Boundary layer height
Wind atmospheric boundary-layer height BLH PBL height
Boundary-layer height describes how far the ground strongly influences the air above it through heating, cooling and friction.
The air closest to the surface
The ground exchanges heat and moisture with the air. Trees and buildings also slow the wind. The layer that responds to these surface effects within hours is called the atmospheric boundary layer. Its height is usually given in metres above the ground.
Why its height changes
- Daytime: Warm ground makes air rise. Eddies can mix a layer several hundred metres to more than a kilometre deep.
- Night-time: The ground cools. With light winds, a shallow, stable layer may develop.
- Strong winds: Friction and eddies can mix the air even without sunshine.
What it means for pollution
If exhaust gases remain within a shallow layer, they have less air to mix with. For the same emissions, their concentration can rise. A deeper mixing layer provides greater dilution. Transport by wind and new emissions also affect the result.
There is no visible ceiling
The boundary is not a solid lid. Weather models estimate it using different methods. A height of 800 metres does not mean that clouds start there, or that air quality is identical everywhere below that level.