Bob Blog 22 May

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 22 May 2022

LISA BLAIR is on target to complete her attempt at the Around Antarctic this
Tuesday see https://youtu.be/TpYWIllg59o

BEAUFORT SCALE
On a wet winter's Saturday afternoon when I was a schoolboy growing up in
Wellington, with nothing better to do, I started reading an encyclopaedia
which our Mum had bought for us. Volume one covered A to C and when I came
across the Beaufort Scale for measuring wind, I became intrigued in
all-things weather. This probably triggered my career choice even when I was
only 11. However, what intrigued me so much was the descriptive definition
given to Force 10 or storm on the Beaufort Scale. There it was in writing
from a reliable source that

Storm is "seldom experienced inland. Trees uprooted; considerable structural
damage occurs."

I was immediately quizzical. The wind in Wellington that day was storm
force, and we seemed to be having a gale every week and a storm every month
in Wellington that winter. In spite of that there was only an odd tree being
blown over and nothing seemed unusual in our storms.

It wasn't until I visited other places that I realised that Wellington's
proximity to Cook Strait made it a wind funnel and funnelled winds have
their own characteristics.

The history of the Beaufort scale is an interesting illustration of how
science works.understanding the pattern of things by observing and
analysing. Analysis requires numbers. So it was in 1805 that the British
navy navigator Francis Beaufort (later Rear Admiral, and knighted in 1846)
developed a code to help navigators to write in the log what the wind force
looks like using a scale of observable effects. Before his scale the ship's
log was filled with indeterminate terms and one mate's "stiff breeze" is
another's "soft breeze". Initially Beaufort used descriptive terms that
applied to a British Man-of-war frigate ship sails, such as "just sufficient
to give steerage". These ships deploy up to 12 sails and the code could
indicate how many sails the ship should unfurl, hence the Force numbers are
from 0 to 12. The important thing to note about the Beaufort scale is that
the levels on the scale are based on increments of observable effect. In
the 1830s the scale became the navy standard. In the first International
Meteorological Conference in Brussels in 1853 it was adopted as a standard
code for all shipping and turned into numbers for ease of reporting.

In 1916, to accommodate steamships, the descriptive terms were shifted from
effects of the wind on sails to effects of the wind on the sea, and a range
of knots for each force was established. In 1923 the director of the UK
Meteorological office added the land descriptors. These are the ones I read
so quizzically early in the 1960s.

The UK Met office shipping forecast still uses the Beaufort scale, but most
marine forecasts are now given with the wind speed in knots. Nowadays the
accuracy of a marine forecast can be around plus or minus 5 knots, and
that's finer than the Beaufort scale where for example the speed range for
each force is around 8 knots. However the definitions used for warning
thresholds (Strong, Gale, Storm and Hurricane) are all still based of the
Beaufort scale.

The Beauford Scale speed in knots do NOT rise in a linear way.
It is as if each step up in "observable effect" gives more grunt.

We know that the force of the wind goes up with the square of the speed.so
a 20kt wind has FOUR TIMES the pressure force of a 10 knot wind. This
explains why sailing in a 20knot wind is fun and a 30 knot wind, just a
small jump in speed, isn't fun anymore.

TROPICS

After a few quiet weeks we had an out-of-season cyclone over Vanuatu on
Friday and Saturday, GINA.
And for the coming week the focus for potential development is shifting to
the China Sea.

WEATHER ZONES
SPCZ stretches from Solomons to Vanuatu to Samoa, recovering after GINA.
A passing trough is expected to move east from New Caledonia to Fiji towards
the end of the week.clip_image010

HIGHS and LOWS
Low L1 is the remains of GINA combined with a frontal zone that extends
southeastwards.
High H1 is moving east along 25S in tandem with L1.
Low L2 is expected to form mid-week between Queensland and New Caledonia and
deepen as it travels south into the south Tasman Sea
High H2 in over Tasmania tonight and expected to travel east along 40S
reaching NZ by Friday.
In the wake of L2 there should be an opportunity to sail from Queensland to
New Caledonia/maybe Fiji
Looks ok for sailing wet from Tahiti this week.

>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>
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
>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>