Bob Blog 15 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 Sunday 15 Jan 2023

A barograph on your mobile.

Barometric pressure. You want to know about wind and rain, but weather
forecasters talk about isobars and fronts. Isobars and fronts show a pattern
that is easy to draw and follow. Isobars are those lines on a weather map
joining together places with the same surface pressure. The shape of the
isobars describes the weather pattern, and the way this pattern changes can
be used to forecast wind and rain.

All this starts by measuring pressure change.
What causes pressure to fall?
. maybe there is an approaching low-pressure system (marked as an L on a
weather map).
. maybe the air is getting warmer (and less dense)
. maybe the air is becoming more cloudy/humid (YES- damp air weighs less
than dry air).
. maybe the air above is being removed faster than it can be replaced (this
happens when rising air is vented off by stronger winds).
. maybe it is just the time of the day. clip_image002This is due to a
solar-induced atmospheric tide and called the "diurnal pressure change".
Pressure rises between about 5 to 9 am or pm and falls between 11 and 3 am
or pm. Daylight saving alters these hours.

Any iPhone includes a pressure sensor which can turn even an old model 6 (or
later) into a sophisticated barograph (sim card not needed for this to
work). There are two apps from STARPATH for mariners to watch pressure
changes.

The basic free Marine Barometer app records time, date, Lat, Lon, and both
sea level and station pressures manually with the press of a button.

The Marine Barograph app, for a small charge, expands on this basic
functionality by storing pressure and navigation data automatically whenever
the app is showing. It creates versatile graphic plots of pressure versus
time, plus all data can be exported as CSV files. Your vessel track can also
be exported as a GPX file for navigation analysis. These tracks can load
into Google Earth. Great for recording your sailing adventures. For a full
youtube discussion on how to use this app see
//www.youtube.com/watch?v=ZdVHWTF-Gs4


TROPICS
The latest cyclone activity report is at zoom.earth and tropic.ssec.wisc.edu
and Tropical Cyclone Potential is from
www.ssd.noaa.gov/PS/TROP/TCFP/index.html
This is the last week of this MJO and there is good potential for cyclone
development in the South Pacific, especially around Vanuatu,

WEATHER ZONES
South Pacific Convergence Zone
The SPCZ is expected to be very active and somewhat erratic with tropical
lows L1 travelling from south of Fiji to east of North Island, and L2
deepening over Vanuatu /New Caledonia mid-week then going southeast Avoid.
HIGHS
HIGH H1 now east of the South Island is expected to travel east along 45S
and help steer L1 southwards .
HIGH H2 is expected to travel slowly north across Tasman Sa and fade by end
of the week. is travelling east along 45S to east of the South Island.
Trough crossing Tasmania/ New South wales mid-week is expected to cross NZ
at end of the week followed by a southerly change.

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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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