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» Online Classroom   »   » Public Discussion of Marine Weather   » Heavy Weather Avoidance Fig 4.1a

Author Topic: Heavy Weather Avoidance Fig 4.1a
Dan Cline

 - posted January 17, 2010 07:59 AM      Profile for Dan Cline           Edit/Delete Post 
I purchased Chesneau and Chen's "Heavy Weather Avoidance" a while back and it has finally reached the top of my things to read pile. I'm having a hard time grasping figure 4.1a on page 30 of the book. It looks to me as if the 3-dimensional drawing representing the equal pressure surface is running parallel with the equal height lines. Shouldn't the surface be running perpendicular to these lines, similar to a three-dimensional drawing of the surface represented on a topographical map?
From: Midland, MI
David Burch

 - posted January 17, 2010 03:34 PM      Profile for David Burch           Edit/Delete Post 
OK, good. I have asked him to give us something here, just in case. He is traveling at the moment, as he usually is. But I think he will be back here for Seattle Boat show, end of this month.
From: Starpath, Seattle, WA
David Burch

 - posted January 22, 2010 03:51 PM      Profile for David Burch           Edit/Delete Post 
Lee is tied up for now, so i took a look. The top picture here is the one you refer to and below it i put one that is almost corresponding from our book Modern Marine Weather.

As you point out, it is difficult to get the perspective needed here. Ours is also wrong in the sense that it looks like the 500 mb surface showing several iso-height lines is running parallel to the sea surface.

This is not the case. The 500-mb surface is not parallel to the ocean surface at all. It is sloped and it has rolling hills and valleys in it that reflect the altitudes that define it.

It is as you say, a topographic map. Does this answer your question?

I have made a note to clarify our drawing with annotations in the caption to explain this in our next printing and have added to the online errata. Thanks for pointing it out.


From: Starpath, Seattle, WA
Dan Cline

 - posted January 23, 2010 04:27 AM      Profile for Dan Cline           Edit/Delete Post 
Thank you. It"s a challenging drawing to make. The top drawing shows undulations, but the left edge of the equal pressure surface looks to me as if each of the height contours is the same distance above the ground, which should not be the case.
From: Midland, MI
David Burch

 - posted January 23, 2010 09:42 AM      Profile for David Burch           Edit/Delete Post 
Yes. the undulations in the figure are actually misleading in a sense as the surface does not undulate in that manner.
From: Starpath, Seattle, WA

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