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....
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.
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:-)
The computerised unit that automatically calculates the cloud height using a l@ser beam echo thing would have to be modified for each installation to add on the height of the airport. Too much stuffing around.
The computerised unit that automatically calculates the cloud height using a l@ser beam echo thing would have to be modified for each installation to add on the height of the airport. Too much stuffing around.
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.
Last edited by darkroomsource; 28th Jun 2012 at 15:39.
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.
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.
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.
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!".
Quote:
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...
The original question related to cloud on the ATIS. Those who have said it is measured by l@ser are barking up the wrong tree. A l@ser 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.
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.
Last edited by le Pingouin; 29th Jun 2012 at 06:31.
The 8340 l@ser 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 l@ser 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...
Or:
The manual way.
Quote:
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)
