Jet stream
A river of fast wind about 10 km up; when it's overhead, the seeing is usually poor.
Jet streams blow at 150–300 km/h near the tropopause. The turbulence along their edges is a main cause of bad seeing, so planetary observers watch the jet stream forecast as closely as the clouds.
In AstroPilot
AstroPilot's seeing estimate uses the forecast wind at 250 hPa (the jet stream's height).