A high-pressure center, or high (H), often contains an airmass of well-defined characteristics, such as cold temperatures and low humidity. When different airmasses finally move and interact, their mutual border is called a front, named by analogy to the battle fronts of World War I.
Fronts are usually associated with low-pressure centers, or lows (L, covered in the next chapter). Two fronts per low are most common, although zero to four are also observed. In the Northern Hemisphere, these fronts often rotate counterclockwise around the low center like the spokes of a wheel (Fig. 12.1), while the low moves and evolves. Fronts are often the foci of clouds, low pressure, and precipitation.
In this chapter you will learn the characteristics of anticyclones (highs). You will see how anticyclones are favored locations for airmass formation. Covered next are fronts in the bottom, middle, and top of the troposphere. Factors that cause fronts to form and strengthen are presented. This chapter ends with a special type of front called a dry line.