I've been away and just noted this thread.
On 24th December 1988, in Artarmon a northern Sydney suburb, I saw a
flock of 150 - 200 Needletails flying in two, neat, concentric
circles, one group of birds above the other. A majority of the birds
were flying anti-clockwise with the balance flying clockwise. They
were quite high up and as they flew the flock drifted slowly to the
north. They were probably 'drifting' against a north-easterly breeze
so possibly not in a thermal, however it was a spectacular and
interesting sight.
Andy Burton
Jenny Spry asked "When seen from below, which way do birds spiral
around a thermal as they rise? Do they spiral clockwise? Do they
spiral anti-clockwise? Is it random? Do thermals rotate or rise
vertically with no rotation?"
Low pressure cells (cyclones) in Australia (well the whole southern
hemisphere actually) circulate clockwise due to the coriolis effect.
I
would have been surprised if a rising column of air, which is (to
some extent) what a low pressure cell is on a large scale, doesn't
behave the same way - and a quick google encountered this link from
a site for glider pilots which confirmed that view (in perfect
conditions), and the best flight was achieved against the rotation.
(Although the article then starts to get a bit complicated with
indications that winds can cause the thermal to spin in an
anti-clockwise direction!) http://www.soartech-aero.com/Thermals.htm
Hence the birds circle against rather than with the air flow, which
is more efficient as they let the air do the work of both forward
flight and lifting them as it rises.
What would be interesting is whether northern hemisphere birds
circle (mostly) clockwise - one would assume that they do.
Cheers
Tom Wilson
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