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A Black-browed Albatross soars over the Southern Ocean.
Photo: Chris Johnson

April 15, 2002
"How Albatrosses Sail in Three Dimensions"
  Real Audio
  28k

This is Roger Payne aboard the Odyssey with a description of how albatrosses manage to fly for hours at a time without flapping their wings-as far as I'm concerned, the most consistently beautiful sight one ever gets at sea.

The supreme sailors of the world are albatrosses, along with their cousins the shearwaters and petrels. These magnificent seabirds sail in three dimensions, the albatrosses gliding for hours over the sea without flapping their wings any time there is a strong wind. This form of sailing in three-dimensions is called dynamic soaring. Humans are too big to have any reasonable hope of ever harnessing this form of energy, for albatrosses appear to be about the largest animals that can be expected ever to achieve it.

When you watch an albatross doing dynamic soaring, what you observe is this: the albatross starts at perhaps 50 feet above the water. It turns and glides downwind, getting up to very high speeds. When it is going its fastest it is practically touching the water with its wingtips. At this point it often rockets into the lee of something-usually a wave, but sometimes a boat, and shortly thereafter turns sharply into the wind and rockets upwards until it is at about 50 feet above the water again. It glides near this altitude for a while before repeating its plunge back down towards the water's surface and subsequent rise back up to about 50 feet. All this time it never flaps its wings.

Just what is it doing?

The trick to understanding how it does it, is to keep two things in mind:

  1. When wind blows across any surface, including water, it drags against that surface. Which means that in a 40 mile per hour wind, the air close to the sea surface is almost at a standstill, while the air 50 feet above that surface is going 40 miles per hour across the water.
  2. The other thing to keep in mind is the difference between air speed and ground speed. Air speed is how fast the air is passing over the bird's body. Ground speed is how fast its body is passing over the ground.
Let us suppose you were the albatross and that when you were gliding just fast enough to stay aloft in still air with no wind blowing you had to fly at at least 30 miles an hour through the air-giving you a 30 miles per hour airspeed. If the wind was still, your ground speed would also be 30 miles an hour. If you made the same 30 mile per hour airspeed glide downwind in a wind of 40 miles an hour you would soon be going 70 miles an hour over the surface of the ground (the water in this case) even though your airspeed would still be only 30 miles per hour, because your body would still be going through the air at only 30 miles an hour even though the wind was adding another 40 miles an hour to your ground speed.

A Black-browed Albatross glides above the waves.
Photo: Chris Johnson

Suppose that in such a case you could command the wind to stop, and it would obey you. "Stop" you would shout to the wind as you were gliding at 70 miles per hour over the sea. And when the wind did so your body would hurtle forward through the air until you were passing through it at 70 miles an hour (not just at 30 miles per hour as before). At that point you could convert that new air speed into altitude by tilting your wings up and climbing as high as possible until your air speed had fallen back to the 30 miles per hour you need in order to stay aloft.

The albatross can't command the wind to stop but it can fly extremely close to the ocean where the wind is all but stopped anyway, owing to its drag on the sea's surface. When the albatross does so its air speed will go up to close to 70 miles per hour. (If it ducks in behind a wave or a boat where the air is even stiller its air speed will be still higher). If the albatross gets very very close to the surface it will find the air much the stillest there. That is why you see albatrosses, fulmars and petrels skimming over ocean waves and missing them by less than an inch. They do this because every inch counts if they are going to get into the stillest air and therefore end up with the highest airspeeds which they can then convert into the greatest altitudes before they have lost air speed back to the 30 mile per hour airspeed they require to stay aloft. Once they start to lose altitude, they immediately start another dive into the still air just above the sea surface, and when they have reached a very high speed they will turn once again into the wind and rise back up to the greatest altitude they can reach.

So why can't people do this? It's because any plane that is big enough to support a human isn't small enough to get close enough to the water's surface to get into that slowest moving air found only at the water's surface.

So that's how dynamic soaring is accomplished, and this is how Roger Payne bids you farewell, while hoping that you get to be one of the lucky few who ever get to watch albatrosses at sea doing their dynamic soaring. For it is they, really who are the ultimate sailors and they use it to get around and not just for funů or do they?

I'll answer that question next time.

(c) 2002 Written by Roger Payne

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