Bob blog 8 Aug 2021

Bob McDavitt's ideas for sailing around the South Pacific.
Disclaimer: Weather is a mix of pattern and chaos; these ideas are from the
patterned world.

Compiled Sunday 8 August 2021

SWELL PERIOD
For about a year now, MetService have been mentioning when swells have a
long period (more than 12 seconds). A blog explaining why is at
blog.metservice.com/Swell-Period

Basically, swell is waves coming from over the horizon. Storms can generate
a spectrum of waves of different directions and heights, and these "sort
themselves out" over time to organise swells that may travel for days. The
swell being generated by a wintry SW flow in the Tasman Sea tonight will
take a great circle and arrive in San Diego around 14 days from now. The
period of a waves is the time it takes to travel past a point. Longer period
swells are further apart and thus have gentler slopes, however, they travel
faster and contain more energy. Thus when they roll onshore they break more
heavily, making beach, bar and reef conditions more hazardous. Also, vessels
longer than one wavelength are prone to resonant motions causing rolling and
pitching.

Sea waves have short periods (7 seconds or less) producing choppy conditions
and hazards to shorter vessels. The shorter the period the steeper the wave.
When the wave is too steep it becomes unstable and BREAKS.

Using the graph calculated by Dr. Dilip Barua and published in NATURE at
widecanvas.weebly.com/nature/archives/06-2018
For a 2m swell steepness becomes unmanageable when period is less than 7
seconds.
Then the relationship is approximately linear, so add six to height in
metres to get minimum safe period in seconds..
For a 3m swell minimum safe period is 9 seconds. 4m,10secs. 5m,11 sec. 6m,
12sec.
(see https://metbob.wordpress.com/2019/02/17/bob-blog-17-feb/
Write these down in your logbook for future reference.

A sharply inclined seabed encourages breaking waves. You can feel this
increased choppiness when approaching land and crossing the continental
shelf.

TROPICS /EUROPE
Athens is surrounded by flames:
www.theguardian.com/world/2021/aug/07/apocalyptic-scenes-hit-greece-as-athen
s-besieged-by-fire

Two cyclones near Japan as the Olympics come to a close: LUPIT and MIRINAE.

Also TC KEVIN off west coast of central America, and a zone of high
potential for formation across the Atlantic ocean.

WEATHER ZONES
SPCZ=South Pacific Convergence zone.

The SPCZ is expected to build and stretch from Solomona to Rotuma to
Samoa/northern Tonga.

Passing trough over Fiji and Tonga tonight travelling east and expected to
be over Southern Cooks by mid-week.

Convergence zone over Fiji by midweek expected to reach Samoa to Southern
Cooks by end of the week.

HIGHS and LOWS

Low over central NZ tonight is travelling off to the southeast. 4+m swell
following this low is expected to get as far north as 25S by midweek. Avoid.


HIGH above 1030 off New South Wales on Monday expected to be in mid Tasman
by mid week and then fade at 30S north of NZ by the weekend. It is expected
to make a squash zone of enhanced easterly over Coral Sea/New
Caledonia/Vanuatu area as it moves east.

Active trough s expected to reach Sydney by Wednesday and NZ on Friday,
followed by vigorous southwest flow. Avoid.

Another High should follow path of previous High from Friday 13th.

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If you would like more detail for your voyage, then check metbob.com to see
what I offer.
Or Facebook at /www.facebook.com/metbobnz/
Weathergram with graphics is at metbob.wordpress.com (subscribe/unsubscribe
at bottom).
Weathergram archive (with translator) is at weathergram.blogspot.co.nz.
Contact is bob@metbob.com or txt 64277762212
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