Sunrise, sunset, sunrise. The daily cycle of radiative heating and cooling of the Earth’s surface during clear skies causes a daily cycle of sensible and latent heat fluxes between the land and the air. These fluxes modify the bottom of the troposphere — a layer called the atmospheric boundary layer (ABL) because it is influenced by the bottom boundary of the atmosphere (Fig. 18.1).
The ABL experiences a diurnal (daily) cycle of temperature, humidity, wind, and pollution variations in response to the varying surface fluxes. Turbulence is ubiquitous in the ABL, and is one of the causes of the unique nature of the ABL.
Because the boundary layer is where we live, where our crops are grown, and where we conduct our commerce, we have become familiar with its daily cycle. We perhaps forget that this cycle is not experienced by the rest of the atmosphere above the ABL. This chapter examines the formation and unique characteristics of the ABL.