Bob Blog 31 July

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 31 July 2022

Nature abhors a void “Horror Vacui”

This was the level of understanding prior to Galileo who in 1632 published his Dialogue Concerning the Two Chief World Systems (

The following year this book was condemned by the Vatican but praised by Evangelista Torricelli, one of his young students (then 25 years old).

Evangelista was the son of some poor textile workers, and was sent as a lad to his uncle, a monk, for training. At age 16 his father died, and his uncle sent him to Rome under the tutelage of Benedictine monk Benedetto Castelli. Castelli was a student of Galileo and also entrusted by Pope Urban VII with hydraulic experiments. This combination is what led him to the invention of the barometer in 1643. His name lives on in the naming of the metric unit for absolute pressure TORR.


Here is how Torricelli invented the barometer. Galileo observed that a common suction pump could not raise water to a greater height than about 10m (32 ft). He considered that the “abhorrence” was limited to 32 ft and discussed this with Torricelli. After Galileo’s death in 1642 Torricelli asked what if we live in a “sea of air” that has weight and exerts pressure like water. The suction pump lifts water up a tube — the piston closes off access to air pressure at the top of the tube, but the air still pushes down on the surface of the water outside the tube. This push causes the water to rise until the weight of the lifted water equals the weight of the air push.


Now Evangelista had access to large vats of mercury in the lab basement collected by students heating local rocks with Bunsen burners. He knew the weight of mercury is 13 times heavier than water so that would reduce the height of the lifted column from 10m to 76 cm. He asked a glass maker to produce a glass tube about a quarter of an inch in diameter and 4ft. long, and hermetically sealed one of its ends; he then filled it with mercury and, applying his finger to the open end, inverted it in a basin containing mercury. When he released his finger, some mercury sank into the basin, but a column remained in the tube nearly 76cm (30inches) above the surface of the mercury in the basin. Also, at the top of the tube was a vacuum, the first ever humanly produced sustainable vacuum, which is now called the Torricellian vacuum. This experiment is sometimes known as the Torricellian experiment.


The following year he wrote up this experiment in a letter to Cardinal Michelangelo Ricci that “We live submerged at the bottom of an ocean of air”

He died 3 years later maybe from typhoid.

A thorough write up on this experiment can be found at

(Original is in Italian but has an English translation).

Stay tuned to see how this led to the barometer and then to weather forecasting.


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


FRANK and GEORGETTE are west of Mexico and SONGDA is near Korea.


Weather Zones Mid-week GFS model showing isobars, winds, waves (purple), rain (red), STR (Subtropical Ridge), SPCZ (South Pacific Convergence Zone) Convergence zone (CZ) and high CAPE (green), Highs and lows.


CAPE mid-week as seen by ECMWF and GFS from


Different models seem to produce different values.

SPCZ=South Pacific ConvL1ergence zone and STR (Sub tropical ridge).


Rain Accumulation next five days from

The SPCZ is active in the Coral Sea to Vanuatu.

Convergence zone slow-moving near Fiji on Monday and Tuesday.

A convergence zone CZ is expected to linger between Samoa and Just south of French Polynesia.

A passing trough is expected to form over New Caledonia on Tuesday and then travel east forming L1 between Fiji and NZ on Wednesday. L1 then moves SE and fades. Avoid L1. Associated trough crossing Fiji/Tonga at weekend.

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.


High H1 across northern NZ on Monday expected to travel east along 35S with a squash zone at around 25S.

L2 is deep in Australian Bight early this week and expected to travel east along 45S reaching southern NZ by end of week. Constant moist NW winds for NZ Alps this week, strong until mid-week.

H2 forming in Tasman Sea by mid-week is expected to cross northern NZ this weekend.


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


Published by metbob

Pattern and Chaos

Join the Conversation


  1. Maybe Torricelli died from Mercury poisoning…! Keep up the good (and often interesting work)

    Cheers, Penny

    Penny Mackay 35 Rainbow Drive Atawhai Nelson

    Phone: 03 545 0411 Mob: 021 402 172


    1. Good point Penny . None of the historians I read have admitted to that possibility , but dipping your hand into a bath of mercury is extremely dangerous.

Leave a comment

Please log in using one of these methods to post your comment: Logo

You are commenting using your account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s

This site uses Akismet to reduce spam. Learn how your comment data is processed.

%d bloggers like this: