Wind Behavior: 0400 - 0600

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Byron Angel
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Wind Behavior: 0400 - 0600

Post by Byron Angel » Thu Apr 02, 2020 2:36 am

Noticed an interesting item in the KTB of Prinz Eugen. regarding wind behavior. Prinz Eugen was periodically entering brief weather notes.
KTB entries for 24 May as follows -

0000 - Wind NW at Force 3, overcast, snow.
0400 - Wind NW at Force 4, overcast.

0553 - Hood and Prince of Wales open fire.
0555 - Prinz Eugen and Bismarck open fire.
0600 - Wind E at Force 3, overcast.

Given (a) that this drama unfolded rapidly over terribly few minutes and (b) that it was unlikely the wind waited until exactly the stroke of 0600 hours to change direction, has anyone seen any other references that cite a more exact time for this considerable (135 degree) wind shift?

Byron

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Re: Wind Behavior: 0400 - 0600

Post by dunmunro » Thu Apr 02, 2020 10:02 am

PoW 0400: Wind N Force 4 Sea 32

Norfolk 0400: Wind N Force 5 Sea 32

Suffolk 0400: Wind NNE Force 7 Sea 22

unfortunately, the log entries are made every 4 hours.

hans zurbriggen
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Re: Wind Behavior: 0400 - 0600

Post by hans zurbriggen » Thu Apr 02, 2020 10:49 am

Hello, British Ships Logs report Wind at:
04:00 as North force 4 (Prince of Wales), North force 5 (Norfolk), NNE force 7 (Suffolk)
08:00 as North force 4 (PoW), North force 5 (NF), SW force 3 (SF).
Confusion here as well.

hans

edit: just saw Mr.Dunmunro entry. Sorry for duplicate info at 04:00

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marcelo_malara
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Re: Wind Behavior: 0400 - 0600

Post by marcelo_malara » Thu Apr 02, 2020 4:33 pm

Hi guys. This chart shows the general synopsis for May 24th 1941, reconstructed from historical records. Bear in mind that meteorology then was not what is now, there were no computers, no globally coordinated surface and upper atmosphere measurings, etc...

The most significant thing in the chart is the low pressure system labeled T, 990 mb north of UK. Winds circles anticlockwise these systems, so you would expect N winds if you are to the S of Icelend, or E winds if you are to the N of the UK. The exact location of the low pressure is certainly unknown, and the low also moves, so you can expect different winds directions at different times in a moving ship.

Moreover, was the described wind directions true wind or felt wind? In a moving ship the felt wind, adequately named the resultant wind, is the vector component of the true wind minus the ship movement. The same with the speed. Because of the low winds mentioned, Force 3 and 4, which are impossible in the presence of a low pressure system, I would conclude that relative winds and velocities are used.




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Re: Wind Behavior: 0400 - 0600

Post by Byron Angel » Thu Apr 02, 2020 9:45 pm

Hi Marcelo,
Interesting post; two comments -

1 - Re the question of the recorded wind direction being relative or true, my bet would be on true. True wind would be important for accurate gunnery.

2 - I disagree that Force 3/4 winds cannot be part of a low pressure system. Wind velocity is related to the steepness of the barometric curve over distance - i.e., the closer together the pressure isobars on the chart, the greater the wind velocity implied and vice-versa. Just as there are weak high pressure systems with weak wind velocities, there can be weak low pressure systems with weak wind velocities.

I'm no professional meteorologist, but I have studied this topic in my own amateur way. Anyone with more intimate knowledge of the subject is more than welcome to comment further on this subject.

FWIW.

B

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marcelo_malara
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Re: Wind Behavior: 0400 - 0600

Post by marcelo_malara » Thu Apr 02, 2020 10:41 pm

Hi Byron, I am an amateur meteorologist too. True that the wind speed depends on:

-the pressure gradient
-the latitude, this is related to the intensity of the Coriolis force

The question is that low pressure systems produce steeper gradients than high pressure ones. So the velocity in a low is strongly influenced by the central pressure of the system. In the extreme case, there is no way that a tropical depression would generate mild winds. I don´t believe that a low with a central pressure of 990 mb would generate 15 kt wind.

These days we have one of the rare occasions in the year that a low forms in the River Plate, and coincidentally it has a central pressure of 992 mb, as you can see in the wind color scale there is no major area around with winds lower than 30 kt.

Image

Moreover, with the chart as seen in May 24th, and with the German ships and the trailing cruisers sailing generally to the south, and PoW and Hood to the W/SW, the ships vectors would subtract a fair portion of the real moderate to strong wind speeds that one would expect close to a low, and give the impression of the fair winds described. I so conclude that the wind data is relative and not real.

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Re: Wind Behavior: 0400 - 0600

Post by Byron Angel » Fri Apr 03, 2020 2:46 am

Hi Marcelo,
I did a little hunting on the interwebnet. If the heading and speed of your own ship is known, it is possible to convert relative wind direction and velocity to true wind direction and velocity.

FWIW,

Byron

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Re: Wind Behavior: 0400 - 0600

Post by marcelo_malara » Fri Apr 03, 2020 3:05 am

Oh yes, it is easy, just use the formula:

relative wind = true wind - ship movement

All the quantities are vectors, and the minus sign implies that ship movement is represented by a vector of opposite direction to the ship´s.

In the example:

-red vector = minus ship speed (ship is actually moving to the top)
-blue vector = true wind
-green vector = relative wind

Image

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marcelo_malara
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Re: Wind Behavior: 0400 - 0600

Post by marcelo_malara » Fri Apr 03, 2020 3:09 am

To find true wind you have to re arrange the formula:

relative wind + ship movement = true wind

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Re: Wind Behavior: 0400 - 0600

Post by dunmunro » Fri Apr 03, 2020 3:16 am

I'm certain that they stated true wind in the log. Here's a device to extract true wind from relative wind:

http://dreadnoughtproject.org/tfs/index ... d_Dumaresq

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marcelo_malara
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Re: Wind Behavior: 0400 - 0600

Post by marcelo_malara » Fri Apr 03, 2020 3:25 am

Another synoptic chart:

Image

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Re: Wind Behavior: 0400 - 0600

Post by dunmunro » Fri Apr 03, 2020 3:35 am

marcelo_malara wrote:
Fri Apr 03, 2020 3:25 am
Another synoptic chart:

Image

Norfolk 0400: BP 997.9

Norfolk 0800: BP 999.5

Suffolk 0800: BP 1003.3

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Re: Wind Behavior: 0400 - 0600

Post by marcelo_malara » Fri Apr 03, 2020 3:41 am

That´s good data, quiet agree with the charts, we can disregard Suffolk´s, may be the barometer was not correctly calibrated.

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Re: Wind Behavior: 0400 - 0600

Post by marcelo_malara » Fri Apr 03, 2020 3:44 am

Can you guys find wind data for that day in the UK? I can´t get any. I would like to see how these isobars spacing translates into wind.

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Re: Wind Behavior: 0400 - 0600

Post by hans zurbriggen » Fri Apr 03, 2020 11:46 am

Hello,
thanks to Mr.Malara for the meteo charts, They point to a probable NE wind (as per first battlemap from F.O.Busch).
I see your point about wind in a low pressure area. I hope someone can find the wind in UK / Iceland that day.
However, looking at PG film and photos, we don't get any feeling of strong wind that day: look Bismarck funnel smoke, guns smoke drifting in the air, aspect of waves on sea, very rare 'white horses', shell splashes appearance, etc.
Difficult of course to exactly estimate wind, but I would not say it was much more than 10 knots. Estimation of force 3 seems more correct than 4.

hans

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