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bobbyjoh

Definition of Weather Ceilings

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Hi AS Team,A very interesting question has come up among some associates of mine about weather terminology. The ceiling for a airport is always listed in the METAR. The question I have is how is the ceiling determined? If there is an overcast layer generally it the ceiling. Hoever if there are broken cloud layers, what is the methodology to determine what the ceiling is, is it 4/8 coverage etc. Should would appreciate info on this subject.ThanksBob


 

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Guest Balrog

Not a member of the AS team, but doing weather observations and issuing METARs was part of my job for 15 years....MK1 Human eyeball is the best instrument, backed up by some bells and whistles.First of all a ceilometer. Basically a laser pointing straight up, firing off shots every x seconds (x varies from make to make), mostly we used one that fired every 30 seconds. Time between shot and reflection determines height. Results are shown on a computer screen, giving you a graphic display of the shots.Low tech instrument at night is a spotlight shining straight up. Then sight an inclinometer at the reflection, read off the angle and hit a table to figure out the height. (Trigonometry - finally found a use for it!)During daylight only used the ceilometer to check eyeballs, working in a TWR you need to know the distance and height of all hills, mountains, tall structures etc in the vicinity of the airport. Gives you a good indication of cloud heigh too. For coverage, ie few, scattered, broken etc, use eyeballing. At nigh the history on the ceilometer screen gives a good indication too. Or watching the clouds drift through the spotlight. Throw in experience, and your good to go.The term "ceiling" is not really a meteorological term, but it`s used to describe the height of the highest coverage. Example:If my METAR says few clouds at 500 feet, scattered at 1500 feet and broken 2000 feet, then the ceiling would be 2000 feet.Cheers,Kjell.

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Hi Bob,Here is a little bit:The CEILING is the height above ground or water of the lowest layer of clouds and is reported as BKN (broken) or OVC (overcast). A minus sign preceding the designation BKN and OVC indicates that the sky cover is thin. BKN and OVC do not constitute a ceiling. If clouds are present but cannot be distinguished because of obscuring phenomena, the ceiling is reported as obscured (X). A sky condition reported as partially obscured (-X) indicates that some span of the sky or cloud layer is visible through the obscurationHope this helps,JimActiveSky Supporthttp://www.hifisim.com/images/as2004proudsupporter.jpg

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Thanks Guys these are really informative. I have just one more question. Is there a NWS definition for ceiling. Until I was asked this question I never gave it much thought, just excepted what was presented in the METAR.CheersBob


 

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Guest hlm65

Hi Bob,official NWS definition of ceiling: The height of the lowest layer of broken or overcast clouds.

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Enrico and others. any idea on what coverage of broken clouds NWS uses to determine the ceiling? For example would 2/8 broken mean a celing of would 7/8 scattered be the ceiling for the same airport?ThanksBob


 

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Hi Jim,This subject has been a real learning expereince for me. What my post was attempting to ask would a half full sky of scattered clouds constitute a ceiling more than a 1/3 full sky of broken clouds. After the post I went through a number of METARS from the NWS, in every case, as you stated in a post obove, the lowest "broken" layer was always the ceiling reported, even when there were scattered clouds at a lower layer. I never knew this before.I hope my point is a little clearer, my question was this, is there ever a condition where a scattered clouds layer is designated the ceiling when there is also a broken cloud layer present?ThanksBob


 

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Hi Jim,I think you are right on this. Would any of the weather guys confirm what we think is correct.CheersBob


 

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Guest John_Hirsch

Jim's reply is correct. But lets not forget that we are dealing with two issues here. The height of the ceiling as well as the amount of sky cover. Herein lies the problem, what determines whether the sky cover is broken? The definition of BROKEN sky cover is from 0.6 to 0.9 (to the nearest tenth) of coverage. The coverage is cumulative as an observer would see clouds from the ground upward. This can best be explained with an example. Let's first consider a low layer of stratus clouds extending from the eastern horizon to overhead at a height of say, 4500ft above ground level (AGL). By itself then, the sky conditions would be considered as 4500 SCATTERED sky cover. Note that only one half or 0.5 tenths of the sky is covered.Now add to this picture another cloud layer, above the stratus layer, that you can see from below that covers the remainder of the sky and is at a measured height of 8000ft AGL. The sky conditions now are: 4500 SCATTERED, CEILING MEASURED 8000 OVERCAST. Here, the sky is completely (10 tenths) covered with clouds, the CEILING height is 8000ft AGL and the sky cover is OVERCAST.When talking about ceiling, we must consider both the height of clouds as well as cloud amounts.So much for the fourth week of our CLOUDS 101 class.John

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Guest hlm65

No, the ceiling is the LOWEST BKN or OVC layer. So you could have more than one FEW or SCT layer and not have a ceiling.Example: FEW001 SCT003 SCT005 = no reported BKN/OVC layer, so NO ceiling; FEW002 SCT003 BKN007 = the ceiling is 700ft (the BKN layer).

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