The lowest part of the atmosphere; as such, it is shaped through contact with the planetary surface. J. C. Kaimal and J. J. Finnigan (1994) define it as the ‘lowest 1–2 km of the atmosphere, the region most directly influenced by the exchange of momentum, heat, and water vapour at the Earth’s surface’. However, R. B. Stull (1988) notes that this layer ‘responds to surface forcings with a timescale of about an hour or less’. Acta Geographica 5, 2012 is devoted to the atmospheric boundary layer.