The flow of wind past surface obstacles, and daytime heating of the ground, both induce a random stirring we call turbulence. It is important in transferring sensible and latent heat between the Earth's surface and the free atmosphere, and in the dispersion of air pollution (Chapter 14).
Turbulence depends on both the wind speed and the atmosphere's lapse rate, so it is categorised as in Table 7.1. Note that the first three lapse rates mentioned (A, B, C) are all superadiabatic. The last column shows how instability means a great variability of wind direction, and the same applies to wind speed, creating gustiness (Chapter 14). The reason is that parcels of the stronger winds at the top of the planetary boundary layer are stirred to the surface by strong instability. The circumstances in which each category of Table 7.1 occurs are shown in Table 7.2. As an example, the ELR near the surface is close to neutral on windy days because of the constant vertical stirring, whereas the lapse rate is superadiabatic during a calm day, changing to an inversion at night.
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