Do a search for "boundary layer" or "laminar flow" on this site, and you'll find plenty of things to read.
They'll talk about "the boundary layer", "flow separation" and so on, as though it were completely obvious what they actually were. Sometimes, they'll even be illustrated with diagrams representing the strata in the flow of air.
I'm no physicist, and this talk and these diagrams make me imagine the fluid equivalent of plywood, or expensive paper handkerchiefs, constructed of discrete layers of material that can be readily identified as such and peeled away from each other.
Is that really how these layers of moving air work? Is the boundary layer over an aircraft's wing like a discrete sheet, with a clear and distinct boundary between the next discrete layer? Or is there a smooth continuum, rather than a distinct step, between the layers and their properties?
Do distinct layers exist more in the conceptual analysis of aerodynamic behaviour than they do in readily identifiable physical entities?
If you were to plot a graph representing some property of the air flow (say its velocity, direction, pressure or turbulence) against distance from the surface of the wing, what sort of shape would the curve have?
In short: how should we imagine these layers?