Bob Blog 19 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 Sun 19 July 2020

 

Your barometer:

That barometer you got last Christmas can be put to use as a weather forecaster.

OK, all it does is read air pressure, or the weight per unit area of the column of air above it. But by following these readings you can tune into the vibrations of the weather pattern as it changes. Like whiskers on a cat.

Pressure. Why is it so important? Because imbalances in air pressure cause wind and weather. You want to know about wind and rain, but weather forecasters talk about isobars and fronts. This is because isobars and fronts have a pattern that is easier to draw and follow. Isobars are those lines on a weather map joining together places with the same surface pressure. The shape of the isobars describes the weather pattern, so changes in the weather can be forecast by tracking the changing isobar pattern or by observing pressure change.

What causes pressure to fall?

Several things can cause this.

• maybe there is an approaching low-pressure system (marked as an L on a weather map).

• maybe the air is getting warmer (and less dense)

• maybe there has been an increase in the moisture or cloudiness in the air (YES- damp air weighs less than dry air)

Cricket players know that a passing cloud increases humidity and reduces air pressure allowing the cricket ball to spin better. Watch a spinner bowler watch for a passing cloud next time you watch a game of cricket, may well lead to an “out!”.

• maybe there has been a decrease in the amount of air above (this happens when rising air is removed by strong winds aloft faster than it can be replaced. increasing the speed of the rising air).

• maybe it is just the time of the day. There is a twice-daily roller coaster as shown here from windy.com, looking 9 days of pressure changes at Cairns (early January 2019)

 

This is due to a solar-induced atmospheric tide and called the “diurnal pressure change”. Pressure rises between about 5 to 9 local am or pm and falls between 11 and 3 local am or pm. Daylight saving alters this.

The amount of this diurnal change is more in the tropics (about 3 hPa per tide) than over New Zealand (about 1 hPa). The easiest way to remove diurnal change from your calculations is to read your barometer at the SAME TIME of the day, preferably with the “flat tops” at about 10am (and maybe10pm).

 

Note that the average atmospheric pressure is 1013.25hpa, and the closest isobar to that is 1012. It turns out that 1012hpa (mostly) divides the higher pressures (drier, more settled weather) from the lower pressure s (unsettled weather). It was labeled as the change area on the old-time barometers. It is a useful tool for weather forecasting.

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The Tropics

The latest cyclone activity report is at tropic.ssec.wisc.edu and Tropical Cyclone Potential is from www.ssd.noaa.gov/PS/TROP/TCFP/index.html

There are currently NO active tropical storms. There are, however, several potential areas for propagation.

WEATHER ZONES

Weather Zones (see text) as expected Wednesday 00UTC showing isobars, winds, waves(magenta), Rain (Blue), CAPE (pink), STR, and SPCZ.)

 

SPCZ=South Pacific Convergence zone.

This is likely to be nudged to the north a little this week and just be from Solomon island d to north of Vanuatu. due to the STR being knocked north over much of the South Pacific by Lows over NZ.

However, there are likely to be passing troughs over Fiji and Tonga /Niue on Monday/Tuesday ,and another passing trough Se of French Polynesia.

 

Accumulated rainfall for next week from windyty.com

Subtropical ridge (STR)

HIGH 1026hPa should move into central Tasman near 35S on Friday and over northern NZ for the weekend.

The record-breaking rain over Northland from a Low in the NW Tasman is now clearing as the Low goes NE then SW (as a new low) south of Fiji on Monday.

Band  of rain crossing NZ on Tuesday.  Another Low, 986hPa, south of NZ is expected to travel NE along east coast on Tue-Wed, bringing strong SW and rough seas onto the North island west coast.

Winds around Northland should start easing again on Wednesday but THURSDAY is looking to be the day this week for departing North.

From Panama: Not as squally as last week as ITCZ may ease a bit. 10 to 15kt west to SW winds as far as 90W. Further west, the winds swing slowly to S then SE. Tropical features may form off west of central America, but not a bother on voyage to Marquesas.

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

Or Facebook at /www.facebook.com/metbobnz/

Weathergram with graphics is at metbob.wordpress.com (subscribe/unsubscribe at bottom).

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

Contact is bob@metbob.com or txt 6427 7762212

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