The use of the word "front" to describe the boundaries that separate air of different characteristics has historical roots. During World War I, the boundary separating the opposing armies was commonly referred to as the "front." Shortly after the war, a group of Norwegian meteorologists, visualizing the atmosphere as a battleground of distinct volumes of air, adopted "front" to describe the boundaries between huge masses of air of different temperature and moisture characteristics. They recognized that storms, or low pressure systems, frequently developed and matured along these confrontation zones, and thus these fronts were often the site of dramatic and rapidly changing weather. Indeed, numerous large-scale pushing and shoving matches between huge volumes of air are occurring each day in the atmosphere. A significant challenge to weather forecasters is to determine which volumes gain territory and which lose ground in this never- ending game of atmospheric advance and retreat.

When two fluids of markedly different properties (such as oil and water) are placed together in a container, they don't mix very much. A distinct boundary usually forms between them. To understand why the boundary forms, it is necessary to understand the differences between the properties of the two liquids. Similarly, fronts in the atmosphere are boundaries between huge volumes of air of different characteristics. Thus, to understand the structure of atmospheric fronts, we begin by exploring the characteristics of the huge masses of air that they separate.