Comment by wat10000

They're flying in mountain wave. Basically, the wind flows over the mountains and you get ripples downstream. You can ride the upward parts of those ripples. They eventually ran out of mountains.

Winter tends to have more favorable conditions for creating these ripples. It needs more than wind, you also need the right temperature profile in the atmosphere.

Darkness is a big problem. Normally you just have to wait for sunrise to fly, and land before sunset. These guys used night vision goggles to avoid that limitation.