Met Question

[Kiev23]

Hello everyone,

One met question I am struggling to understand....

The surface wind at an inland airfield at night is 240/05

What would the surface wind be at 2000ft above the airfield?

A 270/10
B 270/20
C 210/05
D 210/ 10

I realise that the answer is B but why does surface wind act differently during the day than at night ? Day theory would suggest answer A but infact the correct answer is B.

Thanks in advance

[Anonystude]

Convective turbulence. Surface heating during the day increases mixing across the boundary layer, making the surface wind closer to the 2000' wind. At night this doesn't happen, so the wind backs more and slacks more.

[pitot_noob]

Yup.

It's called Diurnal variation.

[Whopity]

What would the surface wind be at 2000ft above the airfield?
The surface wind will never be 2000 ft above the airfield!

[IO540]

Is any of this happening in reality? Surely the coriolis effect, which is responsible for the altitude wind turning right relative to the surface wind, is still there at night?

I don't fly much at night but have never noticed any big difference in the winds aloft v. surface. What does happen is that both of them tend to be much weaker.

[Anonystude]

It does, on a still night. From memory -- when I was last night flying at an inland airfield -- we had something like 210/10 at surface, 270/40 at a couple of thousand feet. The problem is the surface friction causing the wind speed to reduce, reducing in turn the magnitude of the Coriolis force (which is something like 2 x wind velocity x angular velocity of rotation...).

I think!

[Whopity]

Is any of this happening in reality?Some years ago the RAF dumped a number of paratrops in the Kiel Canal as a result of this effect, a number were run down by a ship.

Can be quite noticable when conducting night training.

[Fuji Abound]

Of course it is a lot less noticeable at sea, but I find it interesting that in off shore racing yachts we recognise there is sufficient gradient even between the boom and the top of the mast that it is worthwhile changing the shape of the sail to accomodate the wind direction and strength not being unifrom over the whole sail area.

While talking about the sea it is also worthwhile remembering what a signficant effect the sea can have. The onset of sea breezes in the morning and their reversal in the evening has a significant bearing for a surprisingly large distance in land which means winds will be very different both during the day and at night for any airport that is influenced by sea breezes.

[bookworm]

What would the wind be at 2000ft above the airfield?

A 270/10
B 270/20
C 210/05
D 210/ 10

It's a lousy question. The precision of such rules of thumb are insufficient to differentiate between A and B.

Is any of this happening in reality? Surely the coriolis effect, which is responsible for the altitude wind turning right relative to the surface wind, is still there at night?

It's more that the drag of the surface is responsible for turning the surface wind left relative to the geostrophic wind. With that in mind, a geostrophic wind of 270/20 and a geostrophic wind of 270/10 are both likely to result in a surface wind of 240/not-much.

I don't fly much at night but have never noticed any big difference in the winds aloft v. surface. What does happen is that both of them tend to be much weaker.

Like you I have relatively little experience at night, but tend to notice the effect. What tends to happen on a clear night is that the boundary layer is also thinner. So by the time you get to even 1000 ft, you get the full force of the geostrophic wind.

[vw_nutter]

Hi

Out of all the questions in the met unit this was the one I struggled with the most. Not because I don't understand diurnal variation but because there seems to be different ways of working it out depending on what text you have read. I read both Thom and Pratt books which offered different answers based on the same question.

The question came up in my met exam a few weeks ago and in the end it was the only one out of all the questions I had to take an "educated" guess at so I chose the one that had the biggest safety margin and as luck would have it it was the correct answer. I think if you ever get totally stumped in the exam, go for the one that offers the biggest margin of safety.

[Vone Rotate]

I seem to remember from my PPL days for exams they expect you to veer or add 30' and double the wind speed between surface wind and 2000ft.

We all know there are lots of factors which effect it in the real world but try not to think tooooooo deep on these type of questions...

V1

[IO540]

These questions have bollox value for real flying knowledge, because those who want / need to know the winds aloft will get them off the internet (from Form 214, GFS, etc) and most real flying to places is done with a GPS / radio nav which gives you continuous lateral guidance, making the winds aloft irrelevant except for total range / fuel requirements computations.

And the winds aloft forecasts are often total bollox anyway. I did some 800nm flights in June, to/from Croatia, on which the forecasts were severely out (e.g. forecasting a 10kt tailwind but actually it was a 10kt headwind).

[fireflybob]

The interesting thing is that, all things being equal, you can expect more windshear when flying an approach at night compared to day because the change from surface wind to geostrophic (free stream) wind is spread over a thinner layer.

It's a trap in a way because a light surface wind can lull you into thinking that there isn't much wind at circuit height when flying at night.