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knocker

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Posts posted by knocker

    The North Atlantic Current Is Gone

    in Spring Weather Discussion

    Posted · Edited by weather ship

    if it is by any means weaker than last year, then surely that gives us another good chance of a winter similar to the last or much worse. gfs last year did well in winter, by predicting all those cold spells or the whole cold winter if you like, even if it did go very OTT at times. now we seem to be seeing, some developments from gfs showing some classic winter setup, yet we are only in september.

    they cannot even get lrfs right let alone tell if the gulf stream is slowing. there are people on here like GP etc and meteorologists such as JB who are more reliable than them. i have full confidence in what the more knowledgeable ones like GP are predicting for the autumn and winter.

    The heading of this thread is The North Atlantic Current is gone. This is a subject for oceanographers I would have thought. I have no doubt ther are experts on here, and I bow to their superor knowledge, but predicting autumn and winter weather on ocean heat transfer (Gulf Stream) and reliability of the jet stream, plus interuption of the convergence zone is fraught with danger. My confidence is not high. Classic winter setups can be caused by other reasons. Why are you assuming last winter was caused by the weakening of the Gulf Stream? I asume your experts will tell you.

    The North Atlantic Current Is Gone

    in Spring Weather Discussion

    Posted · Edited by weather ship

    Does anybody have any idea of the temps down towards the seabed of the Channel and the North Sea? If the figures are correct, the waters must be getting churned up in some way i.e. colder bottom water coming to the surface - Looking particularly at the difference between Brighton and Bournmouth.

    Does anybody have any idea of the temps down towards the seabed of the Channel and the North Sea? If the figures are correct, the waters must be getting churned up in some way i.e. colder bottom water coming to the surface - Looking particularly at the difference between Brighton and Bournmouth.

    I don't know if this is any good to you. I was going to post it in reply to another post that I can't find at the moment. I don't know of any mechanism that can cause cold deep water to warm and rise. Of course near the coasts the water will be shallow in comparison. Anyway it'sa fairly interesting diagram.:)

    The Fastnet Race Disaster

    in Historic Weather

    Posted · Edited by weather ship

    Neither the formal report of the Fastnet Race inquiry nor the official meteorological analysis seemed to explain all that had happened during this disaster. In particular, although it was a very severe storm, it wasn’t exceptional, so why did this unprecedented carnage take place? Was there something going on that was slipping under the radar? Very many experienced competitors stated that the wind strength was not unusual, but that the sea conditions were the most dangerous they had ever experienced. Most of the damage done to the yachts was due to the waves and not to wind. Perhaps the clue lay here.

    Allan Watts, a meteorologist particularly concerned with small boat navigation, decided to dig a little deeper and the result of his research, Fresh Evidence on the Fastnet Storm, was published in the Journal of Navigation in 1982. For © reasons I can’t upload the article but his original description of the storm can be found in the Fastnet Race Inquiry Report and is worth a read.

    http://www.blur.se/i...ace-inquiry.pdf

    To continue. Initially Allan Watts found that there was a divergence of opinion between the Met. Office who were taking their wind speed from their charts (48-55 knots) and many competitors who were convinced that the top strength was into Force 11 (56-63 knots). How best to resolve this difference.Watts took a line of action that at the time was probably unique.

    He analysed the barometric records of the yachts themselves having obtained as many as possible from the competitors. The readings of the yachts were checked for accuracy whenever possible by comparing the given readings with the known values at St. Mary’s as the yachts were passing.

    He plotted the results along the rhumb line from Scilly to the Fastnet Rock at four synoptic hours, 1900, 2200, 0100 and 0400 BST. Something quite surprising showed up. A series of troughs in the isobars which would not be detectable from the course plot of barometric readings from the professional reporting stations in the Fastnet area. What Allan Watts called a meteorological oddity. Some of these troughs were 3 to 6mb below what otherwise might have been expected Allan Watts considers that the role of these troughs in the disaster may well have been crucial. Another major factor involved was not the average height of the waves, but the number of ‘episodic’ waves that were generated by the storm. An episodic wave is one of those normally rare waves of excessive height that may well be bred by superposition of wave trains in any storm of severe gale proportions that blow for sufficient length of time to make the proposition a probability. It would appear that in the Fastnet there may well have been dozens of these waves.

    Back to the troughs. The revelation of the troughs in the airstream over the Fastnet fleet provides a possible cause for locally enhanced winds. Allan Watts noted that when yachts were reporting mean winds of up to 55kts and gusts into hurricane force (64+ knots) they were mainly to be found close to the troughs. At the same time boats within a few miles were only recording winds of force 7 and, importantly, AW believes that it is this variation of speed across the wind field that caused the strange seas and contributed greatly to the disaster.

    Sea and wind conditions are extremely difficult to analyse but Stephens, Kirkman and Peterson have done some very pertinent work in the area. The part that is of particular importance to the Fastnet shows that when the wind speed suddenly increases the seaway follows without a significant time interval. In other words winds suddenly generated by local meteorological conditions induce immediate heightening of the sea under them-an increase that will not be experienced in surrounding parts of the wind field not subject to the local increase.

    Allan Watts goes on to say that analysis of waves capable of capsizing yachts in the way they were doing during the Fastnet yacht race shows that they must be very steep and that there is a fast moving ‘jet’ at the crest of the wave that is moving at perhaps twice the speed of the water in the lower part of the profile. Apparently such waves were responsible for capsizing several NOAA weather buoys between 1977-9.

    A couple of his main conclusions were:

    (i) In the troughs that developed over Fastnet the wind increased on the periphery of the trough, but there was a lowered wind speed near the axis and outside the periphery of the trough.

    (ii) Thus surface ‘jets’ only a few miles wide were flanked by lower wind speeds. As the seaways builds without appreciable delay, so massive waves developed along the jets, but lesser seas existed on their flanks. The transverse wind shear may therefore account for the high seas experienced by the crews of the yachts in the 1979 Fastnet storm.

    Refs.

    Watts, A., Fresh Evidence of the Fastnet Storm, Journal of Navigation, 1982.

    Stephens, O. J. Kirkman, K. L., & Peterson, R. S., (1981). Sailing Yacht Capsizing. Society of Naval Architects & Marine Engineering.

    AVHRR 13/08/79 0903UTC

    post-12275-029615600 1284473873_thumb.jp

    The lowest pressure reading recorded in the UK

    in Historic Weather

    Posted · Edited by weather ship

    925.6mb at Ochertyre, near Crieff, Perthshire on the 26th January 1884

    Rslp18840127.gif

    Here are some barometer readings of this remarkable depression.

    London: barometer fell to 28.529" (966mb) at 7.30pm

    Dr Beverly of Aberdeen at 11.30pm recorded a barometer reading at sea level of 27.381" (927.1mb)

    Forest of Glen Tana, a reading of 27.39" (927.4mb) was recorded

    Hitchin: 28.32" (959mb)

    North Shields: 27.63" (936mb)

    Cargen: 27.66" (936mb)

    Oban: 27.39" (927.4mb)

    Dublin: 28.15" (953mb)

    Croydon: 28.548" (966mb) at 7.40pm

    Worksop: 28.292" (958mb)

    Some other weather reports

    London: Violent SW gale

    Stanmore: Terrific gale from SW to W after 4pm

    Reigate: Violent storm with very low pressure

    Croydon: Strong gale, max hourly velocity of wind 40 miles

    Hythe: From 5.30 to 9.30pm, terrific hurricane, with heavy thunderstorm rain and hail

    Littlehampton: Awful gale

    St Lawrence: Severe SW gale with vivid lightning

    Oundle: Hurricane with heavy rain, snow and lightning

    Diss: Tremendous gale from west, thunderstorm in evening

    Torquay: Fearful SW gale with rain, thunder and lightning

    Coventry: Much lightning, violent wind and unusually low bar

    Mansfield: Very violent gale with snow, vivid lightning and thunder

    Macclesfield: Gales, snow, rain and lightning

    Morpeth: A great gale with very low pressure

    Penrith: Great storm with extremely low barometer

    Elterwater: Great gale with snow, sleet and rain

    Llanfrechfa Grange: Violent SW wind, thunder, lightning and rain

    Girvan: Severe storm; pressure fell almost 2.5" very rapidly and rose again almost as quickly.

    Cassillis: Storm of extraordinary severity with snow and rain

    Keith: About two inches of snow fell with fearful drifts on the following day.

    Aviemore: Severe gale from west, highland railway blocked by snow

    Forsinard: High winds, snow and drifts

    Castle Lough: Great storm with heavy rain followed by snow

    Kilconnell: Violent storm from SW, veering to NW

    Castlebar: Violent storm follwed by a fall of snow

    Enniskillen: A very severe storm with remarkable fall of pressure; old trees which had stood for 130 years were blown down.

    Newtownlands: The most terrific storm since 1839, wind 78 miles an hour.

    Douglas: Hurricane from 4pm on 26th to 4am on 27th.

    I believe this was the same storm that recorded a pressure at the Ben Nevis Summit Observatory at 2030 of 784.7mb (1343m). The story of the observer tying a rope around himself to attempt to read the temps and avoid being blown away is quite amusing. Succeeded at the third attempt!

    • Like 2

    The Weather Observations Of Surgeon Menzies

    in Historic Weather

    Posted

    I don't know whether this is of interest, or even if it has been posted before,

    On Vancouver’s historic voyage around the world from 1791 to 1795, Archibald Menzies served as surgeon and naturalist. During the voyage, he kept a weather log, including in it observations of air temperature, sea-surface temperature, barometric pressure, wind direction and wind force (using terminology similar to that used by Admiral Beaufort a decade later in the first published version of his famous scale of wind force).

    The log is important historically, for it includes the first weather observations ever made systematically along the west coast of North America (from Mexico to Alaska), among them observations made whilst at anchor at San Francisco and Monterey. The sheets on which Menzies recorded his observations have recently been returned to the archive of the Royal Meteorological Society. The story of how they came to be in the possession of the Society in the first place will be told in this presentation.

    http://ams.confex.com/ams/Annual2005/techprogram/paper_83619.htm

    http://ams.confex.com/ams/pdfpapers/83619.pdf

    Major Hurricane Igor

    in Hurricanes, Cyclones and Extreme weather worldwide

    Posted

    Thanks to those who answered my query. I've since realised that there was an article in weather on this very subject by Professor Robert Pearce, Department of Meteorology. University of Reading in 2005.. The article has been revisited by Roger K. Smith andthe latter is available online. I think I'll stick to the explanations on here as the complexities of the dynamics I freely admit are beyond me.

    http://www.meteo.physik.uni-muenchen.de/~roger/Publications/Weather_2005_Smith.pdf

    The North Atlantic Current Is Gone

    in Spring Weather Discussion

    Posted

    As I understand it this isn't about another ice age but whether global warming is affecting the the very important convergence zone south of Greenland. If so it would of course lower temps in north west Europe somewhat. The $64,000 question is this going to happen? Or indeed is happening. It creates circular discussions (elswhere in another thread where one could actually age years reading it) and in my very humble opinion you could yap about this for weeks without as yet coming up with an answer. But it certainly is a possibility.

    The North Atlantic Current Is Gone

    in Spring Weather Discussion

    Posted · Edited by weather ship

    From Wikipedia http://en.wikipedia....iki/Gulf_Stream

    "Typically, the Gulf Stream is 100 kilometres (62 mi) wide and 800 metres (2,600 ft) to 1,200 metres (3,900 ft) deep. The current velocity is fastest near the surface, with the maximum speed typically about 2.5 metres per second (5.6 mph).[20] As it travels north, the warm water transported by the Gulf Stream undergoes evaporative cooling. The cooling is wind driven: wind moving over the water cools it and also causes evaporation, leaving a saltier brine. In this process, the water increases in salinity and density, and decreases in temperature. Once sea ice forms, salts are left out of the ice, a process known as brine exclusion.[21] These two processes produce water that is denser and colder (or, more precisely, water that is still liquid at a lower temperature). In the North Atlantic Ocean, the water becomes so dense that it begins to sink down through less salty and less dense water. (The convective action is not unlike that of a lava lamp.) This downdraft of heavy, cold and dense water becomes a part of the North Atlantic Deep Water, a southgoing stream.[22] Very little seaweed lies within the current, although seaweed lies in clusters to its east"

    Explanations seem to vary slightly

    The most prolific producer of deep water in all oceans is a region south of Greenland in the North Atlantic, where several major surface currents converge. Here, the warm salty Gulf Stream converges with the cold, not-so-salty East Greenland and West Greenland Currents. These merging surface waters sink for two reasons. First, "down" is the only place they can go. The second reason is more subtle. Because the lines of constant density on a T-S plot are curved , when waters of the same density but different temperatures and salinities are mixed, the resulting water mass is denser than either of the original waters. So the denser mixture would tend to sink even if it wasn't being forced downward by the convergence. This sinking of merging surface waters is sometimes referred to as caballing. The water mass produced by these converging currents south of Greenland is called North Atlantic Deep Water. It is produced so abundantly that it fills most of the Atlantic Ocean. The North Atlantic Deep Water flows southward beyond the equator and all the way to the Antarctic Circumpolar Current, which mixes with it to produce another water mass, called Common Water. This mixture is carried by the Antarctic Circumpolar Current around the Cape of Good Hope into the Indian Ocean, and around Australia and New Zealand the Pacific, where it fills most of these two oceans as well.

    Courtesy “Exploring Ocean Science†by Keith Stowe. (second edition)

    I think I would need some compelling evidence to convince me that the status quo is under serious threat.

    Edit.

    I should have added that as density is dependant on temperature and salinity this can't be ruled out if there is a major change in either of the latter.

    The North Atlantic Current Is Gone

    in Spring Weather Discussion

    Posted

    I don't know if this of interest.

    Thermohaline Catastrophes

    and the Younger Dryas

    During the most recent ice age, the North Atlantic component of the Global Thermohaline Conveyer was partially shut down and the ocean is thought to have operated in a different mode to that of the present day. The northern North Atlantic was considerably cooler and the transport of the North Atlantic current (the northward extension of the Gulf Stream) much reduced. The North Atlantic component of the conveyer was reactivated at the end of the most recent glaciation, at about 14,000 years BP.

    Recent research on deep-sea sediment cores has shown that this reactivation of the conveyer was not without hiccups! Part of this conveyer stopped abruptly at about 11,000 BP -a period known as the Younger Dryas. This led to a catastrophic cooling of the North Atlantic region and caused the build up of small glaciers in the British mountains in what geographers call the Loch Lomond glacial re-advance, This cooling only appears to have lasted a few centuries, but it developed very rapidly over decades. There are several theories about what exactly led to the shut down of the conveyer. One view is that a sudden influx of fresh melt-water from the Laurentide ice sheet into the North Atlantic could have stabilised the vertical stratification and reduced the rate of formation of North Atlantic deep cold water. This, in turn, could have shut down the North Atlantic conveyor circulation, resulting in a cooling of the surface waters of the northern North Atlantic. The effect of changes in fresh water fluctuations on the thermohaline system is an example of positive feedback. The fresh water input weakens the thermohaline circulation, which makes the circulation more susceptible to further weakening. This process has been investigated in recent years using mathematical models of ocean circulation. The models show that there are a number of different states of the thermohaline circulation, some of which are stable. There are, however, transitions between stable states, which occur over periods as short as 40 years. It has been speculated that the present North Atlantic Ocean may be close to one of these transitional states, of which the Younger Dryas is an example. One study has suggested that the transition between states is not necessarily symmetrical. The change from strong to weak thermohaline circulation may be more rapid (40 years) than the re-establishment of the strong circulation (500 years). Further study of the Younger Dryas and similar events in the palaeoclimate record may give us important clues to the likely response of the present-day thermohaline circulation in the North Atlantic to global warming. (Remaining stable or flipping to another state).

    Source: Oceanography-an illustrated guide, C.P. Summerhayes and S.A. Thorpe.

    To think BP may affect the set up is to my mind laughable.

    Steephill Cove

    in Archived non-weather Photography

    Posted · Edited by weather ship

    It's along the coastal path by the Botanical Gardens at Ventnor. Be prepared for some steep descents and climbs though (hence the name!), there is only a small footpath and no road down to the beach.

    www.steephillcove.com

    Perhaps I should be keeping it all to myself? :D

    Nice set of photos Coast. I know I'm being quite thick here but how does one ensure that the attached thumbnail is reproduced in a reasonable size when clicked? As usual I'm no doubt missing the obvious.

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