On Wed, 14 Sep 2005 19:41:01 +0100, Graham Easterling wrote in


Well, if there's no indication of a tsunami, nor an earthquake in the
oceanbottom; how is the phenomenon Roger described and started this thread
to be explained? Or is it just imagination?

I still think it's a long wavelelength swell, which weather system created
it is open to debate.

Yes, I'd agree with that as the only possible explanation. Such swell can
travel vast distances and retain the most important wave features. Swell
waves become very smooth and regular, and lack the irregularities seen in
waves nearer the generating zone. Knowing their frequency/time period
might help confirm that idea.

--
Mike 55.13°N 6.69°W Coleraine posted to uk.sci.weather 14/09/2005 20:48:57 UTC

"Mike Tullett" schreef in
bericht ...
On Wed, 14 Sep 2005 19:41:01 +0100, Graham Easterling wrote in


Well, if there's no indication of a tsunami, nor an earthquake in the
oceanbottom; how is the phenomenon Roger described and started this
thread
to be explained? Or is it just imagination?

I still think it's a long wavelelength swell, which weather system
created
it is open to debate.

Yes, I'd agree with that as the only possible explanation. Such swell can
travel vast distances and retain the most important wave features. Swell
waves become very smooth and regular, and lack the irregularities seen in
waves nearer the generating zone. Knowing their frequency/time period
might help confirm that idea.

Thank you, Mike and Graham
Of course I'm not familair with the coast line near Eastbourne nor northern
Cornwall; but is it a bad thought this long wave swell is more obvious in
deep water along the British coastline than the far more shallow North Sea
coasts of Holland and Belgium, with its sandbanks? A swell like this would
disappear in the wide surf. Apart from that a swell coming from the south;
flowing through the Channel and the street of Dover, it would spread out
into the North Sea, losing lots of its amplitude??
Wondering, as I put up the subject in a Dutch/Belgium weatherforum, just to
ask if this phenomenon is known by someone along our coasts, but they
thought I was drunk )

Wijke
The Netherlands

On Wed, 14 Sep 2005 23:20:23 +0200, Wijke wrote in


Of course I'm not familair with the coast line near Eastbourne nor northern
Cornwall; but is it a bad thought this long wave swell is more obvious in
deep water along the British coastline than the far more shallow North Sea
coasts of Holland and Belgium, with its sandbanks? A swell like this would
disappear in the wide surf. Apart from that a swell coming from the south;
flowing through the Channel and the street of Dover, it would spread out
into the North Sea, losing lots of its amplitude??

I think you are probably correct, Wijke. Big swell waves can arrive at
times of no local wind or waves and would be best seen on coasts exposed to
the long Atlantic sea fetch. Once they pass through the Straits of Dover,
their amplitude would diminish as the waves fan out in many directions.
Add to that the distortions caused by such features as you mention and they
would soon lose the identity they had conserved over 1000s of miles.


--
Mike 55.13°N 6.69°W Coleraine posted to uk.sci.weather 14/09/2005 21:47:46 UTC

In message , Nick G
writes
Deep water waves that have the greatest wavelengths and longest periods,
travel the fastest and are the first to arrive in regions distant from the
storm which generated them

Methinks it might be something like this if the wave was generated by a
storm in deep waters (assuming the wave length to be double the depth
available):

The square root of (9.8 m/sec (acceleration due to gravity) X 4,000 m (an
estimate of the average depth of the Atlantic Ocean)) which works out at
roughly 198 meters per second,

Therefore the wave would take about 5.6 hours to travel 4000 km.
________________
Nick G
Exe Valley, Devon
50 m amsl

Actual wavelengths are a lot shorter than you have assumed. The
longer-period swell waves from a storm typically have periods up to
20-25 seconds. This gives deepwater wavelengths of something like
600-1000 metres. Individual waves in this period range have velocities
of 60-75 knots while the group velocity is half that i.e. 30-37 knots
approximately. The waves would therefore take approximately 60-72 hours
to travel 4000 km.

The majority of storm waves have periods rather shorter than discussed
above and therefore take longer to travel the same distance.

Another point that has to be considered when looking at swell travelling
over large distances is that swell travels on a great circle track. From
conventional synoptic weather charts, or from navigation charts, it is
not possible to determine how swell will propagate. What is needed is
charts with a gnomonic projection. On such a projection great circles
are straight lines. As an example of this great circle travel, a SW'ly
storm off the NE coast of USA will produce a swell that would reach the
west of the British Isles from a direction slightly north of due west
i.e. not from the SW.

Norman.
(delete "thisbit" twice to e-mail)
--
Norman Lynagh Weather Consultancy
Chalfont St Giles 85m a.s.l.
England

In message , Wijke
writes

"Mike Tullett" schreef in
bericht ...
On Wed, 14 Sep 2005 19:41:01 +0100, Graham Easterling wrote in


Well, if there's no indication of a tsunami, nor an earthquake in the
oceanbottom; how is the phenomenon Roger described and started this
thread
to be explained? Or is it just imagination?

I still think it's a long wavelelength swell, which weather system
created
it is open to debate.

Yes, I'd agree with that as the only possible explanation. Such swell can
travel vast distances and retain the most important wave features. Swell
waves become very smooth and regular, and lack the irregularities seen in
waves nearer the generating zone. Knowing their frequency/time period
might help confirm that idea.

Thank you, Mike and Graham
Of course I'm not familair with the coast line near Eastbourne nor northern
Cornwall; but is it a bad thought this long wave swell is more obvious in
deep water along the British coastline than the far more shallow North Sea
coasts of Holland and Belgium, with its sandbanks? A swell like this would
disappear in the wide surf. Apart from that a swell coming from the south;
flowing through the Channel and the street of Dover, it would spread out
into the North Sea, losing lots of its amplitude??
Wondering, as I put up the subject in a Dutch/Belgium weatherforum, just to
ask if this phenomenon is known by someone along our coasts, but they
thought I was drunk )

Wijke
The Netherlands

There was a very notable event, probably 20 or so years ago, when a very
long-period swell produced very high waves along the south coast of
England on a day of light winds. The waves came over the top of Chesil
Bank. The event was written up in 'Weather' magazine. I don't have time
to search for it today but I'll try to locate it over the weekend.

Norman.
(delete "thisbit" twice to e-mail)
--
Norman Lynagh Weather Consultancy
Chalfont St Giles 85m a.s.l.
England