Bob Blog 24 Nov



Compiled Sun 24 Nov 2019

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.


Th e following notes have been summarized from a blog by Marshall Shepherd at

Most lightning flashes (roughly 80% or so) are intracloud (IC), either cloud-to-air (CA) or cloud-to-cloud (CC). It is the cloud-to-ground (CG) lightning flashes that pose a risk to us.

Lightning is a giant spark of electricity in the atmosphere between clouds, the air, or the ground. In the early stages of development, air acts as an insulator between the positive and negative charges in the cloud and between the cloud and the ground. When the opposite charges builds enough, this insulating capacity of the air breaks down and there is a rapid discharge of electricity that we know as lightning.

Here are the 5 steps that occur in a cloud-to-ground strike:

1. A channel of negatively charged ions surge toward the ground in very distinct steps. This is called a stepped leader.

  1. As the stepped leader (and its various branches) move toward the ground, it attracts streamers of positive charge from the ground.
  2. The electrical potential of the stepped leader connects to the ground, tree, building, or whatever is available in the pathway. The negative charge starts to flow down the pathway.
  3. A return stroke explodes “up” the pathway or channel, and this is what we actually see as the lightning stroke. It happens so fast that you may not easily discern the direction of propagation.
  4. If there is enough charge left in the cloud, subsequent dart leaders can use the same pathway created by the initial stepped leader. The return strokes associated with dart leaders are why lightning seems to flicker.


5 step cloud-to-ground lightning process

The atmosphere is a very good insulator, so lightning seeks the path of least resistance. Because the atmosphere is a good electrical insulator, lightning is looking for the path of least resistance. Lightning often strikes taller objects like trees, buildings, antennas, and metal poles For this reason, we often advise you to get inside if possible. If you are caught outdoors during lightning, and if no shelter is available, crouch low, with as little of your body touching the ground as possible.

  1. Stay away from plumbing or water pipes. Metal piping conducts electricity.
  2. Don’t get near electrical equipment like televisions, stereos, or the other fancy smart electronics in homes today.
  3. Avoid concrete walls and flooring. Concrete is often supported by metal rods or frames.
  4. Don’t use a corded phone, however cellular or cordless phones are fine.

Here is a summary of “A Detailed Analysis of Lightning Deaths in the United States from 2006 through 2018.” (John Jensenius, Jr)

During this 13-year period fishermen accounted for more than three times as many fatalities as golfers, while beach activities and camping each accounted for about twice as many deaths as golf. From 2006 to 2018, there were a total of 38 fishing deaths, 23 beach deaths, 19 camping deaths, and 17 boating deaths. Of the sports activities, soccer saw the greatest number of deaths with 12, as compared to golf with 10. Around the home, yard work (including mowing the lawn) accounted for 18 fatalities. For work-related activities, ranching/farming topped the list with 19 deaths.

While these results may be surprising, there were other results that were not. For example, males account for more than 80% of the fatalities, and weekends are the time of week most likely to experience a lightning fatality.

Myth: If it’s not raining, there will be no lighting

FACT: Lightning can occur without rain—often happens in Australia triggering bush fires.

Myth: Rubber tires on a car or rubber shoes protect against lightning

FACT: It’s the metal surrounds of a car that acts as a Faraday cage, protecting you from lightning when inside (just don’t touch the radio).

Myth: people struck by lightning carry an electrical charge and shouldn’t be touched.

FACT: No, they don’t. It’s safe and necessary to help them immediately. Lightning can disrupt or stop the heart rhythm and may require a defibrillator. If someone has stopped breathing after a lightning strike, start CPR and call 111.


The Tropics

The latest cyclone activity report is at and TCFP tropical Cyclone Formation Potential at

Active cyclones at present are FUNG WONG and SEBASTAIN (now in mid-North Atlantic and remains are aiming for UK).

Large potential for development this week over the Guam area and east of northern Vanuatu (along 70E).



Weather Zones (see text) as expected Wednesday 00UTC showing isobars, winds, waves(magenta) STR, and SPCZ. Pink area = lightning likely (high CAPE)


SPCZ=South Pacific Convergence zone.

The SPCZ is very active to north and NE of Vanuatu with a tropical Low NE of northern Vanuatu. This Tropical low is likely to deepen and may become a Cyclone for next few days as it travels south along around 170E, skirting the eastern edge of Vanuatu. Should fade by end of the week and drift west across mid or south Vanuatu.

Another Convergence zone is expected to hover around southern Cooks/ southern French Polynesia .


Accumulated rainfall for next week from (isobars are for Sunday night).

Subtropical ridge (STR)

HIGH over northern NZ on Monday is expected to travel off to the east along 40s this week.

Another HIGH is expected to enter the Tasman sea on Wed and cross northern NZ on Sun 1 Dec.

Tasman Sea /NZ/Aus

Trough crossing northern NZ on Wednesday night followed by S/SE winds on Thursday /Friday then light winds over the weekend from incoming High. Next trough is likely mid-next week (3-4 Dec) preceded by NW winds that may get strong west of northern NZ on Tues 3 Dec. OK for sailing from Fiji /Tonga to NZ this week, with waypoints to handle the changing winds near NZ.

For Noumea to Aus: OK to o next few days but remains of that possible cyclone on Sun /Mon /Tues 1/2/3 Dec with variable winds.


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 6427 7762212


Published by metbob

Pattern and Chaos

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