> Where's the gradient? Do they fly at the boundary between different wind speeds? At the surface of the water, windspeed is lower than at higher altitudes, so

> Where's the gradient? Do they fly at the boundary between different wind speeds? At the surface of the water, windspeed is lower than at higher altitudes, so that's where the gradient is. If you watch an albatross soaring over water, you'll watch it dive so low its wingtip touches the water surface, and then it'll climb in altitude, before turning to dive again. Here's a good diagram of the whole process: https://www.researchgate.net/profile/Sameh-Eisa/publication/325217254/figure/fig1/AS:627553081503744@1526631722598/Dynamic-soaring-cycle.png https://www.researchgate.net/profile/Sameh-Eisa/publication/...