The planetary boundary layer (or atmospheric boundary layer) is the lowest part of the Earth's atmosphere. It is directly influenced by its contact with the Earth's surface. The planetary boundary layer responds to heat transfer, pollutant emission, and other surface forcings in a timescale of an hour or less.
The depth of the planetary boundary layer depends on, for example, the location, season, time of day, and weather. Typically, the planetary boundary layer extends 50 … 3000 m (164 ... 9840 ft) from the Earth's surface. Fog, haze, mist, smog, and air pollution are typical phenomena in the planetary boundary layer.
The planetary boundary layer contains several layer types:
- Convective boundary layer: Layer of air in which particles mix well due to mechanical and thermal forces. The depth of the convective boundary layer is called the mixing height.
- Nocturnal boundary layer: Stable layer of air that forms around sunset. Its top is often marked by a temperature inversion. The layer usually dissolves by convection in the morning hours, but it can also stay during daytime when solar heating is not sufficient to disperse the nocturnal boundary layer.
- Residual layer: Layer of air containing the particles left from the previous convective boundary layer after sunset or from long-range transportation by winds.
- Surface layer: Layer of air that is situated closest to the ground. Its thickness is typically 50 ... 100 m (164 ... 328 ft), about 10 % of the boundary layer height.