What are those twirelly things on top of commercial trucks?

[J. Peterman]

I have seen these devices on commercial and city trucks for a while. Theu are the black windmill like theings that sit on top of trucks to pick up the wind and they twirel around. ( black, sits flat on horizontaly on top of truck, about 5" in diameter ) I can only guess that it is a device to see if the vehicle is moving or not.
Does anyone know what these devices are?

 

[mrten]

I have seen these devices on commercial and city trucks for a while. Theu are the black windmill like theings that sit on top of trucks to pick up the wind and they twirel around. ( black, sits flat on horizontaly on top of truck, about 5" in diameter ) I can only guess that it is a device to see if the vehicle is moving or not.
Does anyone know what these devices are?

I would assume that they are vents, the same as a turbine on a roof but on a smaller scale.

 

[John_Cage]

i am guessing self-cooling system... some trains actually use them.

It cools itself because it moves at high speed, but the wind itself is not enough on its own. It usually have a liquid cooling system as well.