Lines on synoptic charts

[BlueSkyNick]

Always willing to learn, and not afraid to ask stupid questions (as many of you will have already realised), I would be pleased to know the answer to the following.

On the Met Office synoptics, this one for example,
<A target="_blank" HREF=http://www.wetterzentrale.de/pics/brack2.html>http://www.wetterzentrale.de/pics/brack2.html</A>
there is a dotted line from the middle of the west Portugal coast going in a NW'ly direction across the Atlantic, and then down towards the caribbean. It has 546 on it.

I am told by one friend that it is the boundary between the polar and equatorial air masses. Is this correct? What does the number represent? How is it relevant to a Solent/Channel potterer like me?

Thanks in Advance

<hr width=100% size=1>I'm average size, Its just that everybody else is short.

 

[Twister_Ken]

Dunno, but...

... the chart title is MSLP and Thickness. Supposing that the isobars are MSLP, then maybe the dotty lines and numbers (coz there are more than one) are the thickness quotient. But the thickness of what? Well, I found this via Google:

Information for Understanding Forecast Discussions...

...Atmospheric Thickness Levels: Heights in the atmosphere are often measured based on where the pressure is a constant value. The 'thickness' of the atmosphere is the three dimensional depth of the atmosphere between two pressure values. A useful thickness value is the thickness (or depth) of the atmosphere between the pressure of 500 millibars (mb) (about 18,000 feet) and 1000 mb (millibars) (about the earth surface near sea level). The thickness values become higher when the upper atmosphere is warmer and become lower when the upper atmosphere is colder. Thickness values are useful in predicting rain/snow or sleet. They correlate with temperatures at specific heights that are correlated with snow or sleet or rain.

<hr width=100% size=1><A target="_blank" HREF=http://www.writeforweb.com/twister1>Let's Twist Again</A>

 

[StugeronSteve]

Re: Had to be something simple...

didn't it!

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[Roberto]

The dashed lines represent thickness of the atmosphere.

Imagine sending a recording balloon witha barometer from earth level up in the sky, and measuring at what heights it reads 1000mb and 500mb, compute the difference between the two and you have 1000-500mb thickness. The figure on the chart takes off the final zero, so 546 means 5460m.

Briefly, thickness gives an indication of the average temperature of the atmosphere, the higher the reading, the warmer the column of air where the reading is taken.

What are they used for?
Basically
1.Determining mean virtual temperature advection, warm or cold air advection: if geostrophic wind pushes lower thickness values into an area with higher thickness, you have cold temperature advection (and vice versa)
2.Determining the position of surface fronts: where thickness lines are packed there is a strong temperature variation, that is often a front. The chart you see is poor in detail, if you look at thickness charts here

https://www.fnmoc.navy.mil/mywxmap/login.do?username=guest

you can see many more thickness isopleths (curves joining points having the same thickness) and check that most often where lines are packed there is an underlying front

3.Intensity of fronts: the nearer the thickness lines in a front, the higher the temperature variation, the strongest the front

4.Together with isobars, they are useful to determine areas of frontolysis or frontogenesis, areas where a front is likely to build or weaken

5.Also, in winter, if thickness is above 540dam there is usually rain, if lower usually frozen precipitations


hope this helps

rob


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[ParaHandy]

... at a guess they look v. similar to the upper atmosphere isobars which appear on the 500mb weather charts. if that is so, they represent the jet stream and i have read that knowledge of what they are doing determines the pressure at sea level and are thus important ...

otherwise its the met office bod filling his pools coupon in ....

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[BlueSkyNick]

Thanks Rob, that's really interesting and useful!

I haven't had time to investigate further and study in more detail, but you have certainly given me enough info to do so.

Thanks to Ken and Para, too.

<hr width=100% size=1>I'm average size, Its just that everybody else is short.

 

[FlyingSpud]

Look in ‘Heavy Weather Sailing’ for a good basic chapter on understanding the implications of ‘Thickness’ on a synoptic chart.

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[VicS]

You will also find a brief explanation on<A target="_blank" HREF=http://www.franksingleton.clara.net/index.html#me>Frank Singleton's web site</A>

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