Cross ventilation is easiest to understand if you picture how the air actually moves through a room or tunnel. In a well-designed cross-ventilated space, you can imagine—or sometimes see—air entering from one side, flowing across the occupied zone and leaving on the opposite side. The pattern is a distinct stream of air from inlet to outlet, rather than a swirling or random motion.
In a simple room, cross ventilation might look like this: a window or louver on one wall is open to the outside, and a second window or vent on the opposite wall is also open. On a breezy day, curtains on the windward side move inward as air enters. Inside the room, light objects such as papers or smoke from a stick of incense drift steadily towards the opposite wall, where the air exits. People sitting between the two walls feel a gentle flow of air across their bodies.
In a large industrial hall, cross ventilation appears as air moving from one long side of the building to the other. Fresh air enters through a series of intake louvers or doors, travels across the workshop and leaves through opposite wall louvers or high-level roof vents. If you could see the air, it would form broad bands or layers moving across machines and workstations. Dust or welding fume released in the space tends to move along the same general direction, towards the exhaust side.
In underground mines or tunnels, cross ventilation can be seen where air flows from an intake drift or shaft across a crosscut and into a return drift. The air enters through the intake opening, passes through the cross section of the workings and then exits through the return opening. Flags, smoke tubes or anemometer readings show a definite direction from intake to return. Auxiliary fans and regulators may be used to strengthen and control this cross flow.
Cross ventilation also has a visual layout aspect. Floor plans of cross-ventilated buildings show openings on opposite sides, often arranged to align with prevailing wind directions. Sections of such buildings show lower-level inlets and higher-level outlets to make use of thermal buoyancy. In diagrams, arrows are drawn straight or slightly curved across rooms, clearly connecting intake and exhaust vents.
What cross ventilation does not look like is a single open window with no outlet, or an exhaust fan blowing air out of a room with no planned makeup air. In those cases, air may enter and leave through the same opening or through random leaks, and the flow pattern near occupants can be weak or unpredictable. True cross ventilation is recognisable because there is a clear, continuous path for air to travel from a clean intake to an exhaust, providing effective cooling and contaminant removal along the way.
