Bob Blog 22 July

Bob Blog

WEATHERGRAM

YOTREPS

 

Compiled Sun 22 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.

 

After my notes on lightning last week, one of my readers has asked me to try and have ago at explaining the old folk-lore expression “Red sky at night, sailors delight; red sky in the morning, sailors warning”

(land dwellers change the sailor into a shepherd).

 

Reading weather signs is one thing that humans have been doing ever since we have been human enough to detect pattern amongst the chaos. And this sign is a good one, often proving true. In Matthew 16.2, Jesus told the Pharisees that “When evening comes and the sky is red, you say the weather will be fair, and in the morning when the sky is red and overcast, you say today will be stormy—you know how to interpret the appearance of the sky, but not the signs of the times!

 

“A Red sky at night is a delight.” Explanation, it’s as easy as 1,2,3: 1. The sun sets in the west, and, for many parts of the planet, 2 that’s where tomorrow’s weather is usually coming from. Usually the winds at cloud level above us are from northwest/west/or southwest, so weather features and clouds ride this steering field from west to east. If the western horizon is cloud-free, then there is a good chance that our tomorrow’s weather may be cloud-free.

3. Sunlight shining slant-wise loses all but its red colouring.

Air near the ground gets dirty: it accumulates particles of dust and salt and smoke that are small enough to float around (aerosols). Sun shining on these materials undergoes Mie scattering, whereby red light is mostly scattered forwards and blue light is mostly scattered laterally or backwards. As the sun sets, it beams through an increasingly longer path than at noon. It takes on a golden hue, or, if there is a lot of dust about, then the clouds above are side-lit in a rosy red. Colours can be seen to go through the spectrum and some colours are enhanced according to the aerosols of the day. Once the sun is set, red light from the aerosol layer mixes with the blue light scattered above it to form purple light.

 

“A Red Sky in the morning is a warning” Explanation: Sun rises over the eastern horizon. If that’s where the clear skies are, incoming light is rendered reddish by passing through cloud-free but dirty air. If there are clouds overhead, this usually means we are on the back end of a departing area of dry weather, and thus, as is the nature with weather, on the forward side of the next lot of rain approaching from the west. Dawn horizontally side-lights the overhead clouds with a reddish glow, enhancing our senses of pending gloom.

 

TROPICS

 

Tonight, we still have a “zone of interest” between Mexico and Hawaii.

It is a very busy time in the NW Pacific. This is in part to do with a MJO or Madden Julian Oscillation travelling into the Pacific

 

WEATHER ZONES

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 to south of Samoa.

A passing trough over the Southern Cooks is expected to turn into a low near 30S 160W by 23July UTC that should move off to the Southeast.

Another passing trough is expected visit Vanuatu on Mon 23 UTC, Fiji /Tonga on Tues UTC, Niue on Wed UTC and Southern Cooks on Thu 26 July UTC.

And yet another passing trough is expected to affect Eastern Coral Sea/New Caledonia area on Sat/Sun 28/29 UTC.

 

Subtropical ridge (STR)

HIGH is expected to linger in the northern Tasman Sea from Monday to Thursday and then travel off to the east along 30S.

Too many passing troughs for another HIGH this week. Me-thinks this is a sign of an incoming El Nino, that’s if I’m reading the sign of the times correctly.

 

Around Tasman Sea, NZ to tropics.

Developing low over central NZ on Monday. Then a Low is expected to form near the Kermadecs on Tuesday (as associated trough crosses Tonga). This low should deepen to below 980hpa and travel south along 170W on wed/Thu UTC and then go off to the east along 45S, bringing a SW to westerly flow to NZ, OK for departure to the tropics.

Near Australian coast a new low is expected to deepen off Sydney from Sat 28 July

 

Tahiti to Tonga

Winds around the Society islands look OK for departure any day this week. Try and stay north of the passing troughs.

 

Galapagos to Marquess

Useful southerly winds over Galapagos for going west at first. Go to 2S 106W and then 6S 128W then direct.

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If you would like more detail for your voyage, then check metbob.com to see what I offer.

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Weathergram archive (with translator) is at weathergram.blogspot.co.nz.

Contact is bob@metbob.com or txt 6427 7762212

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