The planetary boundary layer (PBL) is the lowest layer of the troposphere, extending from Earth's surface to a variable altitude of 0.5 to 2 km. It is the atmospheric region directly influenced by friction, heating, and cooling from the surface, and responds to surface forcings on timescales of one hour or less.
Diurnal cycle
The PBL has a marked diurnal cycle: during the day, solar heating of the surface generates convection that vigorously mixes the layer, raising its top to 1-2 km (or more in warm, dry climates). At night, radiative cooling generates a stable, thin layer (100-300 m, called the nocturnal boundary layer) where pollutants become trapped, separated from the residual layer above.
PBL height is crucial for air quality: a shallow PBL (typical of winter anticyclonic situations with thermal inversion) traps pollutants near the surface, causing pollution episodes in cities like Madrid, Barcelona, or Beijing. A deep, well-mixed PBL disperses pollutants effectively.
In meteorology, the PBL largely determines the weather "we feel": surface temperature, humidity, wind, and fog are controlled by processes within this layer. Ceilometers and lidars are the key instruments for measuring its height. See also: thermal inversion, vorticity.