Bob McDavitt's ideas for sailing weather around the South pacific

31 July 2016

Bob Blog

Issued 31 July 2016

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.

Today I open with a point to ponder: what wind was that?
In 1805 Admiral Sir Francis Beaufort put forward an empirical wind scale with 13
units of force (0 to 12), based on the behaviour of the sails on a standard
British Navy frigate from 'just enough to give steerage" to "that which no
canvas sails can withstand".

In 1916 the description was changed from how the sea (fully-developed,
offshore), rather than sails, respond to wind speed and extended to land
observations. George Simpson, (then C.B.E and later Sir), Director of the UK Met
Office, standardized these descriptions in 1923. Today many marine wind
forecasts are not in Bf (Beaufort force) and instead use units such a knots, or
m/s (Russia) or mph (USA). However, the words STRONG, GALE, STORM and HURRCANE
as used in marine bulletins still use thresholds defined by the Beaufort scale

Note that the state of sea table described in the Beaufort scale is NOT used by
meteorologists today. After World War I there was a need for the international
standard in reporting weather, and the code for the 'state of the sea' was
reduced to a 10 point scale (0 to 9). Captain H.P. Douglas of the Royal Navy
designed the Douglas Sea Scale in the 1920s and this still the scale used by
marine meteorologists in today's forecasts.

The Beaufort scale was extended in 1946, when forces 13 to 17 were added to
apply to typhoons. Nowadays only Taiwan and China use these extensions.

When I first estimated the life cycle and attributes of tropical cyclones from
satellite imagery back in the 70s I used the Dvorak technique: see This technique was improved in 2004 as
the ADVANCED Dvorak technique came on line in 2004.

At about the same time the Saffir-Simpson scale was introduced for Atlantic and
Central/Eastern Pacific cyclones (Hurricanes), merging meteorological ideas from
the National Hurricane Centre NHC on storm surge and flooding (Robert Simpson)
with civil engineering estimates of structural damage (Herbert Saffir) so the
hurricanes could be categorized according the wind speed Cat 1 to Cat 5. In
2009 -2010 the NHC made moves to drop the storm surge ranges from the

There is now a move for cyclones that produce observed wind speeds over 151
knots as category 6. But according to Robert Simpson, there are no reasons for a
Category 6 on the Saffir-Simpson Scale because it is designed to measure the
potential damage of a hurricane to man-made structures. "...when you get up into
winds in excess of 155 mph (135 knots) you have enough damage . if that extreme
wind sustains itself for as much as six seconds on a building it's going to
cause rupturing damages that are serious no matter how well it's engineered"
This reminds me of Admiral Beaufort's original description for his Bf12, "that
which no canvas sails can withstand"
In our part of the world the Bureau of Meteorology introduced the Australian
Tropical cyclone Intensity scale late 1989. I think it is related to the Dvorak
scale, rather than the Saffir-Simpson scale. If I'm wrong, please someone let me
And so these days there are different wind scales in different parts of the
world. The WMO has its job cut out to seek uniformity:

The Tropics
Last weekend DARBY in its dying stages: got close to Hawaii
Things in central and eastern North pacific have eased now (but there are still
a few tropical lows there), and there is just one cyclone at present over
Western North Pacific, namely NIDA which seems to be heading for Hong Kong.
The rain maps, from ,for the past
weeks shows a few "NW fingers" off the ITCZ relating to tropical cyclones., and
a drop in activity over the central Indian Ocean, no rain at all over the whole
of Australia, and a slight southwards shift in the SPCZ,


SPCZ=South Pacific Convergence zone.
SPCZ is expected to remain draped from north Coral Sea to north of Vanuatu to
Tokelau to Northern Cooks.
This week's SPCZ may be seen on with rain accumulation and ten days.

STR= Sub-tropical Ridge
HIGH in Tasman Sea on Monday is expected to pass over Northern NZ on Tuesday and
Wednesday, then travel off to the east of NZ. Next HIGH, at this stage, is not
expected in Tasman Sea area until Fri 12 August.

Voyage Outlooks:
Tahiti to the west
A trough is expected to pass across Tahiti on local Tue/wed. Avoid those dates
for departure. So if you depart before Tue then you will need to have some
waypoints to sail around this trough. And if you do depart after Wednesday you
will need some waypoints to avoid the strong SE in the enhanced trade winds on
the north side of the HIGH travelling east of NZ.

Between NZ and the tropics
Disturbed SW winds on Monday. Then there is an opportunity to depart as a ridge
passes on Tuesday and Wednesday. But a departure on Tuesday will still encounter
northerly winds from next approaching trough on Thursday.
If you don't mind a few days of waypoint deviations to handle 20 gust 30 knot
northerly winds with up to 4 metres SW swells then you can depart on Tuesday.
Otherwise stay put, and the next opportunity maybe around mid-August.

See my yotpak at for terms used.
See my website for information on tailor-made voyage forecasts
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