INTRODUCTION

The Earth's atmosphere is the gaseous envelope surrounding the planet. Like other planetary atmospheres, it figures centrally in transfers of energy between the sun, the Earth, and deep space. It also figures in transfers of energy from one region of the globe to another. By maintaining thermal equilibrium, such transfers determine the Earth's climate. However, among neighboring planets, the Earth's atmosphere is unique because it is related closely to ocean and surface processes that, together with the atmosphere, form the basis for life.

Because it is a fluid system, the atmosphere is capable of supporting a wide spectrum of motions. These range from turbulent eddies of a few meters to circulations with dimensions of the Earth itself. By rearranging mass, air motion influences other atmospheric components such as water vapor, ozone, and cloud, which figure prominently in radiative and chemical processes. Such influence makes the atmospheric circulation a key ingredient of the global energy budget.

Descriptions of atmospheric behavior

The mobility of a fluid system makes its description complex. Atmospheric motion redistributes mass and constituents into a variety of complex configurations. Like any fluid system, the atmosphere is governed by the laws of continuum mechanics. They can be derived from the laws of mechanics and thermodynamics that govern a discrete fluid body by generalizing those laws to a continuum of such systems. In the atmosphere, the discrete system to which these laws apply is an infinitesimal fluid element, or air parcel, which is defined by a fixed collection of matter.