geostrophic wind scale

[Peter]

This might be a difficult one to answer without actually being shown, but I’ll ask the question anyway. How does one use the geostrophic wind scale show on synoptic charts?

Peter

 

[JamesS]

I haven't got a chart in front of me but as I recall there should be a scale at the top of the chart. Using your dividers transfer the distance between the isobars to this scale and read off.

Hope this helps.

 

[Guest]

There are also scales to be used in a similar manner to calculate the speed of advance of fronts.

 

[NigeCh]

Goto the following URL:

http://www.onlineweather.com/v4/data/metOC1.html

The instructions are above the geostrophic chartlet at the top left hand corner (BTW, it took a fair number of people from this BB and others to get that chartlet included )

 

[ParaHandy]

Peter
Measure the distance between the isobars eg 10mm,
Note latitude at the selected point eg 50North,
In the geostrophic wind scale box, select the horizontal latitude line (select nearest)
Find which wind vector intersects 10mm from left hand on the 50N line

70N I...y....x................
60N I....y......x.............
50N I......y...@...x...........
40N I.........y.......x........
------------25-----15

Hopefully this diagram comes out. Assume it's a Bracknell synoptic chart and "@" is 10mm from left hand line (shown as "I") and "x" is the 15kn vector and "y" the 25kn wind vector. Your wind speed at "@" will be approx 20kn.

Over land, you should knock a few knots off to allow for friction.

To get the wind direction, around a low in the Northern hemisphere the wind rotates anticlockwise and slightly in toward the centre of the low.

I would use www.greatweather.co.uk for synoptic charts. They have, as well as Bracknell, Sembach charts which you will find very good in understanding how fronts move and what weather to expect.

Hope this is useful.




<P ID="edit"><FONT SIZE=-1>Edited by ParaHandy on Wed May 15 15:34:42 2002 (server time).</FONT></P>

 

[yachtcharisma]

I'm sure I've read of a "rule of thumb" for using these based on the distance on the chart between Lands End and the North Foreland. It's something like windspeed = 10 x the number of isobar spaces between those two points.

Just trying that out now on the chart that seems reasonably accurate, at least for wind speeds likely to be of interest to yachtsmen, but does anyone else know if I've remembered it correctly?

Cheers
Patrick

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