concept

Planetary boundary layer

The lowest 1–2 km of the atmosphere, where surface friction and turbulent mixing dominate everything — quite unlike the free atmosphere above, where the wind has all but forgotten the ground exists. Right at the surface, wind speed grows as the logarithm of height, because terrain roughness and canopy drag hold the flow back; knowing the surface roughness is therefore enough to reconstruct the whole profile. At night the ground cools fast, convection shuts down and the layer collapses to a few hundred metres, letting the newly released air above slide free as a nocturnal low-level jet that can run twice as fast as the daytime wind at the same height.