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Bob McDavitt's ideas for sailing weather around the South pacific

13 January 2019

Bob blog 13 Jan 2019

WEATHERGRAM

YOTREPS

Compiled Sun 13 Jan 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.

 

HOW TO READ A 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.

 

Pascals.  Most barometers measure pressure in hectoPascals (hPa). These are the same as what used to be called millibars. A hectoPascal is one hundred Pascals, and a Pascal is the standard metric unit for pressure, named to recall an experiment done under the direction of Blaise Pascal in September 1648 that used a barometer to show how pressure changed with height. This experiment was historically important for it showed the limitations of Aristotelian philosophy and showed how thinking and experimenting (science) can win out over simply modifying an explanation (literature/journalism)  (see www.strange-loops.com/scibarometer.html). Truth vs struth.

Other common pressure units are inches and millimetres of mercury. They refer to the height of a column of mercury which can be supported by the air pressure. To convert a pressure reading from hectoPascals to inches divide the hectoPascals by 33.86. And to convert from hectoPascals to millimetres multiply the hectoPascals by 0.75.

 

Types: Your barometer is most likely an aneroid (= without fluid) or digital (using a pressure transducer) type.

Aneroid Barometers: Inside there is a metal cell only partially filled with air. The size of this airtight cell varies with changes in the surrounding air pressure, and these variations are passed on to an indicator needle by a series of levers. It is all mechanical, so no batteries are needed. If the metal chamber cracks then the barometer will no longer work.

These barometers usually have words such as “Stormy” for low pressures (980 to 1000 hPa) and “Dry” for high pressures (1020 to1030 hPa). These words are at best only a first guide to the weather and date back to Vice-Admiral (Royal Navy) Robert Fitzroy (1805-65) who first visited New Zealand with Charles Darwin on the Beagle in 1835. Fitzroy later became New Zealand’s second Governor (in 1845) and Superintendent of the British Meteorological Department (in 1853).

Digital Barometers: Digital barometers have the advantage that they can display a graph of recent pressure change and the disadvantage that they require batteries. They may give a read out to the nearest tenth of hectoPascal (precision), but their accuracy is usually to around a one hectoPascal, so don’t bother paying more for this feature. Some use the pressure (and temperature) reading to produce an image that forecasts the weather for the next day or so. For a yacht, a digital barometer is better than an aneroid barometer so long as it has a display of recent pressure change.

 

Will it rain? It is OK to tap the outer glass of an aneroid barometer (just enough to see a change). By so doing you shift any recent pressure change stored in the mechanical linkage to the measuring needle. The resulting slight movement indicates whether the pressure is rising, steady, or falling. If the measuring needle goes to the left then there has been a drop in pressure recently. Most digital barometers also indicate if the pressure is rising or falling.

 

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)

Cricketers know that a  passing cloud increases humidity and reduces air pressure allowing the cricket ball to spin better.  Watch a spinner bowler wait for a passing cloud next time you see a game of cricket, sure to be 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.

There is a twice daily roller coaster of pressure 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 maybe at 10pm).

 

As a rule of thumb,

a sustained DROP in pressure is a sign of more chance of rain

a sustained RISE in pressure is a sign of less chance of rain

 

Where to put it?

Anywhere that is most convenient. But when deciding where to put your barometer there are places that you should avoid..

• Avoid placing a barometer in draughty places such as near a door. In such places the air pressure is too variable.

• Avoid direct sunlight on a barometer. This will warm and expand the metal cell causing a false recording of falling pressure. For the same reason, do NOT position your barometer near a heater.

• Avoid placing a barometer in a well-sealed or air-conditioned room. Such places do not respond well to changes in pressure.

SO whether in your house or an your yacht, the best place is on an interior wall.

 

Setting a Barometer.

The main use for a barometer is not so much to read pressure, but to measure CHANGES in pressure over time. Digital barometers usually display this as an arrow or as a bar graph. In an aneroid barometer there are normally two needles. The needle connected to the insides of the barometer is called the measuring hand. The second needle is a movable pointer (sometimes called the setting hand) which is free to be moved around by means of twirling a knob at the centre of the glass. When you arrange it so the setting hand is directly over the measuring hand you have set your barometer. The idea is that you set your barometer early in the day. Then, later, you need only glance at your barometer to see how far the measuring hand has moved. If it has moved to the left (of the setting hand) then pressures are falling.

If the pressure is changing rapidly this suggests that an approaching weather system is moving quickly or becoming more intense. In this case isobars are moving quickly across your area and are possibly getting closer together. This usually results in strong winds, and can be taken as a STRONG WIND WARNING. But sometimes the isobars in your area may not change position much even though they are getting closer together, in which case you may have stronger winds and only a small pressure change.

 

Getting it adjusted (calibrated).

This only needs to be done if you want to compare your readings with others or with the weather map. To do this, adjust your barometer so that it reads Mean Sea Level (MSL) pressure. Mean Sea Level is the standard datum level to which all barometers should be adjusted. This adjustment will automatically apply a correction to your barometer that takes into account its height above sea level. Pressure near the ground drops off at the rate of about 3hPa per 25 metres of altitude. SO if you change the altitude of your barometer by more than 5m , then  re-calibrate it.

Calibrating your barometer is easy. All you have to do, once you position your barometer in a new location, is look for the latest MSL pressure at a location near you (on the Internet or ask Siri or Alexia or Google). Then tweak your barometer to read this value. For aneroid barometers there is normally an adjustment screw found at the back of the barometer.  For digital barometers this usually means pressing the menu or settings button. Try to do this adjustment at a time when the pressure is not changing much and is neither very high nor very low  (around 1010-1020 hPa)  with no fronts coming, and around 10am local (a flat top).

Your barometer may slowly drift out of adjustment, especially after a bumpy voyage on a yacht, so check it every six months or so.

 

THE TROPICS LAST WEEK

Latest cyclone activity and TCFP tropical Cyclone Formation Potential as seen at www.ssd.noaa.gov/PS/TROP/TCFP/index.html or tropic.ssec.wisc.edu

Much quieter now that the recent MJO of extra convection has moved off from the central Pacific. Last week PENNY and MONA faded. There is still a depression near Micronesia,01W ONE, but this is not expected to develop.

GFS model has the idea of a developing tropical low to southwest of Bali by the end of this week, but ECMWF has a weaker feature.

 

WEATHER ZONES

SPCZ=South Pacific Convergence zone.

The SPCZ is active from Solomons to Tuvalu to Tokelau/Samoa then southeast to Southern Cooks. There may be a tropical low over Fiji by Thursday UTC moving across Tonga on Friday UTC and south of Niue on Saturday UTC.

 

Subtropical ridge (STR)

HIGH that has been east of NZ is expected to travel eastwards along 36 to 38S.

HIGH over Tasmania today is expected to travel slowly NE across the Tasman Sea this week.

 

Tasman Sea / New Zealand

The main event of this week is the front that is expected to travel across the North Island on Monday, turning into a cut off /slow-moving low to east of the North Island for the remainder of the week. This system has cold air above it, so that may bring thunder and hail to the North Island on Monday, a change from the settled weather since Christmas.

After this trough of Monday the wind over the eastern Tasman Sea and NZ for the remainder of the week ahead is likely to be southerly and showery. May be a week to stay put in NZ.

 

Australia/ New Caledonia:

With High pressures in the Tasman Sea and low pressures in the Coral Sea this week, the winds favors one-way traffic form Noumea to (southern) Queensland, but winds over 20 knots and swells over 2m at times.

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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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