View Full Version : Some WX Q's

24th Nov 2004, 11:42
I would be grateful fo some indicative answers and brief explanations concering the Q's below. The correct answers will enable me to nail down some concepts I'm having trouble with.

Thanks in advance.

Q: What is the relationship between met viz and RVR in fog?
a) -Met viz > RVR
b) -Met viz < RVR
c) -No relation between the two
d) -Met viz = RVR

Q: An early morning fog over the sea lasts all day. As the land heats up the sea fog
a) may drift in over the land
b) will always disperse
c) will always remain over the sea.

Captain Stable
24th Nov 2004, 11:46
I am sure I shall get nailed to something hard and unforgiving by some of our regulars here if I am wrong, but I believe both answers are (a).

24th Nov 2004, 13:06
JAROPS 3.430 (It'll be the same in JAROPS1) gives the factor by which to multiply the Met Vis. to get an equivalent RVR. This means that the Met Vis. is usually less than the RVR. However, by day with any lighting system other than HI, the factor is 1, which implies that in those circumstances the Met Vis is equal to RVR.

24th Nov 2004, 13:21
so I have been told ...

VIS is measured in eight directions of compass from observer's station by evaluating contrast elements seen by naked eye (can you see the window on the church tower?) compared to a chart.

Unless great variations are observed, the minimum value is given as reported visibility. In the opposite case, minimum and/or maximum values are provided as well. The logic (incomplete, as my memory is fading) being if a single direction is twice as bad or worse than the lowest other direction you'll see it coded as
4000 1800SE in METAR. Decode: reported(!) VIS 4000m significantly worse to south-east 1800m.

On the other hand, RVR is measured by state of the art electronic equipment parallel to RWY edge at prescribed height (1,5m ???, anyone?). Maximum value encoded to METAR would be Rnn 1500P meaning more than 1500m.

The requirement to provide RVR is set by VIS limit which I cannot remember now. (2000m or less?).

Another important part, what fog is. FG stands for visible moisture that degrades VIS below 1000m!

Thus I conlude that VIS and RVR have no relation, with regard to their values. On the other hand, there is some logic and rules for encoding them.

800 R24/1200D FG R31/500N - possible
4000 1600SE BR - possible
1200 R24/500 BR MIFG - possible

It had been my experinece on ATPL that one must read the question very thoroughly and think VERY hard about the TRUE meaning of every bloody SINGLE word. Seem actually harder than learning the stuff as such. Witnessed also on PPRuNe, yes, at maximum speed, you indeed have maximum drag. :ok:

Cheers, FD.

24th Nov 2004, 14:53
c) and a) for me!!

Probably be seeing you on the nailing board, Captain S!

B Fraser
29th Nov 2004, 18:17
Sorry, points of the compass don't come into it as Met Viz is the lowest value in any direction as estimated from the point of the observer.

Each station made up their own "spotting" card with known distances to significant landmarks such as line of trees, church spires, radio masts etc. There was a bit of guesswork when you could see one point at 1200m reasonably well but not the next at 1500m so the observer would say 1400m as an educated guess.

Wasn't RVR just a case of counting the lights ?

Happy Landings


29th Nov 2004, 18:30
Sorry, points of the compass don't come into it as Met Viz is the lowest value in any direction as estimated from the point of the observer.

Sounds like you may need to catch up on AIC Y145 on Prevailing Visibility (http://www.ais.org.uk/aes/pubs/aip/pdf/aic/4Y145.PDF).