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

19 September 2021

Bob Blog 19 Sep

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 19 Sept 2021


Can you tell the difference between a sucker cloud and a puffer cloud?

Clouds that grow upwards into the sky are called cumulus clouds. They start
off as fluffy white ones.
Once they get sufficiently tall they produce a shower of rain.

If conditions are strongly unstable this shower may become heavy, perhaps
with hail, or thundery or squally.

These small cumulus clouds can be called suckers because that have a zone of
calm on their leading edge. As a sucker approaches the wind speed gradually
drops away. When you see these clouds, try and sail around and between them
rather than under them.

As you look into the surface wind, the cloud will creep to your right (in
the southern hemisphere). This is because it is propelled by the stronger
upper winds. Friction causes the surface wind to "leak to low pressure" and
be to be to the left of the winds aloft. When sailing upwind, put the wind
to starboard (go left) to avoid the approaching calm, and aim for the LEFT
edge of the cloud to catch the extra winds on its rear.

The taller clouds can be called puffers. Once the turrets grow to be taller
than their base, they can start producing rain in the colder upper region,
this rain falls and drags with it a downdraft that fans out from the back
end of the cloud like a waterfall.

As the downdraft ahead of a puffer reaches you, surface winds suddenly
increases. There is a direction-shift around the shoulders of the cloud, and
a huge calm zone that trails behind it. When sailing upwind into an
approaching puffer, remember to do the "squall checklist" --- put things
away-reef the rig--- check the compass-and watch the wind on the water
ahead. When the first puff of the downdraft appears, put the wind on
starboard (go left) so that we are sailing away from the puffer. We may get
lifted for a few minutes as we go around the shoulder of the cloud but keep
aiming to the left so we avoid being caught in the trailing calm zone.

Cyclone PETER is expected to peel off to the northeast and may swipe past

SPCZ=South Pacific Convergence zone.
The SPCZ stretches from Solomona to Tuvalu to Samoa to Southern Cooks, in
its normal position.
Trough over Niue to Southern Cooks tonight expected to get onto French
Polynesia by mid-week and form a low that travels off to the south by end of
the week.
Trough reaching New Caledonia by mid-week is expected to reach Tonga/Niue
area by end of week.

HIGH1 above 1024 to north of NZ is moving ESE along 30 to 35S followed by H2
travelling along 30S. This maintains useful trade winds this week north of
LOW1 is expected to form near Lord Howe by Tuesday and deepen to 995 as it
crosses central NZ on Wed/Thursday/by Wednesday in mid Tasman. Avoid.
Associated trough may form new low east of North Island and south of Niue by
end of week.

If you would like more detail for your voyage, then check to see
what I offer.
Or Facebook at /
Weathergram with graphics is at (subscribe/unsubscribe
at bottom).
Weathergram archive (with translator) is at
Contact is or txt 64277762212

Blog Archive