Editing Boundary layer (section)

{{Short description|Layer of fluid in the immediate vicinity of a bounding surface}}
{{for-multi| fluid flow along walls|fluid dynamics|the concept in asymptotic analysis|Boundary layer (mathematics)}}
{{More citations needed|date=March 2020}}
[[File:Thermal-plume-from-human-hand.jpg|thumb|upright=1.3|The boundary layer around a human hand, [[schlieren photograph]]. The boundary layer is the bright-green border, most visible on the back of the hand (click for high-res image).]]

In [[physics]] and [[fluid mechanics]], a '''boundary layer''' is the thin layer of [[fluid]] in the immediate vicinity of a [[Boundary (thermodynamic)|bounding surface]] formed by the fluid flowing along the surface.  The fluid's interaction with the wall induces a [[No-slip condition|no-slip]] boundary condition (zero velocity at the wall).  The flow velocity then monotonically increases above the surface until it returns to the bulk flow velocity.  The thin layer consisting of fluid whose velocity has not yet returned to the bulk flow velocity is called the velocity boundary layer.

The air next to a human is heated, resulting in gravity-induced convective airflow, which results in both a velocity and thermal boundary layer. A breeze disrupts the boundary layer, and hair and clothing protect it, making the human feel cooler or warmer. On an [[aircraft]] [[wing]], the velocity boundary layer is the part of the flow close to the wing, where [[viscosity|viscous]] [[force]]s distort the surrounding non-viscous flow. In the [[Earth's atmosphere]], the [[atmospheric boundary layer]] is the air layer (~ 1&nbsp;km) near the ground. It is affected by the surface; [[Diurnal temperature variation|day-night heat flows]] caused by the sun heating the ground, moisture, or [[momentum]] transfer to or from the surface.