The difference between 000 and 003 is little enough for the alarm bells to ring.
I am not saying one wouldn’t go; that would be ridiculous. But I would be getting weather updates en route and ensuring that the whole area, including alternates, was not about to go foggy (which I have seen, unforecast.)
I think fog is unforecast more often than not.
I am surprised fog needs a bit of movement. Where I live we often get fog among the hills, and not a single leaf on the vegetation is moving.
When you get fog in zero wind conditions it is usually in sky-clear conditions. This is called radiation fog and starts as shallow fog over meadows. This is because the earth radiates its heat to the atmosphere, cooling the lower layers below dew point. When clouds are present in no wind conditions, the heat is reflected by the clouds and temperature raises, fog is very unlikely.
Thanks to the Belgian Air Force, met wing, for 15 months of involuntary education, but it was fun.
When you get fog in zero wind conditions it is usually in sky-clear conditions.
That was the point I was trying, but clearly failing, to make.
Here is, AIUi, the way slight wind helps radiation fog to form.
If there is a no wind, the radiation fog as described by @dirk remains very shallow, since air conducts heat very poorly, and only the layers closest to the ground get cooled below the dew point.
Very slight wind mixes the lower air layers, increasing the depth of the fog layer. The stronger, the thicker the layer, but the lower the average temperature drop as more air has to be cooled, up to the point that no fog forms. Hence radiation fog formation benefits from a couple of knots wind.
First time I’ve seen such a METAR:
EGLC 280720Z AUTO VRB04G19KT 9999 NCD 11/09 Q0990
How do they choose the runway?!
It might be “interesting” to start the approach with 19kts headwind and land with 19kts of tailwind… Probably, quite a few go-arounds?
arj1 wrote:
EGLC 280720Z AUTO VRB04G19KT 9999 NCD 11/09 Q0990
The operative word is AUTO. So there is no observer there. Otherwise you’d most probably see a direction plus variation.
Looking at FR, the only plane at the time was landing normally on 27.
As for the landing direction, ATC would look at the spot winds as well as the METARS before and after.
EGLC 280820Z AUTO 21007KT 9999 FEW049 11/10 Q0990
EGLC 280750Z AUTO 20005KT 130V250 9999 NCD 11/09 Q0990
EGLC 280720Z AUTO VRB04G19KT 9999 NCD 11/09 Q0990
EGLC 280650Z AUTO 21006G16KT 170V300 9999 NCD 11/09 Q0989
it becomes pretty clear that the predominant wind is from 200-210.
41 nm between two airports in the UK, compare the winds (and the weath in general):
EGKB 020820Z 00000KT 9000 -RADZ SCT016 09/08 Q0955
EGMD 020820Z AUTO 20041G57KT 1000 +RA BKN008/// OVC012/// 13/11 Q0957
Certainly remarkable, but with one airport directly on the sea, and the other inland, it is not too unusual.
Those low QNH’s are noteworthy though.
boscomantico wrote:
Certainly remarkable, but with one airport directly on the sea, and the other inland, it is not too unusual.
I know, and someone with more knowledge in Meteo will most like be able to post picture discribing why it has happened, but still some interesting observations.
boscomantico wrote:
Those low QNH’s are noteworthy though.
Yep!
