Cruising and Navigation Association of New Zealand

Archive for September, 2014

Equinox

Welcome to the vernal equinox (last Sunday). That was when the overhead sun shifted from the northern hemisphere to the southern hemisphere. It is only on the equinox that the sun rises due east and sets due west.

In the diagram the word DECLINATION is used to describe the latitude where the sun appears to be directly overhead. This varies in a regular way throughout the year and can be taken as a marker of our seasons.

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Astronomers, logically, time the seasons using the solstices and equinoxes of earth’s orbit around the sun. By this reckoning, spring starts with vernal equinox. Note that this makes our winter last 93 days and summer 89 days because the earth’s orbit is slightly elliptical and the earth speeds up a bit during our summer.

For practical purposes climatologists measure climate data averaged over each calendar month, and by this reckoning the months of September to November are taken as spring. Note that by this reckoning summer lasts 90 days (91 days in a leap year). In New Zealand this method of marking the seasons is more popular than using the equinox and solstice. Fair enough, it adds 1 or 2 days to summer Smile

You may have noted that the time between sunrise and sunset is NOT 12 hours on the day of the equinox, but around 7 minutes LONGER. This is because sunrise is defined at the time the top limb of the sun is just visible on the horizon, and similarly sunset is defined with the top limb of the sun disappears below the horizon. Due to refraction of the atmosphere the sun appears and disappears when is slightly below the horizon. This explains the “extra” day light. The name used to describe the date when time between sunrise and sunset reaches twelve hours is the equilux as described at wikipedia.org/wiki/Equinox

Over the next few weeks, as we get to notice the longer days there will be extra warmth reaching the southern ocean. Sun starts to shine reach Antarctica after six dark months. Just as the coldest part of night for us is just after dawn, so it is that Antarctica gets is coldest temperatures of the year around the equinox. It is the temperature difference across the Southern Ocean that energies the westerly winds found there, and so these at their most energetic, and expand to their furthest north, around the equinox. This is the origin of the term “equinoctial gales”, however I prefer the phrase ”gales of the Antarctic dawn”.

From Bob McDavitt at http://metbob.wordpress.com/

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Weather computer resolution

The global computer models take weather observations and transfer them onto a grid of dots, thereby averaging the weather out. Features that are occurring between the reports of the observation network are missed altogether, but some data such as temperature is observed in a continuous fashion by remote sensing from satellites and is covered well. The model has to “balance” this matrix of dots to ensure that certain rules are observed, for example all the computed upward motions need to be balanced by down ward motions so that the total atmosphere remains intact. Once a captured pattern is obtained, it is then pushed into the future using dynamic equations using small time-steps. Then another balancing computation is done and so forth. SO the GRIB data of a weather forecast is just a mathematical idea based on the extrapolation of a captured patter. In the real world chaos is continuously jiggling the weather pattern in to different direction, so that the weather forecast deviates from the real world.

You may already be aware of the limitations of GRIB data. Sometimes someone will offer you BRIB data with a better resolution, and the question arises if it will have better accuracy. The answer is somewhat mixed, as I hope to show you today:

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The graphic above shows the normally accessible GRIB data (GFS) over Fiji on a typical trade wind day showing wind flow to be from E to SE 10 to 15 knots.

Here is a simultaneous output from the 8km resolution Predictwind version of the GFS model (PWG) as available (under registration) from www.predictwind.com

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This model has captured some terrain effects—lowering the air pressure in the interior of the main islands of Fiji in the heat of the day, and there by forming a westerly sea-breeze around Nadi/Lautoka and causing an acceleration of the wind on the SW end of Fiji near Navula Passage. All of these terrain effects truly happen, even if they are not resolved by the normal GRIB data.

These models with higher resolution and closer dots (and smaller time-steps) require heaps more computing than the normal global models and give a pattern that is somewhat closer to the real world. However there is no increase in the basic weather observation resolution that goes into these models, so they still have limitations. Their output is still just an averaged idea.

From Bob McDavitt  http://metbob.wordpress.com/


Just a wee blow!

We were going to drop the mooring and sail to Rakino but opted out for lunch on Music Point.. wasn’t long before we were very grateful for our decision!