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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www.birding-aus.org
birding-aus.blogspot.com
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