In short, climate is the collection of all statistical information about the weather. It is a convenient statistical packaging of long periods of weather conditions. But unlike weather, climate is an abstraction that cannot be felt, only contemplated. Nonetheless, just as weather controls, to a large extent, many aspects of our daily lives, climate influences our decisions in a more general, longer-term sense: what clothes we buy and wear, what crops we grow, and the specifics of how homes are built, for example.

In this chapter, we explore in depth the weather and climate of Pennsylvania. Why the climatology of Pennsylvania? Admittedly, one reason is the authors' affiliation with Penn State University, and their familiarity with the weather and climate of the state. But there are sound meteorological and climatological bases for the choice as well. The effects of latitude, topography, and proximity to large bodies of water are firmly etched into Pennsylvania's temperature and precipitation patterns. Parts of the state experience a rain shadow, while other areas receive substantial lake-effect snow. Thus, Pennsylvania provides a representative example of a mid-latitude setting in which these mesoscale processes are superimposed upon the larger-scale effects of latitude, topography, and proximity to large water bodies.

In compiling this climatology, we used data from weather observing sites in the surface observing network as well as many cooperative observing stations. However, keep in mind that our analyses cannot be expected to capture all of the considerable local variation that undoubtedly exists on small spatial scales, especially when elevation varies quickly over short horizontal distances. Unless otherwise indicated, the average temperature and liquid precipitation information presented in this chapter is derived from the period 1961-1990, while average snowfall information is derived from the period 1951-1980. Because terrain variations are crucial to understanding climate variations in Pennsylvania, we provide a color plate of the topography in Color Plate 61. Since the color-coding on this topographic map is expressed in feet and observations of precipitation in the United States are still made in inches, we will adopt English units of inches, feet and miles as our primary units of length and distance in this chapter.

Pennsylvania's most notable land form is the Appalachian Mountains, consisting primarily of the Allegheny Mountains but also the northern extent of the Blue Ridge Mountains (see Color Plate 61). Elevations in the state range from sea level along the Delaware River at Philadelphia (this river forms the entire eastern border of the state) to 3213 feet (980 m) at Mt. Davis in the southwestern part of the state. Pennsylvania's peaks are not as lofty as those in other states that share the Appalachians, nor do ocean waters lap any of its borders. Nonetheless, within its nearly rectangular area, Pennsylvania blends the effects of latitude, topography, and proximity to large bodies of water to produce a diverse collection of climates: for example, winter lovers can revel in places with mean annual snowfalls of more than 120 inches (305 cm), while other parts of the state average more than 25 90^oF-days each summer. A map of Pennsylvania, with county boundaries and principal weather observing sites, is provided in Figure 15.1a. The three-character codes that meteorologists use to identify weather observing stations are indicated (note that in most cases, these codes bear some resemblance to the name of the location that they represent). Figure 15.1b shows the principal rivers of Pennsylvania. As we refer to various locations and topographic features in Pennsylvania, both Color Plate 61 (the topographic map) and Figure 15.1 will be very useful as references.