View Full Version : Cloud heights on the ATIS
27th Jun 2012, 11:28
Can anyone help me with a history lesson as to why the cloud height reported on the ATIS is AGL and not AMSL?
My thinking would be it would prove better for the pilot to hear a level and then automatically look at their altimeter for a reference. Granted it doesn't take much to add the elevation to the reported height on the ATIS to get AMSL...but was just thinking....I guess it depends if the cloud height is more important for aircraft departing or arriving....
Thanks in advance.
27th Jun 2012, 11:49
it fits in with TAF's etc? :confused:
Dunno...but it works out OK ;)
27th Jun 2012, 12:19
Because you can't expect people who are too stupid to understand the difference between 'to six thousand' and 'two six thousand' to convert cloud bases expressed in AMSL to AGL and not kill themselves flying an IAL procedure 10 minutes later.
27th Jun 2012, 12:42
When you are intending to land at an AD it's pretty obvious that a cloud height above the rwy surface is what counts due to the varying surface of the Earth with ref to SL.
You might be landing at an AD that is 1500ft AMSL on a plateau and the area forecast cloud base is 2000ft.This being the case it would be easily confused as thinking yr intended Ldg drome is well in the clear when in fact it has a 500 ft cloud base.
Because of the Earths surface being an uneven level all area cloud levels/bases are ref to SL for a common datum.It's then up to the pilot to determine from those levels how high the cloud base is likely to be at any particular drome that doesn't have the spoken word for cloud base.
Just as a tip the spoken word IE Tower or ATIS cloud base/s always ref to the AD's surface:-)
27th Jun 2012, 12:44
Chuckles......You are on fire tonight :}:}:}:}:}
27th Jun 2012, 12:52
The computerised unit that automatically calculates the cloud height using a laser beam echo thing would have to be modified for each installation to add on the height of the airport. Too much stuffing around.
27th Jun 2012, 12:56
And it's StrayLia....
As to original question,
Alternate minima are easier to calculate....:cool:
27th Jun 2012, 13:15
And here I was sitting here thinking that the history behind it was that in days gone by when entering the circuit area, everyone used to swap to QFE.
silly me :eek:
28th Jun 2012, 08:26
VH-XXXThe computerised unit that automatically calculates the cloud height using a laser beam echo thing would have to be modified for each installation to add on the height of the airport. Too much stuffing around.Are you serious or just taking the piss?
28th Jun 2012, 15:37
Simpler is easier, and less likely to induce mistakes.
If a chap years ago looked up and estimated that the clouds were 600 feet above him, it would be easier to report a 600 foot cloud base.
If he had to add the elevation of the field, that would be math - introducing the possibility of an error.
If a chap years ago was about to land at an airport and the tower reported 600 foot cloud base, it would be easier to know if that was above the minimums.
If he had to subtract the field elevation, that would be math - introducing the possibility of an error.
Since the cloud base is really more important for landing than for flying, it makes sense to have the least amount of calculations be at the time of landing.
I don't see how you really care what the cloud bases are if your flying anyway. I mean, if you're IFR, you're flying at a safe altitude. And if you're VFR it's below you anyway (but that's really not a good idea to be over a cloud deck VFR). Or maybe you're thinking of scud running, but then that's not a good idea if the bases are that low, and you want to know how much room you have under the clouds to fly safely, so again, you want AGL.
Additionally, hearing that the bases are at 3500 MSL, might lead you to think that the cloud bases are somehow stable at that height. 600 AGL over one airport is often close to 600 AGL over another nearby airport even if that airport is a different elevation. Clouds often follow the terrain, and they also often slope, so MSL would kind of confuse the information.
28th Jun 2012, 16:24
My thinking would be it would prove better for the pilot to hear a level and then automatically look at their altimeter for a reference.
With the outfit I fly for, we set the decision height for the approach manually at top of descent and this is done usually after hearing the ATIS. I personally feel better prepared mentally for the approach, knowing how high the cloud base is above the DH and will include this in my approach brief as well.
28th Jun 2012, 16:31
I don't see how you really care what the cloud bases are if your flying anyway. I mean, if you're IFR, you're flying at a safe altitude.
.. don't know about you mate, but I really care whether I'll become visual and land, or whether I need to expect a missed approach, and a possible diversion as well.
28th Jun 2012, 21:32
Are you serious or just taking the piss?
The cloud height is an automatic measurement taken by computer. Whether its laser or similar technology I am unsure.
Better put, you really don't think someone sits there all day estimating the cloud height and putting it onto the Internet do you ? :-)
28th Jun 2012, 22:56
Personally I am a bit shocked at the question.
20 seconds thinking about ATIS cloud height on anything other than height above ground level should have people laughing at the original poster's question. :ugh: :rolleyes:
28th Jun 2012, 23:08
By the way I have no idea why every time I type "l a s e r" it substitutes an @<hidden> symbol.
laser laser laser
29th Jun 2012, 01:21
I can just imagine in the other way - "awesome, the Toowoomba AWIS is saying the cloud is overcast at 2100 feet - no problems getting in today though!".
Because you can't expect people who are too stupid to understand the
difference between 'to six thousand' and 'two six thousand' to convert cloud
bases expressed in AMSL to AGL and not kill themselves flying an IAL procedure 10 minutes later.
Can't wait to be as awesome as you one day Chuck...
29th Jun 2012, 01:36
Well I started to tap out a serious technical answer...then thought NAAAH FCKIT :E
I think you should aim higher myself:p
29th Jun 2012, 05:27
The original question related to cloud on the ATIS. Those who have said it is measured by laser are barking up the wrong tree. A laser points straight up, so if there is a hole above it, it can report no cloud. As the ATIS is recorded by ATC, it is a visual assessment using experience, geographical points and where available comparing mode C readout with sighting the aircraft. The computerised ATIS systems can download METAR information but this is often edited.
40 years ATC, mostly towers.
29th Jun 2012, 06:30
VH-XXX, my understanding is the advertising on PPRune is contextual so threads about dickheads shining such light amplification powered devices at aircraft would attract adverts for said devices.
29th Jun 2012, 06:33
Those who have said it is measured by l@<hidden> are barking up the wrong tree.
Oh dear, people need to get out more :ugh:
Measuring Cloud Height | Laser Ceilometer Model 8340 | Weather Instruments (http://www.allweatherinc.com/meteorological/8340_ceilometer.html)
The 8340 Laser Ceilometer measures cloud height and thickness, in addition to vertical visibility, detecting up to four cloud layers simultaneously to a distance of 40,000 vertical feet. Its precision makes it ideal for applications requiring the highest in performance and reliability, such as aviation and meteorological studies.
A laser pulse is emitted into the atmosphere and backscatter analyzed. Using the speed of light, the height of each cloud base and top is determined. Due to poorly defined borders or a sparse composition, some clouds are much more difficult to measure than others. Depending on the current and historical sky conditions, an adaptive algorithm determines the number of returns needed to maintain accuracy.
Accurate measurement of cloud height and thickness in all weather conditions, including heavy precipitation and low clouds, can cause serious errors in other ceilometers. Proprietary algorithms and digital techniques from 20 years of cloud height detection research and manufacturing are applied, allowing the 8340 ceilometer to provide accurate information even in difficult circumstances.
What about the AWIB's?
There's no ATC where there are AWIB's. (generally)
An AWIB unit can measure different levels of cloud base if you've listened to them carefully...
The manual way.
1. Find the difference between Surface Temperature (°F) and Dew Point (°F)
2. Divide the difference by 4.4
3. Multiply the quotient by 1000
4. Add the product to the Field Elevation (ft)
29th Jun 2012, 06:55
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.
29th Jun 2012, 07:06
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.
29th Jun 2012, 07:19
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.
29th Jun 2012, 08:49
This holds true for the four new towers being commissioned using the latest technology
Which is what technology?
An estimate from ATC?
29th Jun 2012, 09:30
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 :ok:
29th Jun 2012, 09:53
See private email.
29th Jun 2012, 12:11
F F S .... !
We are talking about cloud height above the ground!! It's not that hard surely. And we let these people fly aeroplanes ... :ugh:
30th Jun 2012, 06:32
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.
30th Jun 2012, 06:58
You could always use the "reverse dam busters" technique (to work out the height of the cloud) if anyone knows what that is....