Talk:Dry line

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[edit] Dry air is more dense than moist air

If a warm moist airmass was more dense that a hot dry airmass, drylines would move west during daylight hours, yet they do not. Since this is false, it is therefore reverted. Thegreatdr 19:11, 3 December 2006 (UTC)

The warm moist air encountered at drylines is actually MORE dense than the hot dry air near the surface. This creates a capping inversion and a convergence zone where the dryline slope boundary reverses aloft. The peculiarity results from the difference in heat which near the surface is sufficient to make the drier air less dense. The movement of drylines is not a function of this density difference but of the winds aloft, which is dictated by other factors such as the low level jet in the warm moist air, or an approaching upper level trough.Tmangray 17:10, 4 December 2006 (UTC)

An example outside the classic dryline areas which more clearly demonstrates the significant effect of temperature and not just humidity on air density is encountered along the California coast. Here, mild to cool humid air is ALWAYS more dense than the hot dry air inland and always wedges UNDER that air. Another example where the seas are considerably warmer is off the west coast of Africa where the Saharan Air Layer is found ALOFT, capping the warm humid air which would otherwise be busy organizing itself into hurricanes. The situation in the US southern plains in summer is such that the comparative densities of the warm air masses, moist and dry, is usually very close, with the tipping point being literally the tip of the dry air wedge ALOFT, usually no more than 1500 feet above the ground, which BTW, is close to marine layer thickness off the California coast in summer.Tmangray 17:19, 4 December 2006 (UTC)

Here's a link to a vertical cross-section clearly showing the warm humid air wedging UNDER hot dry air at a dryline:

[1]

Here's a link to an AMS journal article which discusses the issue, especially re: how the slope changes diurnally: [2] Tmangray 21:04, 4 December 2006 (UTC)

Ok. I understand that temperature becomes the overriding factor in density over moisture at a certain point, but at what temperature differential is this achieved? The way the article was worded previously made it sound like the laws of physics (when it came to air density between dry/moist air) were thrown out the window at the surface, yet resumed working aloft. Temperature needs to take more of a role in the article if it becomes the more important factor in density, but what temperature differential is required to override the greater density of drier air? 5F? 10F? 15F? Definitely add those references within the article...and we'll need to find a way to make the arguments presented coherent yet understandable. If they had been embedded within the text originally, I wouldn't have completely reverted it. Thegreatdr 21:27, 4 December 2006 (UTC)

It's not quite as simple as a temperature differential. It's a function also of course of the humidity differential, and both as functions of elevation. I'm not sure how to put it other than as is, at least for now. Tmangray 01:47, 5 December 2006 (UTC)

This would be a good place to note that this is why the dryline is denoted with a warm front colored brown, with the half-circles pointed in the direction of overriding at the surface. What I don't understand is why they don't also use a cold front symbol colored brown when the direction changes. Tmangray 01:54, 5 December 2006 (UTC)

We toyed briefly with switching pips when the dryline marched backwards at night, but that ran against the uniqueness of the pip meaning of the dryline...that the dry air is behind the pips. The pips should be standardized to fit the other frontal symbols, I agree. Thegreatdr 20:12, 5 December 2006 (UTC)

[edit] Better Image

We should keep our eyes on the visible satellite for a better example of a dry line than the current image. It's pretty unclear where the dryline is, and even so, there's not a great drop in dewpoint where its labeled. A more distinct example would be great for Wikipedia. ~ Triberocker (talk) 18:48, 17 February 2008 (UTC)