Physics and Dynamics of the Atmosphere

This chapter describes the basic physics and thermodynamics of the atmosphere, starting from the ideal gas law and the hydrostatic equation, from which the lapse rate in the troposphere is derived. The effect of atmospheric moisture on the lapse rate is identified and the Clausius–Clapeyron equation giving the saturated humidity is derived. The effect of moisture on adiabatic vertical transport is shown. Then, the three-dimensional equations of motion are derived in vector form. From these, geostrophic balance and the thermal wind equations are derived. This, with the Coriolis force, gives the physical description of the atmospheric circulation. The driving force behind circulation is identified as the energy difference between the tropics and the extratropics. This is driven by radiation differences, including, at large geological scale, the Milankovitch cycles. Finally, circulation as a three-cell system per hemisphere, and the development of weather systems such as cyclones, are described.