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PROCESSING RADIUS
INTERVAL VALIDITY PARAMETER (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 provide 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 previous 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 an entirely 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 sensor array, ASOS will take only two minutes to report a scattered deck of clouds even though a trained observer may report a broken sky. 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 min- utes 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.
ASOS relentlessly measures the weather and could inun- date 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 spe- cial observations must be prepared and broadcast by a human observer 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 Lockout Period
If you pay attention to the issuance time on METARs, you will notice that many 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 those that run numerical weather prediction models that 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. Consequently, many of the hourly METARs will carry an issuance time of 53, 54, 55, or 56 minutes past the hour.
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, includ- ing SPECIs, independent of what weather may be occurring. The ASOS continuously monitors and records the weather during the lockout period; however, it just can’t issue a 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 to flight crews, air traffic controllers, and your datalink weather.
For example, assume that a severe thunderstorm moves through the automated sensors within this lockout period producing heavy rain, reduced visibility, and strong gusting winds. During any other time period, a SPECI will be issued. However, during the lockout period, this SPECI is recorded by the ASOS but will not be disseminated.
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 condi- tions, and whether or not they are special observations. Even
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