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 Observing 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 for the terminal area. 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 such as Florida where winds aloft are often very light, 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. The NWS WSR-88D NEXRAD Doppler radar image below also shows areas of returns implying cloudy skies. However, many automated stations in this region were not reporting precipitation, but were reporting clear skies. What could be going on?
The Aviation Weather Center surface observation map 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. 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 understand the limitations of the weather products you are using for those routine operational decisions.
Most pilots are weatherwise, but some are otherwise™
Dr. Scott Dennstaedt
Weather Systems Engineer
CFI & former NWS meteorologist