Christmas Bells

Christmas Bells
Christmas Bells - Blandfordia nobilis

Thursday, January 19, 2006

"Wedgies" circling on a gentle easterly updraft

A pair of Wedge-tailed Eagles (Aquila audax) were circling very low today, just across the road from my house. (Go to this link to the Canberra Ornithologist's Group site, for a wonderful image by Stuart Harris, of a "Wedgie" in flight.) My pair were riding a gentle easterly updraft off the ridge along Missingham Parade, and so were circling immediately above my neighbours' houses, at about 100 metres, maybe less - about three times "tree height", which is pretty "low" for a Wedgie to circle, (especially around houses). They were demonstrating masterly control in light wind conditions. Anyway, that "vision" got me thinking about exactly what the weather pattern is which has brought this steady rain to us, in the last few days.

The ever-wonderful, (but oh so difficult to navigate around) Bureau of Meterology website tells me that we have had 26 mm of rain yesterday, 7 mm the day before that, and 25 mm on the previous day (i.e., Wednesday, Tuesday and Monday). I have changed the days, because the Bureau has this quaint habit of reading - as "today's rainfall" the water in the guage at 9:00 a.m. today (The 26 mm I referred as yesterday's rain is actually recorded as Thursday, 19 Jan). In my world, "today" has only just got started at 9:00 a.m. But that is the official standard.

I suggest, if you are interested in weather details for Robertson, go to the following "links" and then save them in your "favourites" or "bookmark" them for future reference.

This hotlink will take you to the permanent webpage for last 7 days rainfall for the South Coast region (although it is called the "daily rainfall bulletin"). Scroll down to "Central Shoalhaven" district, for Robertson's readings. We also appear in "Lower Shoalhaven" too. That is a permanent link, updated daily (after 9:00 a.m., and it also gets updated several times during the day for rainfall "after 9:00 a.m.")

Back to the rainfall. While it was good, soaking, steady rain, it is not the kind which makes Robertson stand out in the record books. But, heck, it was
good rain. Maybe the best kind. Not enough to cause floods, nor much erosion. But it was falling when the soil was warm - making perfect growing conditions for plants (and Fungi) and probably everything else, too.....

The Synoptic Chart for 1000 hrs, EST 19.1.06 from the Bureau's website. The following link will take you to the "latest" synoptic chart (regularly updated).

The Northern Rivers region of NSW is copping a bucketing, with flood warnings, etc.
In "Bureau speak": "A trough over northeastern NSW is directing a deep moist east to southeast onshore airstream" (from NSW Severe Weather Warning IDN28500, issued at 1:10 p.m., 19 Jan 06).

When a "high", (which in the Southern Hemisphere, has an anti-clockwise circulation of air) is sitting off Bass Strait, as seen in the Synoptic chart above, it is able to dump moist air from the Tasman Sea back onto the northern NSW coastal region. If there is a strong pressure difference between that high and a "low" somewhere near it, that increases the force of the wind and the likelihood of storms. There is an excellent "animation" of the circulation of air in, around and between high and low pressure systems on this page of the Bureau's site. Their "glossary" of weather terminology is also very useful.

Currently, in Robertson, we are getting some of the wash of that illustrated weather pattern.

But we can also get a bucketing, if a "low", with its clockwise air movement, is sitting somewhere off the northern NSW coast, directing moist air onto us, from the south coast. This is what happens when the "tail end" of a tropical cyclone comes down from Queensland, or the Coral Sea, and stalls, as a "deep low" (or a "depression", or a "trough"), off the northern NSW coast.

In general, that is why February and March are such high rainfall periods in Robertson.

And heck, the locals will tell you
that, in Robertson, it can also rain because it feels like it!

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