Bob Blog 24 July 2022

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 24 July 2022
HEAT WAVES
Heathrow airport near London hit 40C on Tue 19 July 2022, a new record.
The cause of this heat wave was the movement of air from over the Sahara
Desert northwards to the UK where it descended and warmed "adiabatically" or
without transferring heat via evaporation or condensation. As the air falls
it encounters higher pressure and warms by compression. We are familiar with
this effect when pumping air into a tyre, but the physics of this has only
been sorted out ~200years ago and took ~300years to sort out
This is a logical application of the "Ideal gas law" which is a combination
of Boyles law (When the temperature and amount if gas remain constant , the
absolute pressure exerted by a given mass of an ideal gas is inversely
proportional to the volume it occupies) and Charles Law (When the pressure
on a sample of a dry gas is held constant, the temperature and the volume
will be in direct proportion).
In physics a "law" is a mathematical formula that fits the observations, not
a decree. The gas molecules that occupy a space just bounce around and do
their own thing, bouncing faster when compressed, and it follows these
'laws'. Boyles law was published by Robert Boyle, an English Chemist, in
1662 and Charles law was published by Jacques Charles, a French balloonist,
in the 1780s. The ideal gas law was finally constructed thanks to Amadeo
Avogadro, an Italian Count, in 1811 and to Émile Clapeyron, a French
engineer, in 1834.
Looking at the temperature and relative Humidity measurements around
London on 19 July we can see that they are inversely related. This is a good
indicator that indeed the hotter air had arrived from the drier reaches
aloft.
This coming summer we are expected to have another La Nina, like last
summer. This usually brings tropical northeasterly winds to Aotearoa New
Zealand, and these are moist and produce rising rather than sinking air, so
an initial forecast is that we may be somewhat protected from heat waves
this coming summer (the nearest source of hot dry air is Australia and only
gets to Aotearoa New Zealand in westerly winds).
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Finally, a shout out for Gulf Harbour Radio ZMH286 who run a broadcast on
shortwave radio at 5:15pm Mon to Fri, May to Nov.
In this Patricia takes and tracks yacht positions and David relates weather
information across the South Pacific.
These broadcasts are archived at www.ghradio.co.nz/live-streaming.html
During July David has also been giving weather lessons:
Lows that form following Highs, see www.youtube.com/watch?v=7XA3ZUvdFAk
The MJO, see www.youtube.com/watch?v=dXuX5tXt1b4
Trade wind surges, see www.youtube.com/watch?v=244713K2nis (on Bastille Day)
Understanding Clouds, see www.youtube.com/watch?v=C1Z3dMgxFnY
So, these files are now available for anyone interested in these topics.
TROPICS
There are no cyclones around at present but there are zones of potential
development around Micronesia, west of Mexico and in the mid-Atlantic.
WEATHER ZONES
SPCZ=South Pacific ConvL1ergence zone
The SPCZ is active in the Coral Sea and expected to travel east cross
Vanuatu on Thursday and Fiji over the weekend.
A convergence zone CZ is expected to linger between Samoa and Just south of
French Polynesia.
OK to travel from Tahiti to Fiji this week but take a northern path to avoid
the heavier swells in the south, and maybe aim to cross that CZ near Samoa.
HIGHS and LOWS
High H1 over Chathams is expected to go NE and then travel slowly east along
30south.
L1 budded off the SPCZ in the last few days and is expected to travel across
NZ on Monday and Tuesday.
L2 is expected to deepen off Sydney on Tuesday and then go east across New
Zealand on Wed/Thurs and then off to the SE.
L3 is expected to form in the south Tasman Sea on Friday and travel
northeast to be east of the South Island by Sunday,
followed by cold southerly flow for NZ on Monday 1 August.
H2 is expected to stay over central Australia this week and on Saturday move
east along 30S into the Tasman Sea
L1, L2 and L3 make it difficult to plan trans-Tasman travel this week.
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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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