Condensation often forms at the meeting rail and at the bottom of the lower sash on the interior of the glass. This is because when warm air cools, it falls down across the interior surface of the window at the same time the air’s temperature is falling. The air contacts the horizontal surface of the meeting rail, which acts like a dam, slowing the air’s rate of fall and creating the perfect opportunity for the trapped water vapor to escape and form on the meeting rail’s surface. The air then rolls over the edge of the meeting rail and again gains speed until it encounters the lower handle of the sash. At this point, the water vapor again makes its exit and lies at the bottom of the sash.