ASOS Accuracy, in general

exncsurfer

exncsurfer

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exncsurfer
Do you find ASOS to be accurate in your daily observations/use?

It seems from my limited use that it is pretty bad. For example, I just walked outside, I am probably 3 or 4 miles from the airport, ASOS says CLEAR. I am getting sprinkled on, I can see no blue(sky).

Is the detection just a straight up point source, single sample vs average?(shooting through a hole and reporting CLEAR for that hour). It seems like I see this a lot when there is pretty solid cover.

Ha, I took so long to post this, I think I answered part of my own question. The next METAR came out and reflected what I was seeing. I guess this just leaves me to wonder, how long is the sample size. Does it average 15 min? 5min? 1?
 
Straight up is where the ceilometer looks. And the converse can happen. One small cloud can park itself directly over the instrument.
 
:thumbsup:
GeorgeC said:
http://flighttraining.aopa.org/students/crosscountry/topics/SA09_ASOS.pdf
Click to expand...

Thanks, from that URL:

" The sampling rate is once every 30
seconds, but a single sample is insufficient to
create a report; 30 minutes of data are
required to create a new observation every
minute. The observation is weighted toward
the most recent 10 minutes of data. This
enables ASOS to report rapidly changing sky
conditions."

So I guess the clouds moved in quick and I checked shortly before the next update.
 
I have been in a situation where all the stations in an area were reporting clear for hours when the reality was closer to broken 2500.

You have to understand these are automated stations with a sensor pointing up to a very small patch of sky. It reports whatever it "sees" in that narrow patch which may not always be exactly what conditions really are. In few/scattered conditions there's a lot of blue to be seen around those clouds. There's even some in broken conditions. Also it's entirely possible the wind is blowing in a different direction where the station is than it is across the runway. I've taken off on an 8000'+ strip with a windsock indicating a headwind where I started and by the end I looked down and the windsock on the other end was blowing the other way!

These machines have software to compensate and really they're about 99% right 99% of the time but you can never absolutely take their word as gospel. One always has to keep their eyes open and be ready to turn around if need be.
 
The one thing I don't understand is why Half Moon Bay routinely reports visibility far below what it really is. I've gone there several times with ASOS reporting MVFR (4-5 miles vis) and observed Monterey 50 miles to the south and Point Reyes 50 miles north. It takes far more than 30 minutes to clear 100 miles of marine layer.
 
MAKG1 said:
The one thing I don't understand is why Half Moon Bay routinely reports visibility far below what it really is. I've gone there several times with ASOS reporting MVFR (4-5 miles vis) and observed Monterey 50 miles to the south and Point Reyes 50 miles north. It takes far more than 30 minutes to clear 100 miles of marine layer.
Click to expand...

I didn't think the curvature of the earth allowed you to see that far from ASOS height.:dunno:
 
MauleSkinner said:
I didn't think the curvature of the earth allowed you to see that far from ASOS height.:dunno:
Click to expand...

You can tell if there is THAT much difference in visibility, from pattern altitude or even higher. Easily. You can see the mist below you. From 10 miles away, you would be asking yourself if it's safe to land there, as a blob of 4-5 mile vis looks a lot like a cloud, from a distance.
 
MAKG1 said:
You can tell if there is THAT much difference in visibility, from pattern altitude or even higher. Easily. You can see the mist below you. From 10 miles away, you would be asking yourself if it's safe to land there, as a blob of 4-5 mile vis looks a lot like a cloud, from a distance.
Click to expand...
Sounds like you need to report it as needing calibration, then.
 
I know how people can accidentally find themselves in IMC around here. I was watching some local weather and KLIT reported low ceilings. Then the next METAR was "clear below 12K". WTH?!

In comes a friend in his C172 saying he just had the fright of his life: that airmass moved from KLIT to KSGT (about 15 miles) and he lost sight of the ground. He climbed and was ok.

I know only having 300 hours isn't much - especially over 10 years, but man I still don't have this weather thing figured out ...
 
ASOS has always been problematical for the reasons stated and a number of other reasons.
Never be willing to bet your life on the accuracy of ASOS.
 
Shepherd said:
ASOS has always been problematical for the reasons stated and a number of other reasons.
Never be willing to bet your life on the accuracy of ASOS.
Click to expand...
I'd say that holds true for any weather product...TAFs, area forecasts, icing reports, whatever...taken out of the context of other available information, it can kill you. Used in context, it's simply part of the big picture.

The only time AWOS/ASOS has been a "problem" for me was when I had to do a couple of turns in a hold looking at the airport while waiting for the AWOS to say I could initiate the approach. Otherwise, it's been a very useful tool for me.
 
They lie.

If you have webcams at the airport, between the cams and a ASOS you can get a better idea.
 
MauleSkinner said:
I'd say that holds true for any weather product...TAFs, area forecasts, icing reports, whatever...taken out of the context of other available information, it can kill you. Used in context, it's simply part of the big picture.
Click to expand...
Agreed, aside from the regulatory impacts (no VFR when the official report is IMC regardless of the actual conditions etc) every weather source has inaccuracies that are occasionally significant to a pilot. You have to take it all with a grain of sodium chloride and factor in what you can actually observe. This also means you should never do something that you would consider unsafe if the forecast or report was wrong.
 
Was out on the ramp doing runup.
AWOS (ok, close to ASOS) claimed the wind was 26 kts, but the tower said 31 kts.
Close enough.
Flag was out fairly straight and the windsock was almost stiff
So, the claim of a stiff wind was probably accurate
(wind was almost directly down the runway. Someone did good on the orientation design)
 
At my old airfield the ASOS was almost never correct. The airport was built in a swamp and the station is located over part of it, on a raised platform. It had a tendency to read the winds wrong from a certain direction and almost always had the ceiling off.

Part of the issue is the station is an older AWSS model that the FAA came out with before the ASOS. Very few of them exist, maybe 15, and as such parts and reliability isn't that high. We are on the list to get upgraded to a ASOS in the near future if not already.
 
Also consider that even a manned FSS can be subject to the same accuracy issues.

I was weathered in at KRDU some years ago and spent an hour or two reading the METARS and looking out of the FBO's window. While I can't remember the details it became clear that the instruments call the shots with all the inherent observation issues. There was such a marked difference between what I could see and what was being reported, I actually called the FSS and talked to a briefer at KRDU.

He explained that not only don't they augment the instruments observations but there are no windows available to them.

So there's that
 
Don't forget that you can tune into ASOS/AWOS at other nearby airports to check the weather reported there. Yes, the systems can lie which is why I like checking at least two other nearby fields and compare.
 
Even with the accuracy issues, we're light years ahead of where we were pre-AWOS/ASOS...the nearby airport weather that Lou talked about simply wasn't there 25 years ago.
 
I do that checking around by calling the AWOS at three or so airports, giving greater attention to those upwind or next airport on my route. I have about 15 AWOS numbers in my phone.
 
tinerj said:
I do that checking around by calling the AWOS at three or so airports, giving greater attention to those upwind or next airport on my route. I have about 15 AWOS numbers in my phone.
Click to expand...

I do that while in the air - if I'm a little skittish about the weather. Been surprised a few times to find ASOS reporting clear below 12K when I'm looking at a thunderhead over the airport :eek:
 
Unfortunately, inaccurate (or plain wrong) ASOS/AWOS reports are not uncommon.
Just recently, I watched my buddy land his company private jet long and use up every inch of remaining rwy to stop. They chose rwy 31 despite winds 140@7. The co-pilot (new kid on the block) listened to the ASOS and reported to the captain "winds calm" so they did a straight in from the east. The kid should have listened to nearby airports' weather because then they would have chosen 13 instead.

If the weather reporting station is in error, notify the airport manager asap.
 
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