Weather Wind Strength

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Sailfree

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Can someone please remind me of the approx connection between isobar space and wind strength at sea please or point me to a website that explains it.

IIRC for say Solent, take distance between isobars , plot is on the 50deg Oeostropic Wind Scale chartlet in topLH corner to give wind strength. Do you then multiply this Oeostrophic wind strength by 0.66 for wind at sea level and 0.5 for wind over land?

If I could accurately remember everything I had ever been told otr read I would be clever!!
 
Presumably you are referring to Geostrophic Scales which can be found on some synoptic charts. If as you suggest you take the 50 degree latitude scale, you would measure across the isobars with dividers and then transfer that reading to the scale measuring from the LEFT. The reading will be your wind strength give or take a little.

Have you tried reading Frank Singleton's Weather for Sailors?
 
Get hold of the RYA/Met Office synoptic chart blank (come in pads of 50 I believe) which has the various scales for wind strength as well as speed of movement of warm and cold/occluded fronts.
If you do a RYA shore based theory course you should learn there how to use them properly.
 
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Presumably you are referring to Geostrophic Scales which can be found on some synoptic charts. If as you suggest you take the 50 degree latitude scale, you would measure across the isobars with dividers and then transfer that reading to the scale measuring from the LEFT. The reading will be your wind strength give or take a little.

Have you tried reading Frank Singleton's Weather for Sailors?

[/ QUOTE ]From memory you need to do a correction for height as well.
 
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measure across the isobars with dividers and then transfer that reading to the scale measuring from the LEFT. The reading will be your wind strength give or take a little.


[/ QUOTE ]

That will give you the geostrophic wind. If you want to guess at what sort of wind speed your actually going to experience you need to correct for the centrifugal force and the surface friction. Both of these can be big effects. e.g. in a tight low pressure system, your geostrophic scale might predict a wind of 80knots, though you may only feel 20-30 knots.
 
Strictly, centripetal is the conceptual force involved.

Corrections for gradient are less significant over the sea than land.

Remember, too, that cold fronts move at geostrophic wind speed, warm fronts and occlusions move at 2/3 that value.
 
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Strictly, centripetal is the conceptual force involved.


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Depends how you look at it really; there are (obviously) both types of force. The rotation of the air causes a centrifugal force, but the pressure gradient force (which is centrepetal) will exactly balance this. Maybe it's safer to say that you have to correct for the rotation!
 
So far the answers match what I was suggesting to establish the Geostophic wind speed but only Oen has stated that a front moves at 2/3 of the Geostrophic wind speed.

I appreciate that there are factors to correct it to surface wind speed but what is the best approximation or does it vary so much between whether it a high or depression that there is really no meaningful relationship between the spacing of the isobars and surface wind at sea level?

I have difficulty with one suggestion that the geostrophic wind is approximately the surface wind!
 
I'm not alone here in being fortunate to have studied meteorology and climatology in some depth. These questions don't have straightforward answers, and understanding the weather is as much an art as a science, in some respects!

The relationship between surface wind and geostrophic wind (and vertical and horizontal air movement in the rest of the atmosphere) is complex, and depends (in the case of surface versus geostrophic) upon a wide range of variables, principally, pressure gradient, insolation, atmospheric stability, time of day, surface friction (mountains versus flat sea, etc), presence or absence of inversion (environmental lapse rate)...
 
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Remember, too, that cold fronts move at geostrophic wind speed, warm fronts and occlusions move at 2/3 that value.

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Isnt there a scale for the speed of the fronts.... measure the distance between the bumps then compare with the scale?
 
Nobody's mentioned temperature yet. This is relevant if you are estimating the effect of the wind on your boat and on the sea surface. A force 6 on the Arctic circle packs a lot more punch than a force 6 at the equator.

Hmm. This means that the use of the word "force" to prefix the numbers in the Beaufort scale is wrong. What's the right word. "Velocity" is too posh, and "Speed 10 from Navarone" doesn't quite make the cut.

Must quit and do something useful now.
 
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