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Flight in Birds
In the Gliding
page, we saw how a bird can move through the air without flapping
its wings. Unfortunately, gliding flight always results in a bird
moving downward through the air. Soaring flight is a special kind
of glide, in which the bird flies in a rising air current. Because
the air is rising, the bird can maintain its height relative to
soaring flight, the bird moves downward through a mass of
It can maintain its height, relative to the ground.
can only occur at special places and times. For example, warm air
heated by the sun can rise up from the hot ground and into the sky.
This rising air current is called a thermal. Thermals often rise
up along the slope of a hill, but they can also form over flat ground.
As the air rises, it also expands and cools. (This results from
the lower pressure at altitude.) Eventually, the water vapor in
the thermal may become cold enough to condense, forming the tiny
droplets of liquid water that make up a cloud. When you have a sunny
day with puffy clouds, it's probably a good day for thermals, and
you may see hawks or vultures soaring overhead.
can also find rising air in places where the wind is forced to flow
up the side of a hill. Long ridges produce the best lift. Smaller,
isolated hills don't produce as much lift because the air can flow
around the sides of the hill instead of going over the top. The
amount of lift and where it is found will depend on the speed and
direction of the wind, as well as the shape of the land. Smaller
objects such as trees or houses produce ridge lift too, though it
may not be enough to keep a bird in the air.
is one more soaring technique, called dynamic soaring. Dynamic soaring
does not rely on rising air currents. Instead, it uses the difference
in wind speed between the ground and higher up. Here's how it works:
- First, the
bird must climb up into the faster airflow, ten or twenty meters
above the ground. It climbs facing the wind, so it receives the
benefit of a constantly increasing air speed as it goes up higher.
It may appear that the bird is slowing down, but its speed relative
to the surrounding air is actually increasing.
- Then the
bird makes a turn and heads back downwind. From the ground, it
seems that the bird has suddenly gained a tremendous amount of
speed, because now the bird is flying along with the wind.
- Then the
bird dives down into the lower, slower air. Again its air speed
increases, this time assisted by gravity.
- The bird
turns back upwind and repeats the cycle.
works best in places where there are no upwind obstructions that
would block the wind and cause turbulence. This means flat, open
ground, over the ocean, or at the top of a ridge. The albatross
uses this type of soaring to support its multi-year voyages at sea.
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