Winter Wings: The Science of Soaring
By Paulina Jenney
Broadcast 10.19 & 10.22.2022
Last weekend, some friends and I woke up to a foot of snow blanketing the pines outside our windows and the coldest October temperatures on record in the Blackfoot Valley. We pulled on layers of fleece and Gore-tex in the early morning and trudged away from our little lodge to a pond not far up the road. I hovered at the edge of the sheet of ice, pondering its thickness, gently set my foot down in the snow, and took a tiny step before sliding out onto the frozen water, turning happy circles in the sunshine.
As we hurled the fine, powdery snow into the air to watch it sparkle, I noticed out of the corner of my eye a black shape slowly circling against the blue sky, getting ever so slightly smaller with each turn.
“It’s thermalling!” Zoey said, squinting. As a raptor ecology specialist, Zoey doesn’t seem to spend a moment outdoors not looking or listening for evidence of avian presence. I’m not a scientist, but it so happens that as a paraglider pilot, this time I knew what she was seeing. Since paragliders (consisting of just a sheet of fabric, some lines, and a harness) can’t flap their wings, or use jet propulsion to take flight, we rely on the same soaring technique to remain airborne.
When air warms unevenly near the surface of the earth, it rises in giant bubbles toward colder layers in the atmosphere. Some birds seek out these rising masses of air, riding them like elevators while looking for prey. So as not to fly straight through the lift, but to remain within it, they’ll angle their wings without flapping, circling the confines of the columnar thermal. When the air inside the thermal gets so high that it cools to the temperature of the surrounding air, they will glide out on a line, searching for the next ride upward. Pilots rejoice to see birds flying this pattern, and eagerly follow “locals” into the areas with the best lift, turning in tandem when they feel their wing begin to rise.
It wasn’t until I took to the skies myself that I learned that different types of birds excel at different types of flying. While some migratory birds have pointed wings that allow them to flap for long periods of time without tiring, and coastal birds might have long, narrow wings that allow them to soar in high winds, some inland birds of prey like eagles, hawks, and vultures have short, broad wings, which are specifically designed for catching thermals. These wings, called “passive soaring wings,” allow a bird to fly slowly and with better maneuverability. However, the round shape of their wings can also create drag, which is undesirable for a bird trying to gain altitude as efficiently as possible. Soaring birds solve this problem by flying with their primary feathers extended out like fingers, creating slots that allow the wing to catch rising air while reducing surface area. During migration, hawks can travel hundreds of miles gliding from one mass of rising air to the next.
We watched the hawk—a red-tailed—until it topped out of the thermal, amazed that it had found rising air in such frigid climes. As we walked back to the lodge, a long sinuous string of white floated across the sky. I blinked, wondering if there was something in my eye. No, still there: hundreds of snow geese on their long migration south. And although I couldn’t see them, I could imagine each set of pointed wings, tirelessly flapping their way to warmer weather.
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