Picture an albatross that wants to fly to the west, but the wind is
blowing from south to north. The albatross is five
meters in the air (a bit taller than your ceiling probably is). It points itself to the north and glides quickly downwind.
It gains speed rapidly because it has the wind pushing it and because it
is gliding gradually from a high position to a low position. After a long downhill
glide with the wind pushing it, the albatross turns west, which is the
direction it wants to go. It is moving very fast, just as you would be on a bike
after a long downhill with the wind at your back. Conveniently for the albatross,
the wind moving from south to north is slowed so much by friction with the ocean that the
bird's flight is not affected much by wind (like we said earlier). Compared with the
bird's speed, the wind is not blowing much at the surface. So, the bird can fly a
long distance, maybe a hundred meters (the length of a football field) on the energy it
got from the long downhill glide. It has not been flapping its wings during this
time. Finally it slows down enough that it needs more energy to keep going. It
could flap its wings to increase its speed, OR...It could change the angle of its wings so that it rises up
above the ocean surface where the wind is blowing more strongly, and grab itself some more
wind energy by flying downwind again. During the downwind glide they pick up enough
energy to fly some to the west and also to rise up into the wind again at
the end of their progress to the west.
This alternation
of downwind gliding and cross-wind gliding is the usual way that albatrosses get around,
and all without flapping their wings.
Now, really what they should do if they want to fly west in the situation
above is to glide downwind to the northwest and upwind (just above the
ocean surface) to the southwest. This would have them moving mostly
west even though the wind is moving to the north.
This technique is called dynamic
soaring. It's kind of hard to understand the first time through.
Click the link below to see the movie that has been downloading and remember what
you've just read. Pay attention to the arrows in the animation; they show wind
direction and speed. You'll see that wind arrows close
to the ocean surface don't move as fast as the arrows higher above the surface.
That's a key point. Review a few paragraphs up if you don't remember why.
Here is the video:
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