Troughs & Ridges

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junepath
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Troughs & Ridges

#1 Postby junepath » Thu Aug 04, 2016 8:22 am

Could someone explain these two different things to me, in relatively easy to to understand ways?

I watch this site to sort of keep an eye on an area that we frequent on the US East Coast* as it's a popular target for TS's at least, and the occasional hurricane, and it has come up a few times in the forums about troughs and ridges near the US East Coast and I don't totally understand what that means.

I've watched a few hurricane videos on youtube but they only explain the mechanics of the storms themselves, as well as how they are pushed by trade winds and hindered by shear, and all that stuff that you guys already know. I haven't seen anything that explains ridges and troughs. From what I can gather a ridge is sort of a barrier?

(*I started watching this site to keep an eye on our beloved Hatteras Island and now I can't stop reading about any and all hurricanes/tropical storms. I think I'm one of you...)
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TheAustinMan
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Re: Troughs & Ridges

#2 Postby TheAustinMan » Sat Aug 06, 2016 7:42 pm

I don't know too much about troughs and ridges, but I'll try my best:

Broadly speaking, troughs are elongated areas of low pressure, while ridges are elongated areas of high pressure. Troughs and ridges may also be known as cyclones and anticyclones, but the latter is much more frequently used.

Troughs and ridges are important factors in the movement of hurricanes. You probably know that low-pressure systems spin the air around them counter-clockwise, while high pressure systems spin clockwise, at least in the northern hemisphere. When hurricanes encounter troughs and ridges, they generally follow these paths: they will move clockwise about the periphery of a ridge, and move counter-clockwise around the periphery of a trough. Hurricanes move around troughs and ridges, and not through them - this is where the idea of being a "barrier" comes from.

Troughs and ridges usually come and go as they move with the tradewinds, but some are nearly permanent features. Perhaps the most well-known ridge is the Bermuda/Azores Ridge (also known by a lot of other names, usually in reference to one of those two locations or that it's an area of high pressure). It sits in the subtropics in the Central Atlantic, and steers tropical waves and storms off of Africa westward. The following illustration puts this into context:

Image

There are some steering troughs that move about in the tropics, but oftentimes they are lobes in the polar jet stream. You may have heard troughs referred to as "dips in the jet stream". These troughs usually have a more recognizable feature associated with them: cold fronts. Since troughs rotate counter-clockwise, a hurricane approaching a trough from the east will (in general) move more northward.

You may have heard the idea that the stronger a hurricane is, the more north it will go. This is oftentimes true, but I would more accurately rephrase it as "the stronger a hurricane, the more its track will be influenced by ridges and troughs." Generally speaking, the track of a strong hurricane will be more heavily influenced by ridges and troughs, while a weaker storm will often move moreso along with the tradewinds.

For example, imagine a storm north of Puerto Rico, moving west, while a trough is moving east across the United States. If the hurricane is a powerful major hurricane, it is generally more likely to "feel" this trough and curve north as the trough approaches. If the storm is a weak tropical depression, it may too curve north, but it might also have a chance to continue moving west as the trough passes to its north. That said, the "stronger = more north" rule-of-thumb is more of a general idea rather than a 100% guarantee. Weak storms don't always continue westward when faced with a trough. The combination of many troughs and ridges along the path of a hurricane can throw a wrench into the usual steering patterns.
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Kalrany
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Re: Troughs & Ridges

#3 Postby Kalrany » Sat Aug 06, 2016 10:40 pm

Very nice and concise. Learned a few things from it -- thank you.
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junepath
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Re: Troughs & Ridges

#4 Postby junepath » Sat Aug 27, 2016 11:56 am

Thank you for your reply, that sort of makes sense :) I think by reading threads here, I have an even better understanding of how they help move/block hurricanes.

(Sorry for the delay in replying, life came up and kicked me in the teeth for several weeks, and as it is I won't even need to worry about a ruined vacation this year because we probably aren't even going.)
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