Bob blog 20 Oct

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 20 November 2022

INSTABILITY AND 1012 THE MAGIC ISOBAR

The recent rains in Australia were brought about by the influx of air aloft
from the Antarctic. This cold air last week crossed the Tasman Sea bringing
with it thundery downpours that have recently reached New Zealand. When
colder than normal air resides aloft, meteorologists describe the air as
being "unstable", meaning that any rising pocket of air may be boosted
upwards by its own buoyancy. An interesting way to "defy" gravity. These
unstable clouds are the ones that produce downpours. CAPE is a good
parameter to measure instability

All clouds contain rising moist air but only a few produce rainfall.
Basically, there are three types of clouds: flat ones, wavy ones, and bubbly
ones. The factor that determines the type of cloud and any resulting
rainfall is 'upward motion'.

Flat clouds are the result of gentle large-scale upward motion created by
air rising slantwise along a slope such as a frontal boundary. These layers
of rising air have little and slow-changing buoyancy: a condition which is
called stable air. They produce rain.

Bubbly clouds are a sign of rapid upward motions which occur on a small
scale in time and space when the atmosphere has lots of triggered buoyancy,
and is called unstable. They produce showers.

Wavy clouds are produced when strong winds blow over a range of mountains on
a stable day. Supercooled cloud droplets form in the
lower pressure pockets that ripple downstream. These clouds, called
Altocumulus lenticularis, are usually dry.

In order to be able to share our cloud watching experiences with others, we
need to learn a few cloud names. The names we use were introduced in England
by chemist Luke Howard in 1802. The two main Latin words are 'Stratus' for
flat or stable and 'Cumulus' for lumpy or unstable. Combined, they describe
a flat low layer of cloud with a bumpy base as stratocumulus.

The word 'nimbus' indicates rainfall. It's used as a prefix for flat clouds,
as in 'nimbostratus' but has the suffix 'nimbus' as in cumulonimbus.

Height above the ground is divided into thirds as follows: 'Cirrus'
describes hairy looking clouds high in the sky, with the prefix 'cirro'
attached to other clouds so high that they are made of ice crystals. The
prefix 'alto' is added for middle clouds (the same meaning of the word is
used for the alto voices in a choir). Middle clouds are composed of liquid
droplets and some of these may have a temperature below zero Celsius
(supercooled). Low clouds carry no prefix and are distinguished by being
modified by the terrain or 'feeling the ground'.

Average atmospheric pressure on Planet Earth is 1013.25hPa. The nearest
isobar to this is the 1012, and indeed this is A MAGIC ISOBAR. It's the
dividing line; at any one time the isobars higher than 1012 will always be
balanced by those lower than 1012. Robert FitzRoy selected the word 'change'
to describe the 1000 to 1012 mean sea level pressure zone on a barometer.
Usually, the 1012 isobar is the straightest on the weather map. Isobars with
lower numbers are associated with cyclones called 'cyclonic'. Isobars with
high numbers are associated with anticyclones called 'anticyclonic'.
Usually, but not always, anticyclonic isobars are associated with stable
air, and cyclonic isobars are associated with unstable air.

In New Zealand flat, wavy, and bubbly clouds often co-exist, especially near
the mountains. Even when the weather map and large-scale clouds indicate a
stable day, a fresh breeze blowing onto a range of mountains from the sea
can produce cumulus clouds and showers on the windward slopes. Stability can
change during the day, often reflected in the clouds: a day may start with a
flat deck of stratocumulus which has a lumpy underbelly, and by afternoon
the cloud deck may have changed to a flat underbelly, indicating a change to
unstable air. The clouds may then burn off and break up into cumulus clouds
with lumpy tops. These changes add to the fascination of cloud watching.

Judging stability by reading weather maps and/or watching clouds is both an
art and a science.

TROPICS

There are some lows in the China Sea and southwest of Jakarta producing
near-equatorial westerly winds across Indonesia, but no named storms
tonight.

WEATHER ZONES

The SPCZ South Pacific Convergence Zone is somewhat quiet from Solomons to
north of Fiji, then has an active convergence zone/trough over southeastern
parts of French Polynesia.
The active trough between Fiji and NZ is expected to weaken in a few days.

HIGHS and LOWS
HIGH H1 east of Aotearoa NZ is still a blocking High, staying put and
helping maintain a strong subtropical ridge westwards to Australia
LOW L1 is expected to travel across the south Tasman Sea.
Around Tuesday night/Wednesday its trough should cross NZ, avoid.
And then it should travel east.
The simple zonal pattern this week makes for simple voyage planning.
Traveling from tropics to NZ means crossing the variable subtropical ridge
--- a zone of fickle winds.

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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
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