Calculating Wind Speed

[Walking Beaver]

In the book of Lyle i saw how you can calculate the wind speed.  For every isobar between the high pressure and low pressure you can count 4 millibar (UK meteo maps).  For every isobar you can count 16km/U.
I took contact by mail with the Belgian Forecast Service (KMI) and they told me that on their maps the isobars are every 5 millibar.
How should I calculate the wind speed?  Don't think I can use 16km/U for each isobar.  How can I calculate how much I should use for each isobar (km/U)?

Thanks a lot for any information,

WB

[adi]

The lines drawn on weather charts do the same job as the contour lines on a map they connect equal values of recorded data and are called isopleths and the act of drawing these lines is known as isoplething.

The science of meteorology is long and well established and as with all sciences it terms and equations are standardised across world and in Meteorology that is done by The Royal Meteorology Society in the UK and The Royal Meteorology Institute in Belgian.

Isopleths of different data are know by different names.

The Isopleths for Pressure are called Isobars and are drawn from the base value of 1000 mb at 4 mb intervals (sometimes at 2 mb, very rear)

Isotherms for difference in temperature every 2° or more usually 5° C of F

Isodrosotherms is the dewpoint temperature and is measured in the same way as Isotherms so every 2° or more usually 5° C of F

isotachs which is wind, although normally upper level winds so rear to see a chart like this. the Isopeths are drawn every 20 kts from the base of 30 kts

Now Walking Beaver are you sure the RMI Isopleth their Isobars at 5 mb and it is not the 5°C or F isotherm?

I have just looked at Meteo.be site and although they do not show synoptic charts they do shear their data with ECMEF model and use its data. And I can inform you that the Isobar are in fact spaced ever 4 mb as can be seen here

[adi]

This may help

[Callum]

Great table Adi, already printed it out

[adi]

Lyles description in the book is a little misleading.

In the image he shows the low positioned over Rennes France and looks at the Isobars north across the UK. Low pressure systems form in the troughs of the jet stream, the highest winds in a low pressure system are experienced to the south of the low. This can be seen in the image Lyle used by looking at the bottom of the image you can see the isobars are tighter as they fall off the bottom of the image.

So looking at the chart above at a Latitude of 50° and a space of around 85 miles between the isobars that cross the UK we can see this gives a wind speed of 35 knots which is and average wind speed of 40 mph across the whole country.

I don't mean to contradict the the information in the book but because Lyle has not shown how he came to his conclusion I can't see why there is such a difference.

[Lost Soul]

Does anybody know how to insert a pdf file into a post?  I have something that should add to this thread.

[adi]

As far as i know you can only link a PDF file from, another site.

[Walking Beaver]

adi, thanks a lot for the information.

I checked the mail from the KMI again and the person who sent it to me is formal, the charts of the KMI are always 5 millibar.
You can check them on http://www.kmi.be/meteo/view/nl/123489-Weerkaarten.html

Greetz

P

[adi]

Ok I am not going to argue with that, they know what they produce.

What I do find strange is that they don't issue pressure charts with the pressure on them. They just show Low and High pressure but don't tell you the how many mb each is. You can't work out how deep a low pressure system is, its just lines on a map. They can be what ever scale they chose, even thousands of sausage roles. Without the number the charts are almost useless.

[Lost Soul]

Hmm, pity about the pdf thing.  What I had wanted to include was a Met Office Synoptic Chart that has in in its top left hand corned a graphical tool that allows you to superimpose isobar spacings vs latitude to obtain wind speed.  However, this link will do just as well.  https://secure.metoffice.gov.uk/aviation/bw_charts_sample.jsp

[adi]

Sadly not because it is a log on area.

I think you were alluding to this. To work out the wind speed and direction from the weather chart for a given location, we measure the distance at right angles between the isobars. This can be done with a pair of dividers on a paper chart. We then calculate the wind speed by transferring the dividers to the 'geostrophic wind scale' that is usually found on the corner of a synoptic chart.

As a rule of thumb, the surface wind over the sea will be reduced by 10-20% of the gradient wind that we measure directly off the synoptic chart, with the wind backed (anticlockwise) by 10 to 15 degrees.

Overland the friction is harder to define, based on the roughness of the surface, coupled with the temperature and stability of the air. A rough guide is to reduce the wind speed by around 30-40% with the wind backed by around 30 to 40 degrees.


We have one further modification to make that is a correction for curvature of the isobars or centripetal force. This acts as a negative factor around a 'low' and is a positive around a high pressure? i.e. the wind speed is increased with isobar curvature around a high pressure.

[adi]

The scale can be seen here in the top let corner

[Lost Soul]

Great that is the very chart I was trying to post.  As for access to that web site its free for the basic service.  You just need to register a few details and away you go.

[adi]

Yeah I know mate I have been a member for years, I was just highlighting the fact for others.