Range hoods vent smoke and grease through duct work extending outside the roof with a vent cap covering the end to prevent rain from entering the duct and draining onto the range. The roof vent cap should also have a damper to block return air when not in use.
Depending on the path available for duct routing, sometimes the range hood is vented through an external wall. In this case, the vent pipe still must have a cap to prevent moisture and debris from blowing into the open duct. Recirculating range hoods or duct work that terminates inside of a building do not clear the kitchen of smoke and grease.