Bob Blog 19 Aug 2018



Compiled Sun 19 AUGUST 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.

Why is the Sky Blue?

After my recent blog on twilight colours , I have been asked to offer my explanation of the blue sky.



Sunlight is made up of a spectrum of colour – a bundle of waves of electromagnetic energy all travelling at the same speed but each vibrating at different frequencies. When talking radio stations we speak about its frequency (e.g. 95FM) but when talking about light we speak about its wavelength. Red light has a wavelength of around 650nm and at the other end is violet light with a wavelength almost half that at 400nm. Red Orange Yellow Green Blue Indigo Violet.


A beam of light travels in a straight line until something gets in the way and does one of four things

1) Reflects all of it on a straight path (like a mirror)

2) Bends it (like a prism)

3) Scatters it (read more below)

4) Absorbs some of it and reflects/scatters off the rest (we see this scattered/reflected light and call it that the COLOUR of the object).



Back in 1859, John Tyndall described how wavelengths in a light beam get scattered when it shines thru a colloid (a mixture in which the floating particles do not settle out) e.g. cloud/fog, smoke, milk, ink, paint – the particles in a colloid are 1 to 1000nm or o.ooo1 to 1 micron, This scattering is called the TYNDALL EFFECT. Shorter wavelengths get scattered sideways and only the longer wavelengths get transmitted forwards.



The Tyndall effect: Headlights seen in fog: Example of how all that remains of white light when scattered by the Tyndall effect are the longer wavelengths (yellow and red)



LORD RAYLEIGH, or John William Strutt, the third Baron Rayleigh (1842-1919). Born in Essex, Educated in Trinity College, Cambridge.

In 1871, he described how light is scattered by the molecules of air, and this is called RAYLEIGH SCATTERING


As sunlight travels thru the atmosphere it encounters air molecules and reacts with their electrons so that some light gets scattered. The shorter wavelengths are deflected at a greater angle than the longer wavelengths. Lord Rayleigh described how this works when the scattering is done by molecules showing that “the amount of scattering is inversely proportional to the fourth power of the wavelength” This means that the shorter wavelengths are deflected away into the sky and the longer wavelengths get transmitted forwards.

Or: Blue scatters Best (ten times more than red).

This scattered blue light fills the sky giving us skylight.



Lord Rayleigh also explained why the light from the Blue sky is polarized:



The image on the right was taken thru a polarizing filter that only shows light that is linearly polarized in a specific direction.


OK I hear you: so if the shorter wavelengths are scattered best, then why isn’t the sky VIOLET or INDGO? It’s true that Indigo and violet are scattered even better than blue, but the sun emits more energy as blue light than indigo or violet. That’s only part of the answer for we can see the violet in a rainbow. The rest of the answer lies in the way our vision works.

Humans have evolved with three types of colour-receptors in our retina. They respond most strongly to light at the red, green and blue wavelengths. When they are stimulated in different proportions our visual system constructs the colours we see.

Response curves for the three types of cone in the human eye



When we look up at the sky, the blue cones are stimulated strongly and the red and green cones are stimulated less so, and almost equally. If there were no indigo and violet in the spectrum, the sky would appear blue with a slight green tinge.  However, the indigo and violet wavelengths stimulate the red cones as much as the blue, producing a purple tinge. The net combined effect is what we call the “sky blue” colour.

It may not be a coincidence that our vision is adjusted to see the sky as a pure hue. We have evolved to fit in with our environment; and the ability to put natural colours such as sky-blue or grass-green into the background and focus quickly on anything that does not have natural colouring is a useful survival trait.




Map of current storms is from

TC LANE is SE of Hawaii. It is expected to weaken and go west along 15 to 16N, but may bring some windy rain to western parts of Hawaii in a few days.


See for an update.

TC SOULIK is expected to skirt around the south end of Pan and visit Korea this week, and CIMARON may travel NW towards Osaka Bay in Japan.

The Atlantic continues to remain somewhat quiet, for now.

Looking at the weekly rain maps we can see that the Asian monsoon is active between India and the Philippines, and the ITCZ is active across the Pacific and over Panama. South Pacific Convergence zone has a wet week over Solomon Islands. Kerala, the Southwestern state of India , stands out with recent flooding.






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 expected to stretch from PNG and Solomon Islands to north of Fiji and around the Tokelau/Samoa area.

A passing trough at present over southern Cooks is expected to deepen into a LOW that should continue deepening to below 989hPa by mid-week as it travels ESE/SE to o 35S 125W, Associated trough is expected to cross Tahiti on local Sunday and Tuamotu Islands on local Tuesday.

A passing trough is expected to visit New Caledonia /Vanuatu early this week and stall over Fiji/Tonga during mid-week then move onto Southern cooks late in the week



Accumulated rainfall for next week from

Subtropical ridge (STR)

HIGH is expected to move along 25 to 35S from south of Fiji/Tonga early this week to south of French Polynesia by mid-week, followed by another High later in the week,

Next HIGH from Australia is finally expected to move into the central Tasman Sea late in the week and visit NZ early next week.

Around Tasman Sea, NZ to tropics.

Low is expected to travel across southern NZ on local Monday.

There may be a brief ridge over northern NZ on local Tuesday,

and then another Low from central Tasman Sea is likely to cross the North island on local Wed/Thu/Fri. Avoid these lows.

Tahiti to Tonga

Wait until local Monday or Tuesday for the winds t return to be from SE after a passing trough.

There is expected to be a small enhancement of the trade winds late in the week near 20S as a High to the south travels east.


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