system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How do planes know when there's an air turbulence in front to turn on the seatbelt signs?

When there's a turbulence coming, the lights always go on a few seconds in advance. Can the pilot see the turbulence coming or is there a radar or something like that?
gpt
There are multiple ways that pilots can know turbulence is coming. 

Some kinds of turbulence can be seen on the radar on the plane. Sometimes pilots can see conditions that are likely to cause turbulence (like layers in the clouds). Additionally, pilots will report turbulent conditions on the radio, so at times air traffic control may provide pilots notice of upcoming turbulence.