Subscriber question:
"See the attached METAR. I was expecting a measured ceiling, such as 'Overcast 200,' but it says 'Vertical Visibility 200 feet.' What is Vertical Visibility, and why/how/when is it different from the ceiling?" — Mike J.
Jeff:
“That VV in the METAR is ‘vertical visibility,’ but you’ve probably also heard it on the ATIS or AWOS as ‘indefinite ceiling.’ The latter is probably more useful from a pilot’s perspective: The ceiling has a miasmic zone of diminishing visibility rather than a defined cloud base.
Of course, there’s more to the story. So, I asked my friend and meteorological expert Dr. Scott Dennstaedt for the details. Here’s my nutshell version of what he said.
Ceilings are determined using an upward-pointing laser and measuring the light reflected back. The distance to that reflection is recorded as a height above the airport. To prevent against one rogue cloud causing a low report when it’s actually the only cloud in the sky, the reports are averaged over a period of time with extra weight toward the most recent one. Variability in those heights (and other data) gets crunched in as well. The resulting height is reported as a ceiling, such as overcast 300 (OVC003) or broken 7000 (BKN070).
Sometimes the backscatter from the laser doesn’t provide a clear height. This is almost always associated with poor horizontal visibility at the surface. Makes sense; the beam is getting scattered on its way up to the cloud bases before it really finds one.
That’s why you’ll most commonly see something like what you saw in the METAR (3/4SM BR VV002), which is reported as ‘… visibility 3/4, mist, sky condition vertical visibility 200 ….’ Or, sometimes, you’ll hear ‘indefinite ceiling 200.’
The vertical and horizontal visibility are usually related. So ‘1/2SM VV002’ or ‘2SM VV007’ would be more common than ‘1/2SM VV007.’ But sometimes you have almost no visibility with a definite ceiling (1/4SM FG OVC002) or the opposite (4SM VV002). Fog (FG), mist (BR), or some kind of precip such as light rain (-RA) are usually culprits with a VV report.
Operationally, know that a vertical visibility report hints that the IFR conditions will be slow to clear. Approaches in these conditions can be more challenging, as you won’t break out so much as see the airport environment slowly emerge.
Well, hopefully that’s what you see. Don’t count on it.”
Do you prefer to read your METARs and TAFs coded or in plain language?
