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Automated ceiling reports
Whether in the form of a METAR or by the ground-to-air radio
broadcasts, pilots use surface observations to make many routine
operational decisions during any particular flight. Many of
these observations come from automated weather
systems. 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.
Even though these automated
systems create a completely new observation every minute, they must
have adequate sensor samples to develop an accurate observation. In
order to provide a representative observation at an airport, 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.
This is especially important when
considering the observation for sky cover and cloud
height. When approaching an airport, for example,
pilots don’t necessarily want to know what’s happening instantly over
the sensor since it may not always be representative of the sky
condition throughout the airport’s terminal area and it might vary
quite a bit between reports.
Automated systems employ an
upward-pointing laser beam ceilometer to determine sky cover and cloud
height. The cloud height indicator, for instance, transmits
approximately 9,240 pulses in 12 seconds, but it’s not these individual
samples that are used for the observation. Instead, this data
is collected over a period of 30 minutes before an observation is
considered
acceptable.
Based on field studies, 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 (counting 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 that is valid over a three to five statute mile radius
around the airport.
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. Trained
weather observers can
certainly augment the observation to add these details such as clouds
with bases above 12,000 AGL.
Automated systems can only report clouds that are below 12,000 feet
AGL. 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 the overcast cloud deck at 15,000 feet AGL.
In the more tropical regions,
where winds aloft are often very light, such as Florida, afternoon
cumulus tends to move more slowly. Trained weather observers
have reported up to four-tenths of the sky covered by fair weather
cumulus when the automated system is reporting clear below 12,000 feet.
With a high broken or
overcast sky,
it is common to see automated stations reporting a clear sky.
For example, at 1401 UTC on March 2nd, North
and South Carolina had an obvious overcast sky as can be seen by this
visible satellite image shown above. Click here to view a larger
image. The NWS
WSR-88D NEXRAD Doppler radar image on the right also shows areas of
returns implying
cloudy skies. Click here to view a larger
image. However, many automated stations in this region were
not reporting precipitation, but were reporting clear skies.
What could be going on?
The
Aviation Digital Data Service (ADDS) interactive METAR Java tool mixes
both automated reports and those from a human observer. Any
green square
shown on the image below (valid at 1430 UTC) is an automated station
that is reporting clear skies. Click here to view a larger image.
Since automated stations  cannot
report clouds over 12,000 feet the sky condition is referred to as
“clear below 12,000” to emphasize this point.
This is a case where the sky was indeed broken to
overcast over a widespread area. Most of the stations were
reporting clear below 12,000 feet or a higher overcast or broken
ceiling. This kind of example can make interpretation of the
weather difficult at times. What you see on the
satellite and radar images might seem to conflict with the surface
observations. So it is important to fully undertand the
limitations of the weather products you are using for those routine
operational decisions.
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