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 » Online Classroom   »   » Public Discussion of Marine Weather   » Wind

Author Topic: Wind
 M. C. Rowley posted January 25, 2005 06:37 PM                   I am having a difficult time comprehending the reason why wind flows near the surface at roughly 20 degrees inward of isobars around lows and roughly 20 degrees outward from highs and then flows essentially parallel to contours at higher altitudes. I assume this change is due to friction at the earth's surface, however, at first glance one would think that wind should flow more perpendicular to pressure contours. Since this is obviously not the case, what is the driving force that makes wind flow parallel to contours. Is the Coriolis force entirely responsible for this? Can you direct me to a resource that explains this mathematically? I understand the concept of pressure gradients and that highs normally have weaker winds due to centripetal forces increasing much faster than the Coriolis force. This explains the reduced pressure gradient normally found around highs, however, does not explain why wind flows roughly parallel to isobars.I look forward to your answer, in the meantime, I will keep reading......Mark
 David Burch posted January 27, 2005 01:11 PM                   Mark, i notice from your other post that you have the weather trainer software. If i may direct you to article A2 on What Makes the Wind. Click the book called Wind in the main library page. There are pictures and detailed discussion.The key is wind flows from High to Low due to the pressure gradient (gravity), and coriolis force bends it to the right (NH). With no friction, it continues to bend until pressure gradient balances out coriolis force and this leaves the wind flowing parallel to the isobars.... and indeed this is what happens at higher elevations where there is no friction.at the surface, on the other hand, the bend to the right is resisted by the surface friction and the balance of forces occurs before it bends all the way around, leaving it flowing some 15 to 30° out of a High, which is exactly the same as saying into the Low.The issue of strong winds around Lows and weaker around Highs is another issue. Partly related, as you have detected, but with more nuance. We have actual equations to illustrate this in that same article, see page 20 and 21 region. From: Starpath, Seattle, WA
 Robert Bemben posted September 02, 2005 05:33 PM                   There's an interesting 'twist' to this that I read about in relation to sailing. The wind direction at the foot of the sail is different from the wind direction at the head of the sail, also due to friction of the water surface. That's why some 'twist' is desirable in the mainsail (not just to 'spill wind' in a blow), i.e. the head of the sail is pointed more windward than the foot, because the wind direction there is more windward. I'm wondering if the Coriolus effect at work here too? I think it applies to both starboard and port tacks.