You’re on the verge of reaching your destination, and you dial the ATIS, ASOS, or AWOS frequency to listen in and gather information about the current weather. With broken ceilings at 5,500 feet, you’re set to land under VFR conditions. But how exactly are those ceiling heights reported?
The Celestial Dome
At any weather reporting station, there’s a specific portion of the sky that’s utilized for weather reports. It’s known as the celestial dome. The celestial dome refers to the part of the sky that can be seen by a human observer above all natural obstructions, such as hills and trees. However, if there is a building that partially blocks a person’s view, they’ll make an effort to observe or estimate the sky conditions on the other side of the structure. But celestial domes are relevant only for observation stations with human observers.
Sky Conditions
When the Federal Air Regulations mention “ceilings” in relation to weather minimums, the FAA defines a ceiling as: “The height of the lowest layer of clouds above the surface that are either broken or overcast, but not thin.” Since METAR and SPECI observations don’t include the term “thin,” anything reported as broken or overcast is considered a ceiling.
So how are broken and overcast conditions measured? Through something called “octals,” which are 8 equal sections of the sky. If the sky is covered between 5/8 and 7/8 with clouds, it’s reported as broken.
And if it’s covered 8/8ths with clouds, it’s considered overcast.
Measuring Cloud Altitudes
There are various methods employed by reporting stations to determine cloud altitudes. Here are some of the most common ones:
1) Human Observers are trained to accurately assess cloud types and altitudes. Although this method is becoming less common with the transition to automated reporting stations, if you come across a METAR that doesn’t mention “AUTO,” it indicates that the station has a dedicated weather analyst (often a local ATC controller) who will physically go outside and observe the conditions to confirm the METAR. However, there is a significant drawback to human observations. Observers tend to overestimate sky conditions due to the “packing effect,” where clouds that are farther away laterally appear to be more densely clustered.
2) Light Beam Ceilometers utilize trigonometry and a light beam to determine cloud heights. Essentially, a light beam is positioned at a distance from the station and directed upward. A measurement device at the station senses when the light disappears into the cloud layer. By using trigonometry, cloud altitudes can be recorded.
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3) Cloud Height Indicator (CHI) Sensors employ vertically pointing lasers (LIDAR) to measure cloud heights. You can think of CHI in a similar way to radar. The round-trip time for the beam to return once it hits a cloud layer determines the cloud height. CHI is typically limited to an altitude of 12,000 feet, which is why you might sometimes see “clear below 12,000 feet” in a weather report. CHI is the most commonly used cloud measurement device at airports today, and with technological advancements, it will soon enable cloud measurements up to 24,000 feet.
So how does the CHI determine the cloud formations in the area? Remember how the percentage of clouds overhead determines how the clouds are classified? Based on the percentage of time that “cloud hits” are reported within a 30-minute time interval, the CHI automatically generates a cloud type observation. CHIs are used as the sole cloud measurement tool at most automated observation stations. So the next time you listen to the ASOS or AWOS, there’s a good chance a CHI contributed to the generated report.
Why It Matters for Pilots
Clouds are always reported by weather stations in feet above ground level (AGL). As you plan your takeoff, route, and arrival, pay attention to cloud reports to calculate how high the clouds are around you. This will help you determine the altitude you need to fly at to maintain cloud separation requirements.
To do this, you need to find the cloud height in feet above mean sea level (MSL). It’s as simple as adding the elevation of the reporting station to the reported cloud heights. Once you know the cloud heights in MSL, you can determine where to fly, as altimeters are set in feet above MSL, not in feet AGL.
Example: At Grand Forks International Airport (KGFK), clouds are reported broken at 7,000 feet AGL. By adding the reported clouds of 7,000 feet AGL to the field elevation of 845 feet MSL, you get a cloud base altitude of 7,845 feet MSL.
Surface observations, no matter how advanced the system, have limitations. They are restricted by location and time, so they really only apply to the immediate area around an airport. But once you understand how clouds are reported by weather stations, it becomes easier to comprehend how they will impact your flight.
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