114 | Chapter 6
now gather speed as it glides downwards, perhaps roughly at right an-
gles to the wind direction and therefore parallel to the waves, before
turning into the wind to gain more height and to repeat the manoeuvre.
The net track is, broadly speaking, across the wind.
The second technique is dynamic soaring, manifest in the albatross
ascending into wind, turning at the peak of the climb, descending and
accelerating downwind before beginning another climb. Such repeti-
tive dynamic soaring exploits the fact that drag slows the passage of air
close to the sea. Consequently the speed of the wind is least immediately
above the water, increases with height for about 5– 10 m, and then nearly
levels off. Thanks to this wind gradient, lift is generated when the bird
turns into wind to make the climb since air flows over the upper surface
of the wing faster than over the lower surface, and consequently pressure
The dynamic soaring cycle of an albatross consists of (1) a windward climb
into wind, (2) a curve from windward to leeward at the highest altitude,
(3) a leeward downwind descent and (4) a curve from leeward to
windward at low altitude close to the sea surface. During a
single cycle the bird covers a few hundred metres.1012(^34)
0
–500
–400
–300
y (m)
Altitude (m)
x (m)
–200
–100
0
0 100
Wind
200