A Föhn
bank is formed by a Föhn wind. This is a warm
contour-hugging wind that is blowing across Coronation
Island in the South Orkneys group in this case.
As the warm (relatively to the ice and rock) wind
blows across the land, it causes snow and ice to
sublime. That is to turn directly from a
solid to a gas without passing through a liquid
phase, so causing the cloud layer that can be seen
- the same thing may happen when you open the door
to your freezer and "smoke" comes out.
The overall effect as seen from a distance is that
the land is covered by a very large duvet. The gross
contours can be seen through the cloud layer, but
all of the finer detail is obscured.
Received from
an "Antarctic met man":
The Föhn effect is dominated
by "blocking". Wind, approaching a ridge,
will either go up and over the ridge (normal) or
come to a stop and then flow round the sides. South
Georgia often experiences this
latter.
Which of the two
(up and over or round the sides) depends on the
temperature gradient (stability), and wind speed,
(Web search
Froude number for more technical description).
If it's stable, the air at the ground is cold
and "heavy" and wont flow up and over
the top, it goes round the side. BUT, the air at
the top of the ridge and just above the ridge, flows
over and then down. Air aloft is already warm (because
its stable, cold at the bottom), it then descends
and gets even warmer and dryer ...a Föhn. South
Georgia has these often. If the air is just between
the flow round the sides (Froude > 1) of up and
over (Froude < 1 ) you get waves, which make
the lenticular clouds.
Paul Ward - Pentax
equipment, 200mm lens, 35mm film, K64.
This
picture may not be copied or used in any manner
without prior written permission.
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