I don't understand this METAR

COFlyBoy

COFlyBoy

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COFlyBoy
This METAR says vis is 3sm and then it says that SFC VIS is 8. What the heck does that mean? The METAR decoder I found didn't help much.

METAR KAPA 012153Z 03006KT 3SM -SN OVC012 M07/M09 A3043 RMK AO2 SFC VIS 8 SLP353 P0000 T10671089
​
 
one is the automated observation (3SM) and the SFC VIS 8 is probably an observation from the tower that got put in the remarks?
 
Jim Logajan said:
Trust the robot's entry - it has to follow the three laws of robotics. :wink2:
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Unless it becomes sentient...

picture.php
 
COFlyBoy said:
OK, but now it makes me wonder which one is right.
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It's possible they're both right.

The human observer can see known objects at least eight miles from the observation point throughout at least half the horizon circle.

The ASOS visibility sensor can't "see" anything, it measures the clarity of the air over a short distance and converts that measurement to a visibility value, three miles in this case. There might be a little patch of thin fog at the ASOS site.
 
3 miles is the prevailing visibility ....SFC Vis is the distance seen from the tower to a known landmark


Prevailing Visibility. Prevailing visibility is the greatest visibility equaled or exceeded throughout at least half the horizon circle, which does not necessarily have to be continuous. This is the visibility that is considered representative of visibility conditions at the station.

Surface Visibility. The prevailing visibility determined from the usual point of observation is the surface visibility.
 
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COFlyBoy said:
This METAR says vis is 3sm and then it says that SFC VIS is 8. What the heck does that mean? The METAR decoder I found didn't help much.

METAR KAPA 012153Z 03006KT 3SM -SN OVC012 M07/M09 A3043 RMK AO2 SFC VIS 8 SLP353 P0000 T10671089
​
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We landed about that time today. Coincidentally the tower asked for the conditions on final. We broke out at 7,000' MSL at a distance of 5 miles from the runway. I would say the surface visibility was at least 10 under the overcast. I know they were reporting 3 for a while but the computer was wrong.
 
taters said:
3 miles is the prevailing visibility ....SFC Vis is the distance seen from the tower to a known landmark


Prevailing Visibility. Prevailing visibility is the greatest visibility equaled or exceeded throughout at least half the horizon circle, which does not necessarily have to be continuous. This is the visibility that is considered representative of visibility conditions at the station.

Surface Visibility. The prevailing visibility determined from the usual point of observation is the surface visibility.
Click to expand...

From the tower to just one known landmark? Your statement implies prevailing visibility and surface visibility are distinctly different, but you quote a definition of surface visibility that says it is prevailing visibility.
 
roncachamp said:
From the tower to just one known landmark? Your statement implies prevailing visibility and surface visibility are distinctly different, but you quote a definition of surface visibility that says it is prevailing visibility.
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Its splitting hairs....but the tower takes vis observations using known landmarks...this can differ from the prevailing visibility

The prevailing part should not be there...
 
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taters said:
Its splitting hairs....but the tower takes vis observations using known landmarks...this can differ from the prevailing visibility

The prevailing part should not be there...corrected
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Are you expressing your opinion? According to Federal Meteorological Handbook No. 1 Surface Weather Observations and Reports , "prevailing" should be there:


Prevailing visibility. The visibility that is considered representative of visibility conditions at the station; the greatest distance that can be seen throughout at least half the horizon circle, not necessarily continuous.

Sector visibility. The visibility in a specified direction that represents at least a 45 degree arc of the horizon circle.

Surface visibility. The prevailing visibility determined from the usual point of observation.

Tower visibility. The prevailing visibility determined from the airport traffic control tower (ATCT) at stations that also report surface visibility.
 
So many times the sensor gets crap on it and reads way lower than reality. I'd go with the human!
 
Based on the METAR, the tower vis is 3 miles and the surface vis (as determined by the ASOS or the human observer at KAPA) is 8 miles.

FAAO 7900.5B
9-3. VISIBILITY STANDARDS
Visibility may be determined at either the surface,
the tower level, or both. If visibility observations
are made from just one level (e.g., the air traffic
control tower), that level shall be considered the
"usual point of observation" and that visibility
shall be reported as the surface/prevailing visibility.
If visibility observations are made from
both levels, the lower value (if less than 4 miles)
shall be reported as prevailing visibility in the
body of the METAR, and the other value shall be
a remark.
Click to expand...
 
Let'sgoflying! said:
So many times the sensor gets crap on it and reads way lower than reality. I'd go with the human!
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Yeah, we had blowing snow this morning. It could have blown into the sensor.

It's unclear to me which observation was human and which was machine. I would guess that a surface observation is the ASOS. (maybe I should go find out where it is)
 
COFlyBoy said:
Yeah, we had blowing snow this morning. It could have blown into the sensor.

It's unclear to me which observation was human and which was machine. I would guess that a surface observation is the ASOS. (maybe I should go find out where it is)
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They both could have been made by humans. If the observer feels the ASOS reading isn't correct, he/she can change it before the observation goes out.
 
COFlyBoy said:
Yeah, we had blowing snow this morning. It could have blown into the sensor.
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I'm going with this theory. I have been to another airport where snowblowing was in progress and they were reporting low visibility even though you could see the airport from 30 miles away.

COFlyBoy said:
It's unclear to me which observation was human and which was machine. I would guess that a surface observation is the ASOS. (maybe I should go find out where it is)
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I think the tower visibility is human and the surface visibility is the ASOS. We even joked around about the reported visibility yesterday. In real life it was way better than the reported 3 miles.
 
I think what would concern me more on that METAR, if I was thinking of flying, would be the temperature/dew point spread.

John
 
John Baker said:
I think what would concern me more on that METAR, if I was thinking of flying, would be the temperature/dew point spread.

John
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Well this is where local knowledge of the climate comes into play. There was probably ice up in the clouds (Mari can confirm since she was up there), but below them it was fairly clear with light snow. One could have been doing VFR pattern work with no problems (except that the ceiling was a bit low at times). We don't get fog here in the winter unless it is warming up. And yesterday there was no warming up at all.

Its a bit strange living somwhere where high humidity is 30%.
 
COFlyBoy said:
Well this is where local knowledge of the climate comes into play. There was probably ice up in the clouds (Mari can confirm since she was up there), but below them it was fairly clear with light snow.
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We had the bleed air anti-ice on so none was accumulating on the wings for us to see. I don't remember noticing any on the non-heated parts of the windshield. As I recall there were quite a few layers, the bottom layer being fairly shallow.
 
tonycondon said:
one is the automated observation (3SM) and the SFC VIS 8 is probably an observation from the tower that got put in the remarks?
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I'm with you on this. All "remarks" in METARs are human-generated. What I would do with this info is assume it's 3 to 8 miles... in other words, not so great. :D
 
tonycondon said:
one is the automated observation (3SM) and the SFC VIS 8 is probably an observation from the tower that got put in the remarks?
Click to expand...
taters said:
3 miles is the prevailing visibility ....SFC Vis is the distance seen from the tower to a known landmark


Prevailing Visibility. Prevailing visibility is the greatest visibility equaled or exceeded throughout at least half the horizon circle, which does not necessarily have to be continuous. This is the visibility that is considered representative of visibility conditions at the station.

Surface Visibility. The prevailing visibility determined from the usual point of observation is the surface visibility.
Click to expand...

3 miles = tower visibility

8 miles = surface visibility (while it may have been taken by the ASOS, it isn't considered automated because the human observer is responsible for validating the reading and changing it before it goes out if necessary.)

The tower vis is reported in the body of the METAR because it is lower than the surface visibility and is considered to be the official vis for the airport.
http://www.faa.gov/air_traffic/publications/at_orders/media/SWO.pdf
 
taters said:
Its splitting hairs....but the tower takes vis observations using known landmarks...this can differ from the prevailing visibility

The prevailing part should not be there...
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The tower takes vis observations using known landmarks in various directions in order to determine prevailing visibility.
 
Y'all might also find it interesting that the Tower has a set of sensors that are official up in the Tower Cab, and there's ALSO an ASOS, but they're NOT linked, AFAIK. DIA has similar.

It's always made me wonder about where to find the appropriate weather outlets to see the historical comparison between the "official" weather, and the ASOS online.

Doing the comparison in real-time is easy, dial the phone number for the ATIS right after the hourly change, and then dial the phone number for the ASOS.

I was curious so I started hunting the Net for the historical ASOS information to see what it was reporting for visibility (it's over near Denver JetCenter East, over near Area Hotel), but can't find it. My Google-Fu is weak on historical weather, I guess...

I found this, but it doesn't go back far enough: http://weather.noaa.gov/weather/current/KAPA.html And it's not REALLY clear which source is feeding it...

Paging Scott... Paging Scott... :)
 
denverpilot said:
Y'all might also find it interesting that the Tower has a set of sensors that are official up in the Tower Cab, and there's ALSO an ASOS, but they're NOT linked, AFAIK. DIA has similar.

It's always made me wonder about where to find the appropriate weather outlets to see the historical comparison between the "official" weather, and the ASOS online.

Doing the comparison in real-time is easy, dial the phone number for the ATIS right after the hourly change, and then dial the phone number for the ASOS.

I was curious so I started hunting the Net for the historical ASOS information to see what it was reporting for visibility (it's over near Denver JetCenter East, over near Area Hotel), but can't find it. My Google-Fu is weak on historical weather, I guess...

I found this, but it doesn't go back far enough: http://weather.noaa.gov/weather/current/KAPA.html And it's not REALLY clear which source is feeding it...

Paging Scott... Paging Scott... :)
Click to expand...
You can pull historical metar data on http://www.jesseweather.com
I have pretty much everything back to early 2009. Database is getting kind of big...
 
denverpilot said:
Y'all might also find it interesting that the Tower has a set of sensors that are official up in the Tower Cab, and there's ALSO an ASOS, but they're NOT linked, AFAIK. DIA has similar.
Click to expand...

That set of sensors probably doesn't sense visibility or cloud cover, just temperature, dewpoint, and wind.
 
roncachamp said:
That set of sensors probably doesn't sense visibility or cloud cover, just temperature, dewpoint, and wind.
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The ASOS reports visibility via the phone, if that's what you're saying... if you mean the Tower's sensors don't have visibility... then that doesn't answer the question of where the 3 mile vs 8 mile difference came from in that METAR.
 
denverpilot said:
The ASOS reports visibility via the phone, if that's what you're saying... if you mean the Tower's sensors don't have visibility... then that doesn't answer the question of where the 3 mile vs 8 mile difference came from in that METAR.
Click to expand...

The visibility sensors in the tower are the controller's eyes.
 
The tower at APA happens to have (most towers don't) a Stand Alone Weather Station (SAWS) display. SAWS that provides wind speed and direction, gusts, altimeter, temperature, dew point, and relative humidity. Since APA has a weather observer, the information from the SAWS would only be used is one or more of the ASOS sensors failed, in which case the observer would call the tower for the readings.
 
kkoran said:
The tower at APA happens to have (most towers don't) a Stand Alone Weather Station (SAWS) display. SAWS that provides wind speed and direction, gusts, altimeter, temperature, dew point, and relative humidity. Since APA has a weather observer, the information from the SAWS would only be used is one or more of the ASOS sensors failed, in which case the observer would call the tower for the readings.
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Sounds like the SAWS was put in the wrong place.
 
Blatantly stolen off the web:


Automated airport weather stations use a forward scatter sensor which determines the local air clarity and translates it into prevailing visibility. The sensor uses a beam of xenon light which is sent from one end of the sensor toward the receiver, but offset from a direct line to the receiver by 45 degrees. The amount of light deflected by particles in the air into the receiver determines the clarity of the air. Computer algorithms then translate the amount of scattered light into a prevailing visibility. The current sensors available for deployment cannot detect differences in visibility that are less than 1/4 mile or greater than 10 miles (16 km). Thus, visibilities are only reported at a maximum of 1/4 mile increments, with visibility significantly below 1/4 mile being reported as "M1/4" (less than 1/4 mile); visibilities above 10 miles (16 km) are reported as equal to 10 miles (16 km).
Obscurations to vision

Automated airport weather stations do not have a separate sensor for detecting specific obscurations to vision. Instead, when visibility is reduced below 7 statute miles, the system uses the reported temperature and dewpoint to determine an obscuration to vision. If relative humidity is low (i.e, there is a large difference between the temperature and dewpoint), haze is reported. If relative humidity is high (i.e, there is a small difference between the temperature and the dewpoint), mist or fog is reported, depending on the exact visibility. Fog is reported when visibility is 1/2 mile or less; mist is reported for visibilities greater than 1/2 mile but less than 7 miles (11 km). If the temperature is below freezing, humidity is high and visibility is 1/2 mile or less, freezing fog is reported.


denny-o
 
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