Cloud heights on the ATIS
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ATIS
ATIS is valid for 5nm around the Airport Reference Point. Therefore AGL. Wind direction is in magnetic to correlate to runway directions / RWY in use (ATIS includes which RWY is in use) = if the RWY in use is mentioned, directions are in magnetic (spoken word by ATIS, ATC, Met report for a specific airport)
Automated systems uses multiple readings now = lowest laser reading measured during the last hour (30 min at international airports). Multiple readings eliminates the possibility of incorrect readings when laser beam just so happen to miss the cloud when it takes a (only one) reading e.g. BKN cloud cover. So, it gives a better overall report. Read the footnote in the MET section of the AIP.
ARFOR are above MSL because it covers areas outside the 5nm radius of the airports and the pilot must add ground elevation to ensure safety heights, flying altitudes or Flight Levels. Wind directions are in True = no RWY mentioned, thus enroute winds converted to magnetic using local variation.
Automated systems uses multiple readings now = lowest laser reading measured during the last hour (30 min at international airports). Multiple readings eliminates the possibility of incorrect readings when laser beam just so happen to miss the cloud when it takes a (only one) reading e.g. BKN cloud cover. So, it gives a better overall report. Read the footnote in the MET section of the AIP.
ARFOR are above MSL because it covers areas outside the 5nm radius of the airports and the pilot must add ground elevation to ensure safety heights, flying altitudes or Flight Levels. Wind directions are in True = no RWY mentioned, thus enroute winds converted to magnetic using local variation.
Last edited by Nirak; 29th Jun 2012 at 07:01.
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VH-XXX
I know there are ceiliometers but again the original post was height on the ATIS. Again, it does not come from a LASER measument. Having worked in six towers, I have never recorded an ATIS using a laser measurement. This holds true for the four new towers being commissioned using the latest technology.
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Ceilometers uses a light beam, reflect it against the cloud onto a receiver and the cloud height is calculated using trigonometry.
Laser beams are new technology that beams straight up and measures the difference in thickness of the air straight above.
Laser beams are new technology that beams straight up and measures the difference in thickness of the air straight above.
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If you read the text next to the picture it says that the pulsed laser beam detects the amount of 'backscatter'
Due to the changes in thickness of the cloud (= back scatter pattern) both the height of the cloud base and cloud top can be determined. Can do up tp 4 different layers of cloud straight above and any amount of cloud cover.
The old type 'light beam' could only reflect the base of the lowest cloud that is BKN or more. Is probably done manually (by an observer) and does not tell you anything about the layers of cloud above or the thickness of those layers.
New technology = great stuff
Due to the changes in thickness of the cloud (= back scatter pattern) both the height of the cloud base and cloud top can be determined. Can do up tp 4 different layers of cloud straight above and any amount of cloud cover.
The old type 'light beam' could only reflect the base of the lowest cloud that is BKN or more. Is probably done manually (by an observer) and does not tell you anything about the layers of cloud above or the thickness of those layers.
New technology = great stuff
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Hey fujii, I've worked at a few towers too, and in the absence of pilot reports, we'd frequently use ceilometer measurements when visual observation didn't match what the dew point was telling us. Non-convective cloud can be hard to pick if you don't have any visual reference to compare it to.
Nirak lasers are light beams. Laser stands for "light amplification by stimulated emission of radiation". I don't think anyone was talking about the old cloud searchlights, which were out of date in the 1940s and I haven't seen one in thirty years.
I've always reconciled cloud reports AGL as being target audience - two people care about cloud base: pilots who wish to remain below it, and pilots who wish to make an approach through it.
Pilots wishing to remain below cloud need to know how high it is above terrain to determine if they can satisfy their 500/1000FT minimum height rules and remain in VMC - this is easier if expressed AGL. Pilots wishing to approach through cloud need to compare the base with their approach minima, which are admittedly expressed on most plates both AMSL and AGL, but are usually defined in legislation and ops manuals as figures AGL. Then you have the radioaltimeter, which is only in AGL.
Finally, at most meteorological sites around the country, the observers are not pilots, not controllers, and don't prepare their observation specifically for aviation. For these people, it's easier to determine height AGL and the end user - you - can do what they wish with the information.
Nirak lasers are light beams. Laser stands for "light amplification by stimulated emission of radiation". I don't think anyone was talking about the old cloud searchlights, which were out of date in the 1940s and I haven't seen one in thirty years.
I've always reconciled cloud reports AGL as being target audience - two people care about cloud base: pilots who wish to remain below it, and pilots who wish to make an approach through it.
Pilots wishing to remain below cloud need to know how high it is above terrain to determine if they can satisfy their 500/1000FT minimum height rules and remain in VMC - this is easier if expressed AGL. Pilots wishing to approach through cloud need to compare the base with their approach minima, which are admittedly expressed on most plates both AMSL and AGL, but are usually defined in legislation and ops manuals as figures AGL. Then you have the radioaltimeter, which is only in AGL.
Finally, at most meteorological sites around the country, the observers are not pilots, not controllers, and don't prepare their observation specifically for aviation. For these people, it's easier to determine height AGL and the end user - you - can do what they wish with the information.
