# 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](https://astropilot.space/glossary/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).

**Related:** [Seeing](https://astropilot.space/glossary/seeing.md)

From the AstroPilot astronomy glossary: https://astropilot.space/glossary/jet-stream