though the automated surface observation system or ASOS creates a
completely new observation every minute, they must have adequate sensor
samples to develop an accurate observation for the airport. In order to
provide a representative observation, the automated hardware must
continuously collect the sensorís 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 donít necessarily want to know whatís happening
instantly over the sensor since it may not always be representative of the
sky condition throughout the airportís 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 itís 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 (or counting twice) the last 10
minutes of data.
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."