I bought the Mariner’s Weather Handbook and find it helpful. I have a question, though. On page 123, you attempt to show divergence/convergence on a drawing (bottom of page). I have read it 10 times and don’t understand what you are trying to tell me. It is early in your discussion of this subject and so I do not want to move on without an understanding. Can you elaborate on what that drawing is showing me? Thank you, George
Hi George: Sorry about the confusion. This is a difficult topic to get across. First of all, there is a typo in the last sentence. This should read, “On the warm side the winds diverge on the upwind side and converge on the downwind side.” The drawing itself is correct. What we are trying to explain is the difference between troughs and ridges on their up- and downwind sides. For troughs, you have convergence upwind and typically divergence downwind. The opposite is true with ridges. Where this is important is the effect at the surface. Areas of convergence (regardless if from a trough or ridge) lead to subsiding air and high pressure on the surface. Areas of divergence aloft lead to surface level low pressure. And, both divergence and convergence can originate from troughs or ridges although it is normally the troughs which have the greatest impact in surface low pressure development. A further issue, discussed later on, is the shape of the convergence and divergence, especially the latter. This has an impact on the type of surface frontal structure, and the speed at which it develops. Hope this is helpful.
Posted by Steve Dashew (November 30, 1999)
