Bob Blog 10 Jan

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 Sun 10 January 2021


Rows and floes of angel hair
And ice cream castles in the air
And feather canyons everywhere
I’ve looked at clouds that way
But now they only block the sun
They rain and snow on everyone
So many things I would have done
But clouds got in my way
I’ve looked at clouds from both sides now
From up and down, and still somehow
It’s cloud illusions I recall
I really don’t know clouds at all

© Joni Mitchell, June 19, 1967; Gandalf Publishing Co. (as “From Both Sides Now”)

Basically, over the sea. clouds can be either flat (in stable air) or bubbly (in unstable air).

Flat clouds are the result of gentle large scale upward motion, such as when moist warm air from the tropics rises slantwise along a slope such as a frontal boundary.

Flat layered clouds are Cirrus, Cirrostratus, Cirrocumulus, Altostratus, Altocumulus, and, when thick enough they bring RAIN as Nimbostratus.

Bubbly clouds occur where the air has sufficient buoyancy to rise against gravity. This occurs in unstable air and on a smaller scale in time and space than stable air. Cumulus clouds can grow to towering cumulus and produce SHOWERS. These may grow into Cumulonimbus clouds that may become dangerous, producing thunder and lightning, and/or hail, or squally winds, or maybe tornadoes.

Clouds can also become wavy. These do not occur over the sea, rather they form over land, downstream of mountain.

The Latin names for clouds were introduced by Luke Howard (a Chemist) in 1802.

Isobars: The average surface atmospheric pressure is 1013.25hPa. Nearest isobar to that is 1012, and indeed it is usually the straightest isobar on the weather map. Isobars with lower numbers are generally cyclonically curved and unstable. Isobars higher than 1012 are usually anticyclonic curved with stable air.

The Tropics

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

TC DANILO continues going west across the Indian Ocean.

There are some zones of high potential for tropical cyclone formation around Indonesia and north Australia. . .



Weather Zones Mid-week GFS model showing isobars, winds, waves(magenta), Rain (Blue), STR (Subtropical Ridge), and SPCZ (South Pacific Convergence Zone).


Predict wind CAPE mid-week gives two possibilities for SPCZ


SPCZ=South Pacific Convergence zone.

SPCZ is south of its normal position and stretches from Coral Sea to southern Vanuatu/New Caledonia and south of Fiji/Tonga with another convergence zone from Tuvalu/Samoa to the east.

A low is expected to form south of Fiji on Monday and then travel south to east of NZ North Island by Thursday and then off to the southeast.


Rain Accumulation next five days from

Subtropical ridge (STR)

HIGH 1020 in mid Tasman Sea is expected to stay put this week — good for trans-Tasman travel sailing west to north of 20S or sailing east to south of 35S or motoring at 30to 35S.

This High is expected to spread across central NZ on Monday and form a new centre east of the South Island that should travel east along 40 to 45S. There is expected to be a squash zone of strong winds between this High and the Low from Fiji. This squash zone should form large swells affecting eastern North Island especially on Wednesday and Thursday.

NZ/Tasman troughs

The Tasman High is blocking other features this week, but a passing trough should get across the South Tasman Sea on Thursday and deepen into a low south of South Island by Saturday—pushing the High off to the northeast, and replacing it with a west-southwest flow


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