SPECI. It depends on the height of the broken
layer. In other words, a sudden broken ceiling at
600 feet has a significant operational impact and will generate a SPECI
since the flight category changed from VFR to IFR.
But it will take nearly 10 minutes before the SPECI is
issued.
Each minute an ASOS processes the most
recent 10 minutes of visibility sensor data to obtain a representative
value. Therefore, when visibility drops suddenly (in one minute) from 7
statute miles to 1 statute mile, the ASOS needs about four minutes
before the 10-minute mean values reach the 3 statute mile criteria.
This criterion forces SPECI to alert pilots to a significant change in
visibility in this instance. A total of nine minutes will
pass before the ASOS will report the 1 statute mile visibility.
On the other hand, when the visibility
rapidly improves from 1 mile to 7 miles, the ASOS generates a SPECI
four minutes after reaching the 1.5 statute mile threshold. In about 11
minutes, the ASOS will report 7 statute miles. The system is
intentionally designed to raise surface visibility more slowly than to
lower it. This design provides a margin of safety and buffers
rapid changes when the visibility is widely fluctuating over a short
period.
Hourly and special observations are the
only ones created by human observers. In contrast, ASOS relentlessly
measures the weather and could inundate pilots with more frequent
special observations than a human observer when the weather is changing
rapidly. Thus, the system is purposely throttled to only
provide SPECIs at 5-minute intervals to limit the number of
observations that can be transmitted during the hour. An even slower
response is seen at controlled airports where only the hourly and
special observations must be prepared and broadcast on the Airport
Terminal Information System (ATIS). At uncontrolled airports
pilots can also receive the 1-minute weather by calling the voice phone
link or by the ground-to-air radio broadcasts.
The FAA has recently created a Google
map presentation online of the locations of all automated weather
systems across the country. This includes including
the frequency and phone numbers for each ASOS and AWOS currently in
operation. Go online and visit http://www.faa.gov/air_traffic/weather/asos/.
The lockout period
If you
pay attention to the issuance time on METARs, you will notice that many
routine METARs are issued a few minutes before the top of each
hour. This allows the observation to be transmitted and
ingested into other computer systems such as numerical weather
prediction models. Some models get executed at the top of the
hour or shortly thereafter. Starting at 47:20 past the hour,
the ASOS begins to make its routine observation. By 53:20,
the hourly observation has been prepared and edited and should be ready
for transmission.
This period of time between 47:20 and
53:20 minutes after the hour is known as the lockout
period. During this period, the ASOS is prevented
from issuing any other reports including SPECIs. The ASOS
still continuously monitors and records the weather during the lockout
period; however, it just can’t issue a formal surface
observation. This does not affect the 1-minute weather you
receive by calling the voice phone link or by the ground- to-air radio
broadcasts, but it will affect any formal observations that get
transmitted even if the weather conditions are truly ugly during this
lockout period.
Can
I trust automated observations?
All observations, whether automated or taken by
human observers, should be used with care. Pilots must be aware of how
long ago the observation was taken, under what conditions, and whether
or not they are special observations. Even though automated systems are
totally objective and maintain a certain uniformity among all sites, it
does not mean they match what a pilot sees out the windscreen.
ASOS may occasionally report cloud
decks lower than what is actually encountered. Sometimes precipitation,
lower cloud fragments or fog triggers these lower values. Pilots have
said that these "lower" reported values often indicated the height
below which they had to fly before gaining enough forward visibility to
see an airport and land. The key lesson here is to evaluate all reports
closely before dismissing them as inaccurate.
Even though the visibility sensor is
designed to objectively represent the visibility of the atmosphere over
a wide range of weather conditions, day or night, it occasionally
reports a visibility more optimistic than what a human perceives.
During the day, the human eye can be overwhelmed by bright light
reflected in clouds, light precipitation, fog or haze. Many pilots will
resort to wearing sunglasses to obtain some relief from the glare.
The ASOS visibility sensor is not as
sensitive to this condition and sometimes reports a visibility
approximately twice as high as what an individual may determine. Be
alert for these bright conditions and expect a more optimistic value
from the automated system.
What
will automation not
provide?
We can easily become complacent when it comes to
automation. We learn to trust automation and sometimes don’t
acknowledge that it has real limitations. Therefore, to
finish this discussion, it is just as important to know what automation
will not provide.
Automation systems can only report the weather
that passes through the sensor array. They do not
provide a horizon-to-horizon evaluation of the weather. This
means that weather in the vicinity of the airport will not be
measured. A rain shower that passes just to north of the
airport, for instance, may reduce visibility in that immediate area but
will not be reported by the automated system.
Next, the automated system only reports clouds
that are below 12,000 feet. This means that an overcast cloud
deck at 15,000 feet will be reported as clear. Effectively, a
clear sky report from an automated station means clear below 12,000
feet. For airports with a human observer, this report can be
augmented to include clouds above 12,000
feet.
Automated systems can only report one
precipitation type at a time. For instance, if freezing rain
and snow are detected, snow is reported. Certainly trained
weather observers can edit the observation before transmission to
include additional precipitation types.
Lastly, the system is not designed to
report virga, tornadoes, funnel clouds, ice crystals, snow pellets, ice
pellets, drizzle, freezing drizzle and blowing obstructions such dust
or sand. All of these elements can be provided at locations that employ
a trained observer. Often with drizzle, freezing drizzle, ice pellets
or a mixture, you will see the automated system report an unknown
precipitation type (UP). Nevertheless, automated reporting is
in its infancy so it’s likely new sensors will be added to measure some
of these other weather elements in the near future.
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