Bob Blog 21 June 2020

Compiled Sun 21 June 2020

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.

Here comes winter

I know that most of the world deem 1 June as start of winter-, for convenience, timing it as the calendar months of winter being June, July, August.

However, I prefer to use one of the four “corners” of the year as per our view of the sun. These are the spring and summer equinoxes and the winter and summer solstices. The winter solstice usually falls around June 21 (depending on when was the last leap year).


So, Winter “starts”, using the solstice, on 21 June at 9:43am in Auckland.

The southern hemisphere will be having its shortest “day” lasting, in Auckland, just 9 hours 38 minutes, but it’ll take a week before this duration extends by even a minute, and yet in a few months (spring start day length) will extend to 12 hours.

A good measure of our view of the sun is the solar declination, which is known by marine navigators.

Here is a download from the Internet, sadly with seasons for northern hemisphere, but you should be able to switch these by sight as the Northern Hemisphere summer is the Sothern hemisphere winter, etc. Note that for the solstice, the declination is 23.45 degrees to the north of the equator.


So the angle of the overhead sun in Auckland is 90 degrees minus Auckland’s latitude, minus the sun’s declination that’s 90-37-23.45= ~29 degrees (compared with ~77 degrees at summer solstice). This can be surmised from the Sun path for Auckland from


The coldest days of the average year in Auckland is with a day-time high of around 14C in late July to early August. Hence, the saying “When the days (start to) get longer, the cold gets stronger”.

The Tropics

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

Currently it is quiet

There are a few small low to moderate patches of potential development but these are not really expected to come to much. But be aware.



Weather Zones (see text) as expected Wednesday 00UTC showing isobars, winds, waves(magenta), Rain (Blue), CAPE (pink), STR, and SPCZ.)


SPCZ=South Pacific Convergence zone.

The SPCZ is active from PNG/Solomon Islands, Vanuatu, Fiji, Samoa, to French Polynesia, with a convergence zone that stretches off to SE of SPCZ.

A tropical trough is expected to cross New Caledonia on Tuesday, and brush past Fiji and Tonga on Wed /Thurs this week.


Accumulated rainfall for next week from

Subtropical ridge (STR)

Brief ridge expected to cross Tasman Sea on Monday and NZ on Mon/Tues, as a HIGH 1028 to 1026 hPa slips off to east of South Island.

Next High 1024 to 1028 hp expected to enter South Tasman near 50S on Tues, and travel across south NZ by Thurs. There is likely to be a squash zone of enhanced easterly winds between High and Low over the North Island on Thurs/Friday. Take care.

Tasman Sea /NZ/Aus

Low over Northland tonight moving away to east on Monday.

LOW 1006 hPa over Tasmania on Monday expected to travel NE across Tasman and reach northern NZ by Thurs/Friday (along with a squash zone).

From Panama: Not good this week, SW winds or light winds in Panama, and the ITCZ is visiting at times. It is most active/squally 10N to 6N.


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