That black pipe next to this bathroom skywindow is the plumbing vent stack or what some people refer to as the "stink pipe". This pipe serves a few purposes, one is to equalize the air pressure in the drainage system so that water can drain properly, which also prevents siphoning of water from the plumbing traps. The water in the plumbing traps prevents sewage gas from entering up through the plumbing fixture drains. The reason some people call this the "stink pipe" is because sewage gas vents out of it. So when this skywindow is open and the wind is blowing in the right (wrong) direction, sewage odors will enter into the bathroom.

Photo taken by Dennis Robitaille of Able Home Inspection, Inc.