Bob Blog 15 July



Compiled Sun 15 July 2018

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.

Kayaker Scott Donaldson made successful land fall in new Plymouth early last week after departing from Coffs harbour in early May.



I have been helping Scott with twice daily txt messages looking at currents, wind and swell. Scott is an adventurer motivated to raise money to help those with live with Asthma (such as he and his son)

If this success inspires anything in you then please show it by funding his donation page:— Do this soon as the page closes by end of July.

Surely such an adventure is worth a $10 donation?


WINDY.COM have added a new layer, called THUNDERSTORMS, which comes from the global ICON model from DWD as a combination of rain and lightning density. More resolution than the CAPE parameter.


Technical details are at


Auckland’s lightning forecast in today’s passing front from windy.

Lightning is static electricity. Any object that spikes upwards from an otherwise flat horizon is asking to be struck by lightning. When updrafts in a shower cloud exceed around 25 km/hr (13 knots), air and water molecules are rubbed together, creating static electricity (making lots of free electrons). In the updraft, rain, ice, and snow are jumbled, and the lighter particles are lifted to the cloud top, whilst freed electron tend to attach to the heavier particles at base of the cloud. Like charges repel, and so this reservoir of electrons in the base of the cloud causes the free electrons in the ground beneath the cloud to go elsewhere. This forms an area of positive charge in the ground under the cloud. When the cloud gets blown around, this positively-charged area follows the cloud like a shadow. When the reservoir of charge in the cloud gets large enough, an explosive discharge occurs.

First, groups of electrons leap outwards and downwards in all directions towards the positive area near the ground. This happens in jumps, called leaders, each taking around a nanosecond, and at the end of each jump the pause may break the leader into two. When one of these leaders is within about 100 metres of the ground a group of positively-charged particles streams up from the earth to meet the leader. These are called streamers and are best made from a spike or tower (or tree). When the streamer meets the leader, the stored energy (100 million volts) is released suddenly enough so that the air in the stream is ionized. It turns to plasma, 20,000C+, or 10 times hotter than the surface of the sun for a fraction of a second. This ionization works its way up the streamer (so the light starts at the bottom and ends at the top), and this is called the return stroke. On its way up all the places where the streamer has divided get lit, hence the spread-out pattern of some lightning photos. After the first strike, the main pathway between the ground and the cloud reservoir of charge can go through another ionization episode (or strike). This is the flicker seen in a lightning strike.

A normal lightning path or bolt may be many kilometres long and is only as thick as a thumb. The speed of sound is 1 kilometre in about 3 seconds or 1 mile in about 5 second, and you can use the time difference between the flash and the bang to measure the distance between you and the strike. Lightning may be seen when it is 60 kilometres away, but thunder is only audible for maybe 10 to 20 kilometres. In the 19th century, thunder was thought to be caused by the implosive collapse of the partial vacuum of the lightning bolt. Nowadays, the consensus is that it is the shock wave of the sudden thermal expansion as the bolt forms plasma (air breaking the sound barrier). The rolling sound after the initial clap is sound arriving later from parts of the lightning bolt that are further away. Over land, rolling thunder also echoes off surrounding terrain.

80% of lightning discharges occur from top to bottom of a cloud, or between two nearby clouds (C2C or clouds to cloud). The ones that reach the ground are called C2G or cloud to ground.

I handle weather forecasts for around 400 yachts each year, and in the past year two have lost their navigation instruments due to lightning damage (both in tropical waters). This seems to be a higher rate than the normal quote of 1 in a 1000 per year.

A yacht mast “spikes out from a flat surface” and thus tends to act like a lightning rod. Normally it is connected to the keel reasonably well by its rigging. Since lightning is static electricity it tends to flow on the outside of a surface, and so a metal mast and rigging should take most of the charge to the keel and surrounding seawater. Some extrapolate from that to say there is a cone of protection underneath the mast, but some say that’s a myth (see Installing a proper lightning rod and connecting this via copper plate to the keel may just divert some of the lightning charge so that it blows thru the copper plate, holing the keel. Opinions differ on this point.

When you see a lightning squall coming, the standard advice is to get the sails down and the engine going on auto helm, and standby in the companion way or below deck and cross your fingers. I’d like to add that you protect your electronic equipment: Disconnect your antenna, and gather any mobile navigation equipment such as GPS, satellite phone, tablet, laptop, etc., and toss them into the galley oven (or a microwave if you have one) as an approximation to a Faraday cage.



As seen at

Tonight, we still have BERYL in the North Atlantic (now out of the tropics), and a “zone of interest” between Mexico and Hawaii.

If we compare the past week’s rain with the previous week we can see an increase in the intensity of convection around Malaysia and Indonesia. This is part of a MJO event, a trend to increased convection spreading east into the NW Pacific. This is likely to trigger tropical cyclone activity around Micronesia over the next week or two.



– see



Weather Zones (see text) as expected Wednesday 00UTC showing isobars, winds, waves (magenta). STR, and SPCZ. Pink area (none this week) = lightning likely (high CAPE)

SPCZ=South Pacific Convergence zone.

The SPCZ is expected to hover over PNG and Solomon Islands, with a weak stretch across the Wallis and Futuna area.

A passing trough is expected to visit Tonga on Monday UTC and then travel east to visit Rarotonga on Thursday UTC and linger there until Sunday 22 July UTC.


Accumulated rainfall for next week from

Subtropical ridge (STR)

HIGH near 40S and east of NZ near 180 tonight is expected to slowly travel east along 40S this week. Associated squash zone of enhance d trade winds is expected to peak near 20 to 25S over Gambier Islands on wed to Fri 18 to 20 July UTC.

Next HIGH is expected to travel east along 25S across the north Tasman Sea on from Mon to Thu and then linger over northern NZ on Friday and Saturday UTC.

Around Tasman Sea, NZ to tropics.

Trough moving off NZ on Monday followed by a westerly flow. A weak trough is expected on Wednesday followed by a SW flow on Thursday.

Light winds with a passing ridge on Friday and then another trough is expected from the Tasman Sea on Saturday/Sunday.


Australia to tropics

A slow-moving ridge over the northern Tasman Sea this week.

Tahiti to Tonga

Winds around the Society island may reach a peak on local Sunday/Monday 15/16 July as a squash zone (associated with a passing HIGH) travels east. For the remainder of the week there should be an OK east to ESE flow from Tahiti to Tonga with a minor squash zone along 25S.

South of 18S, A passing trough should reach Rarotonga area from Thursday/Friday UTC.

Galapagos to Marquess

SE to E to NE winds, mostly less than 20 knots. To use available currents, go to 2S 93W and then 6S 125W and then direct.


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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1 Comment

  1. Hi Bob

    Great explanation of Lightning thanks. Will show my AvMet students.

    When I did a trip from Jamaica to Tahiti on a Cat through the Panama Canal 5 years ago there was a very active ITCZ and the cruising word out was that 12 boats had been hit so far with 2 I knew of having to fly someone out from the States for electrical repair – we talked to them in the Marina at Colon.

    Cheers, Penny

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