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PostPosted: Wed Jan 04, 2006 9:28 am 
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I see all the time when I check observations on NavCanada's website about the RVR. What I don't understand is if the prevailing or runway visibility changes, say improves from 3/4 to 1 sm, why would the observer indicate Downward trend? Or, the opposite, if it deteriorates from say 1 sm to say 1/8 sm and the RVR drops from 6000 ft to 1800 and Upward trend is indicated, what am I missing? :?:

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PostPosted: Wed Jan 04, 2006 5:01 pm 
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The remark U/D/N is generated from the RVR itself, its not from the observer themselves.

What may have occurred in the case of RVR dropping from 6000 to 1800 with an Upward trend was that the RVR actually dropped to (for example) as low as 1400. And in time span it took the observer to go outside, look at the clouds, right it out on the form, then punch it into the computer the RVR improved slightly. Thus the upward trend.

hydro



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PostPosted: Wed Jan 04, 2006 5:21 pm 
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So manned observations sites are the only ones capable of reporting RVR's, which I assume are taken from the ceilometers? I say this since AWOS reports don't include RVR's, just minute by minute specials for reduced vis's. Even on occasion ni a METAR with no specials, an RVR can stay the same with either Upward or Downward trend and no chg whatsoever during the past hour or two.

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PostPosted: Wed Jan 04, 2006 6:06 pm 
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The RVR info is either fed directly into the weather report from the RVRs or input by the observer.


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PostPosted: Wed Jan 04, 2006 7:06 pm 
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ceilometers and RVRs are two different instuments, reporting two very different things.

RVRs (at least the ones I'm familiar with) update every minute, and they are also very heavily affected by localized pockets of higher or lower visibility, since they only report visibility for the area between the light source and the sensor (very short distance). If a pocket of fog, or some other obstuction to vision like smoke, drifts past the sensor, one minute the reading can be fairly high, then the next drop to almost nothing, then go back up to a higher number, but not quite as high as the first. On the last reading, it will indicate the value and an upward trend, even though the prevailing visibility is going down.



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PostPosted: Wed Jan 04, 2006 9:25 pm 
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What Pygmie said. And ceilometers (oh, how I want one. ceiling projectors are bad) measure the height of cloud, so as far as visibility goes, they're pretty much useless for anything but vertical visibility. They shoot a laser pulse up into the air, and record when they get a reflection. Like radar, but with light, and only looking at the 1cm square area directly above the laser. Which is part of the reason why AWOS stations suck.


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