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You're approaching your destination, and you dial the ATIS, ASOS, or AWOS frequency to listen for the current weather. With broken ceilings at 5,500 feet, you're set to land under VFR. But how were those ceilings reported? And why is it important to understand how they're measured?
At any weather reporting station, there's only a portion of sky used for weather reports. It's called the celestial dome. The celestial dome is the part of the sky that's visible to a human observer above all natural obstructions, including hills and trees. However, if there is a building that blocks part of an observer's view, they'll make an effort to see or estimate the sky conditions on the other side of the building. But celestial domes only apply to observation stations with human observers.
When Federal Air Regulations refer to "ceilings" for 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." But since METAR and SPECI observations don't include the term "thin," anything reported as broken or overcast is treated as a ceiling.
So how is broken and overcast measured? By something called "octals", which are 8 equal segments 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 overcast.
There are a variety of methods that reporting stations use to measure cloud altitudes. Here are the most common ones:
1) Human Observers are trained to accurately estimate cloud types and altitudes. While more and more uncommon with a transition to automated reporting stations, if you read a METAR that doesn't contain the phrase "AUTO," the station has a dedicated weather analyst (often a local ATC controller) that will physically walk outside and view the conditions to confirm the METAR. But there's a major flaw with human observations. Observers tend to overestimate sky conditions because of "packing effect," where clouds further away laterally look more packed together.
2) Light Beam Ceilometers use trigonometry and a light beam to determine cloud heights. Essentially, a light beam is placed at a position away from the station and directed upward. A measurement device at the station senses when the light disappears into the cloud deck. Using trigonometry, cloud altitudes can be recorded.
3) Cloud Height Indicator (CHI) Sensors use vertically pointing lasers (LIDAR) to measure cloud heights. You can think of CHI like radar; the round trip time for the beam to return once hitting a cloud layer determines cloud height. CHI is normally limited to 12,000 feet of altitude, which is why you'll sometimes see "clear below 12,000 feet" in a weather report. CHI is the most common cloud measurement device used at airports today, and with technological advances, it will soon allow cloud measurements up to 24,000 feet.
So how does the CHI determine the cloud formations for the area? Remember how percentage of clouds overhead determines how the clouds are classified? Based on percentage of time that "cloud hits" are reported within a 30 minute time interval, the CHI auto-generates a cloud type observation. CHI's are used as the sole cloud measurement tool at most automated observation stations. So next time you're listening to the ASOS or AWOS, a CHI probably helped generate the report.
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. That will help you determine what altitude you need to fly at in order to maintain cloud separation requirements.
To do it, you need to find the cloud height in feet above mean sea level (MSL). That'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 you should fly, since 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. Add the reported clouds of 7,000 feet AGL to the field elevation of 845 feet MSL to get a cloud base altitude of 7,845 feet MSL.
Surface observations, no matter how advanced the system, have limitations. They're limited by location and time, so they only really apply to an airport's immediate area. But when you understand how clouds are reported by weather stations, it's easier to understand how they'll affect your flight.
Swayne is an editor at Boldmethod, commercially licensed pilot with multi-engine and instrument ratings, and a commercial aviation student at the University of North Dakota. He's the author of the articles, quizzes and lists you love to read every week. Swayne's experience ranges from international flights in a King Air F90 to ferrying a 1943 Grumman Widgeon across the country. You can reach Swayne at email@example.com, and follow his flying adventures at http://www.swaynemartin.com.