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Atis Message Altitudes?

Sat Feb 05, 2005 3:53 am

I did a search for this topic, but the relevant postings did not specify ATIS, ASOS/AWOS, METAR, etc. Based on my particular experience related below, I'd like to ask this question anyway.

Sample ATIS recording: "Denver International Airport Information India, 1753 Zulu. Visibility 10. Temperature 10. Dewpoint -4. Altimeter 3018. Light rain, mist. Ceiling 7000 broken...."

Does the "seven thousand" in the ATIS broadcast refer to altitude AGL or MSL? I used to think it was MSL, but DEN has a field elevation of 5431, and I could swear I once heard something like "ceiling three thousand broken." This doesn't make sense because the cloud ceiling can't be below the field elevation. I only heard it once though, on a particularly bad weather day. Can someone please clarify this for me?
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RE: Atis Message Altitudes?

Sat Feb 05, 2005 4:07 am

It's an AGL altitude...

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RE: Atis Message Altitudes?

Sat Feb 05, 2005 5:00 am

I have the same problem with the local TV forecast. When he says snow down to six thousand feet I'm thinking there will be snow in my yard a thousand feet deep.
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RE: Atis Message Altitudes?

Sat Feb 05, 2005 5:40 am

Thanks for the responses. I guess in a lot of places, there's little difference between AGL and MSL with respect to cloud levels. How are these measured, anyway? Is it a precise WX radar measurement, pireps, or a combination?

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RE: Atis Message Altitudes?

Sat Feb 05, 2005 8:23 am

Couldn't find any technical details on the ASOS/AWOS website, but info on one type of ceilometer can be found here

I don't know if that particular sensor is used for the ASOS/AWOS installation though...

If it comes from an ASOS and you have the AUTO annotation in the METAR then it's entirely off the ASOS sensor, there is no human intervention, and no PIREPs are used.

So sometimes you can have a cloud sit on top of the airport right over the sensor, and it's reporting a 1000ft ceiling and there are no clouds anywhere else in sight for 10 miles around the airport, but the ASOS will report that cloud at 1000ft. Or the other way around, clouds all around the airport, but the hole over the ASOS sensor will have the ASOS report CLEAR BELOW 12000.

[Edited 2005-02-05 00:23:49]
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RE: Atis Message Altitudes?

Sat Feb 05, 2005 2:03 pm

Technically, the cloud would have to sit on top of the sensor for at least ten minutes without moving for it to report the clouds erroneously.

A way to think of how an ASOS measures clouds is that its basically reporting the average of the weather over a 30 minute period, the last ten minutes weighed more heavily.

This is a good explanation of how an ASOS works:

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