Question about rooftop structure?

Question

Can you identify this? I've always wanted to know what it's called or what it does.

I'll try to describe it as best I can:

I always see them on rooftops of industrial buildings/factories when I drive to NJ to Newark airport.

They're small (maybe 10 inch tall) and there is a rotating spiral object.

They kind of look like this:

http://asymptotia.com/wp-images/2008/01/roof_vent_spin_thing.jpg

Answer 1

That's the output of a vent. Depending on size, it could be a bathroom vent, or a kitchen, for a home or a restaurant or business.  The slots are supposed to help the air flow.

Answer 2

that's a roof vent. it spins, creating suction from the interior of the building.

Answer 3

The picture is a gravity ventilator. The are used to allow air to exit a building. They are used in a number of building types, but mostly in warehouses, and sometimes in residential attic ventilation.