I put my fingers toward the window jamb and feel a slight draft of cold air coming from the silver-colored metal rail that the lower sash slides on (See picture).
By holding a piece of cardstock at the corner between the upper sash and that rail and putting my hand on either side of it, I've determined the cold air is coming from the silver rail area, as opposed to from between the upper sash and the jamb. The window sashes have some fuzzy weatherstripping between the sash and jamb.
So it feels like air is infiltrating through the side of the window. Why would this happen, and is there anything that could be done? I have several windows, and they are all similar.
Note, in learning about window anatomy, I've observed that "rail" refers to the top of a window sash. But here, by rail, I mean the silver metal pictured upon which the window sash slides. I don't know what it's called. Maybe channel is the correct term?