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


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.

Here is the  Beaufort scale showing the minimum speed in knots  for each force (empirically derived).


Note that it isn’t linear, almost looks like a square graph. It is as if  each step up in “observable effect” gives more grunt.

And here is a graph of the square root of those knots


This is more linear (especially after force 4). We know that the force of the wind goes up with the square of the 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.


The latest cyclone activity report is at and Tropical Cyclone Potential is from


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 st=hifting to the China Sea.


Weather Zones Mid-week GFS model showing isobars, winds, waves (green to red +arrows), Rain (Blue), STR (Subtropical Ridge), SPCZ (South Pacific Convergence Zone) and CAPE (pink)


CAPE mid-week as seen by ECMWF and GFS from

Different models seem to produce different values.


SPCZ=South Pacific ConvL1ergence zone.

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

Rain Accumulation next five days from


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.


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