You need a damper for your duct. If you can see straight through to the sky, you also need a roof cap so that rain, snow, vermin, etc. do not come down your duct.
Go to www.google.com and search for a "roof cap with damper." Many roof caps have built-in dampers. You need to get a roof cap that fits the size of the duct in your vent system, and you need to install the roof cap so that it makes a watertight seal with your roof.
If you have a duct pipe that sticks up above your roof sufficiently, then you may be able to find a roof cap with damper that sits on top of the duct -- this would be the best solution because you will not have to touch your roof.