A couple of folks have noted that during a storm (low atmospheric pressure) that high tides run higher than normal -- the well-named "storm surge." This surge can run several feet, as it has on many occasions on the Gulf Coast. Along with the huge waves generated by hurricane-force winds, the storm surge causes massive damage to coastal areas. Alas, it is not the direct effect of the low pressure that raises the height of the tide by several feet. Even in the center of the worst storms, when the barometric pressure drops below 29 inches of mercury, the equivalent rise in column of water is only a foot or so at the maximum (density of water is about 13.6 times smaller than that of mercury). Normal sea level air pressure is about 29.9 inches of mercury. A low pressure of 28.5 inches mercury (extremely low for the atmosphere at sea level -- about what you get in the eye of a hurricane) only changes the tide height by 19 inches. In short, it is a different effect, mainly the effect of the storm center's winds, that pushes a bulge of sea water ashore along with the storm center. Here is a pretty good description of the effect: http://www.comet.ucar.edu/nsflab/web/hurricane/313.htm complete with animation (takes a while to load). Note that the highest levels of tide (worst storm surges) are to the right of the eye of the hurricane (Northern hemisphere), and __not__ at the location of lowest pressure. On the coast of Oregon, storm surges are common, and run 1 to 2 feet. When the tide is at its monthly maximum, and a heavy rainfall is on us, the combined effects near the mouths of rivers make for a lot of flooding in the lowlands. Just debunking the myth. -- Dave Kruger Astoria, OR *************************************************************************** PaddleWise Paddling Mailing List - Any opinions or suggestions expressed here are solely those of the writer(s). You must assume the entire responsibility for reliance upon them. All postings copyright the author. Submissions: PaddleWise_at_PaddleWise.net Subscriptions: PaddleWise-request_at_PaddleWise.net Website: http://www.paddlewise.net/ ***************************************************************************
I wasn't perpetuating a myth, just using a common natural event to illustrate a point. If you look at the bottom paragraph in the web page you cited you will see that low barometric pressure is an important component of storm surge. The first part seems to gloss over atmospheric pressure so if you don't read the whole page you miss a good bit. In any event, the question was, does barometric pressure have an effect on sea state? The answer is yes, it does. BTW, a standard day is defined as 29.92 inches at 70 degrees F. I'd like to see a more factual definition of the height of water rise in a low pressure center than a comparison of water vrs mercury density. There's a good bit more to it than that. Regards, Dave G. At 05:56 PM 12/23/2002 -0800, Dave Kruger wrote: >Just debunking the myth. > >-- >Dave Kruger >Astoria, OR *************************************************************************** PaddleWise Paddling Mailing List - Any opinions or suggestions expressed here are solely those of the writer(s). You must assume the entire responsibility for reliance upon them. All postings copyright the author. Submissions: PaddleWise_at_PaddleWise.net Subscriptions: PaddleWise-request_at_PaddleWise.net Website: http://www.paddlewise.net/ ***************************************************************************
I must be missing part of the web page, for the three factors that appear to be cited are: - winds. - waves, i.e perodic surface phenomena - sea-level rise (ie. the overall mass of water displaced by the storm). Am I correct that you ascribe #3 primarily to barometric pressure as opposed to an overall (DC component if you like) effect of surface wind on water? What the material on the website truly appears to say is that you can have storm surge without an explicit hurricane eye, and that the onset or maximum water level of storm surge need not coincide with an idealised center of the storm. I offer for discussion the example of the lagoon of Venice, where there are no hurricanes as such. It sits at the north end of a narrow sea, at the silted-up mouth of a river, and storm winds tend to be southerly. One sees flood-level tides almost reliably from October to April. Dave Gorjup <dgorjup_at_cox.net> wrote:I wasn't perpetuating a myth, just using a common natural event to illustrate a point. If you look at the bottom paragraph in the web page you cited you will see that low barometric pressure is an important component of storm surge. The first part seems to gloss over atmospheric pressure so if you don't read the whole page you miss a good bit. In any event, the question was, does barometric pressure have an effect on sea state? The answer is yes, it does. BTW, a standard day is defined as 29.92 inches at 70 degrees F. I'd like to see a more factual definition of the height of water rise in a low pressure center than a comparison of water vrs mercury density. There's a good bit more to it than that. Regards, Dave G. At 05:56 PM 12/23/2002 -0800, Dave Kruger wrote: >Just debunking the myth. > >-- >Dave Kruger >Astoria, OR --------------------------------- Do you Yahoo!? Yahoo! Mail Plus - Powerful. Affordable. Sign up now *************************************************************************** PaddleWise Paddling Mailing List - Any opinions or suggestions expressed here are solely those of the writer(s). You must assume the entire responsibility for reliance upon them. All postings copyright the author. Submissions: PaddleWise_at_PaddleWise.net Subscriptions: PaddleWise-request_at_PaddleWise.net Website: http://www.paddlewise.net/ ***************************************************************************
Dave Gorjup <dgorjup_at_cox.net> wrote: >>I wasn't perpetuating a myth, just using a common natural event to illustrate a point. If you look at the bottom paragraph in the web page you cited you will see that low barometric pressure is an important component of storm surge. The first part seems to gloss over atmospheric pressure so if you don't read the whole page you miss a good bit. In any event, the question was, does barometric pressure have an effect on sea state? The answer is yes, it does. BTW, a standard day is defined as 29.92 inches at 70 degrees F. I'd like to see a more factual definition of the height of water rise in a low pressure center than a comparison of water vrs mercury density. There's a good bit more to it than that.>> I should have been more explicit. There is a very small rise in local sea level in direct response to a drop in air pressure. The density of water is 13.6 times smaller than that of mercury. That makes it easy to calculate what the rise will be: a column of water that exerts the same pressure as 29.92 inches of mercury will be 13.6 times 29.92 inches (about 407 inches = 33.9 feet of water). Consequently, if local air pressure __drops__ by an inch of mercury, then the local sea level will rise by 13.6 inches of water. That's the effect Dave and others described. That's no myth. It is, however, very small, albeit enough to drown out your campfire if you have it no higher than a foot or so above the surface of a large lake or sea. The myth is that larger rises are directly due to the local air pressure. Read that web site again and you'll find that it is the __wind-generated waves__ associated with a storm center that cause the much larger (many feet -- sometimes up to ten feet) rise called "storm surge." And, that rise is __not__ at the center of low pressure, it is off to the side. In addition, a big surge demands a specific type of local shore : shallow slope. One could say that the low pressure caused the surge, but it's really the effects of the storm. Wind-generated waves from a distant source can cause the same effect, so the local ari pressure can remain relatively constant while a surge occurs. Hope that sorts it out. -- Dave Kruger Astoria, OR *************************************************************************** PaddleWise Paddling Mailing List - Any opinions or suggestions expressed here are solely those of the writer(s). You must assume the entire responsibility for reliance upon them. All postings copyright the author. Submissions: PaddleWise_at_PaddleWise.net Subscriptions: PaddleWise-request_at_PaddleWise.net Website: http://www.paddlewise.net/ ***************************************************************************
This is my last post on this subject. Some have said that talking about the low pressure effects on sea state are a myth and/or have minimal effect. An example of an approximately 13" rise in sea level for a one inch drop in mercury was given. While barometric pressure is not the most significant component of storm surge, it is certainly worthy of consideration. If one takes into consideration the size of a storm low pressure system, which can be many hundreds, if not thousands of square miles, the amount of additional water weight and/or pressure added by that one foot addition of water, can add to the effect extensively. Take it from me, as some one who lives about 4 feet above mean low water on the Chesapeake Bay, every inch makes a difference. When the ditch in the back yard is full of tide water, I know that we are in spring or fall neap tide and a nor'easter is brewing. ;-) Dave G. Poquoson, Va. *************************************************************************** PaddleWise Paddling Mailing List - Any opinions or suggestions expressed here are solely those of the writer(s). You must assume the entire responsibility for reliance upon them. All postings copyright the author. Submissions: PaddleWise_at_PaddleWise.net Subscriptions: PaddleWise-request_at_PaddleWise.net Website: http://www.paddlewise.net/ ***************************************************************************
----- Original Message ----- From: "Dave Gorjup" <dgorjup_at_cox.net> > Some have said that talking about the low pressure effects on sea state are > a myth and/or have minimal effect. An example of an approximately 13" rise > in sea level for a one inch drop in mercury was given. > > While barometric pressure is not the most significant component of storm > surge, it is certainly worthy of consideration. Since I started the whole thing about barometric pressure affecting the height of tides, I think I should pipe in again. Barometric pressure was one of a number of influences on the height of tides that was included to be complete in terms of how tide (and currents) prediction tables are almost always going to be off in terms of heights (and speeds) and timing. The reason is that you cannot predict a year ahead of time variables such as wind speed, wind direction, rainfall, snow meltoff, and barometric pressure. Also unpredictable or constantly changing are manmade structures that can affect currents and even tides. For example, here on the Hudson, a number of piers have been pulled out and this changes how water moves along the river. As for tides, in a narrow passage such as the Kill Van Kull that leads to Newark Bay, dredging operations can improve flow of water and tides but they can also hinder it while the dredging is being done as the barges and apparatus used during the dredging impede passage of water. ralph diaz *************************************************************************** PaddleWise Paddling Mailing List - Any opinions or suggestions expressed here are solely those of the writer(s). You must assume the entire responsibility for reliance upon them. All postings copyright the author. Submissions: PaddleWise_at_PaddleWise.net Subscriptions: PaddleWise-request_at_PaddleWise.net Website: http://www.paddlewise.net/ ***************************************************************************
I once experienced a storm surge on Lake Superior. I was camped with friends on a low beach in a bay in Pukaskwa National Park when a rainstorm passed by the mouth of the bay. Suddenly the water rose several inches and swirled onto the beach, putting out the fire in the Coleman double-burner stove I was cooking dinner on. The water remained high several minutes, then slowly drained away. I always assumed that low pressure associated with the rainstorm caused the surge. An alternate explanation would be the wake from an ore or grain ship that was over the horizon. However, Pukaskwa is far from the shipping lanes, so that explanation seems unlikely. Chuck Holst *************************************************************************** PaddleWise Paddling Mailing List - Any opinions or suggestions expressed here are solely those of the writer(s). You must assume the entire responsibility for reliance upon them. All postings copyright the author. Submissions: PaddleWise_at_PaddleWise.net Subscriptions: PaddleWise-request_at_PaddleWise.net Website: http://www.paddlewise.net/ ***************************************************************************
Chuck Holst said: > I once experienced a storm surge on Lake Superior. I was camped with friends on a low beach in a bay in Pukaskwa National Park when a rainstorm passed by the mouth of the bay. Suddenly the water rose several inches and swirled onto the beach, putting out the fire in the Coleman double-burner stove I was cooking dinner on. The water remained high several minutes, then slowly drained away. Chuck - You may have experienced a seiche. These happen when the air pressure is lower in one part of great lake than other. It sets up something that is usually described as being similar to water sloshing in a bowl: it goes up on one side and down on the other, then switches. These are not all that rare on the Great Lakes. I once almost got fired from my lifeguard job at a beach in Chicago because the water had seiched out and my boat was high and dry with about fifty yards of sand between me and the water. The captain's attitude was that I should have pushed the thing out rather than tell the swimmers to stay out of the water. Jim Tibensky > *************************************************************************** PaddleWise Paddling Mailing List - Any opinions or suggestions expressed here are solely those of the writer(s). You must assume the entire responsibility for reliance upon them. All postings copyright the author. Submissions: PaddleWise_at_PaddleWise.net Subscriptions: PaddleWise-request_at_PaddleWise.net Website: http://www.paddlewise.net/ ***************************************************************************
[Moderator's Note: Content unaltered. Excessive quoting (i.e. headers/footers/sig lines/extraneous comments from previous posts, etc.) have been removed. Please edit quoted material in addition to removing header/trailers when replying to posts.] Or water accumulation needing a few minutes to equalize. Or wind blowing water toward shore delaying equalization. Or combination thereof. Check out the shallow beach in Terrace Bay for this sort of thing. Chuck Holst wrote: >I once experienced a storm surge on Lake Superior. I was camped with >friends on a low beach in a bay in Pukaskwa National Park when a rainstorm >passed by the mouth of the bay. Suddenly the water rose several inches and >swirled onto the beach, putting out the fire in the Coleman double-burner >stove I was cooking dinner on. The water remained high several minutes, >then slowly drained away. I always assumed that low pressure associated >with the rainstorm caused the surge. An alternate explanation would be the >wake from an ore or grain ship that was over the horizon. However, Pukaskwa >is far from the shipping lanes, so that explanation seems unlikely. > >Chuck Holst *************************************************************************** PaddleWise Paddling Mailing List - Any opinions or suggestions expressed here are solely those of the writer(s). You must assume the entire responsibility for reliance upon them. All postings copyright the author. Submissions: PaddleWise_at_PaddleWise.net Subscriptions: PaddleWise-request_at_PaddleWise.net Website: http://www.paddlewise.net/ ***************************************************************************
>> Chuck - You may have experienced a seiche. These happen when the air pressure is lower in one part of great lake than other. It sets up something that is usually described as being similar to water sloshing in a bowl: it goes up on one side and down on the other, then switches. These are not all that rare on the Great Lakes. >> I have experienced seiches on Lake Superior, but they were periodic and much slower. This was a one-time event, and very rapid compared to the seiches. But this was not the only strange thing I have seen in Pukaskwa. On another, very still, day on the same trip we were paddling along the shore north of Michipicoten Island when I noticed tall clouds developing over Michipicoten and the mainland to the north. Gradually these clouds grew higher and higher, forming tall pillars. Finally they reached a thin layer of mackerel clouds, where the cloud pillars began to spread out fpr miles in all directions, a band of clear blue sky always separating them from the pre-existing layer of clouds. Ihad never seen this phenomenon before, and have never seen it since. Chuck Holst *************************************************************************** PaddleWise Paddling Mailing List - Any opinions or suggestions expressed here are solely those of the writer(s). You must assume the entire responsibility for reliance upon them. All postings copyright the author. Submissions: PaddleWise_at_PaddleWise.net Subscriptions: PaddleWise-request_at_PaddleWise.net Website: http://www.paddlewise.net/ ***************************************************************************
Chuck Holst wrote: >I noticed tall clouds developing over >Michipicoten and the mainland to the north. Gradually these clouds grew >higher and higher, forming tall pillars. Finally they reached a thin layer >of mackerel clouds, where the cloud pillars began to spread out fpr miles >in all directions, a band of clear blue sky always separating them from the >pre-existing layer of clouds. > Probably the result of poutine based flatulence by people having eaten at the White River chip stand. *************************************************************************** PaddleWise Paddling Mailing List - Any opinions or suggestions expressed here are solely those of the writer(s). You must assume the entire responsibility for reliance upon them. All postings copyright the author. Submissions: PaddleWise_at_PaddleWise.net Subscriptions: PaddleWise-request_at_PaddleWise.net Website: http://www.paddlewise.net/ ***************************************************************************
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