Nearly all of Pennsylvania lies within the Appalachian region of the United States. The only non-Appalachian areas are narrow lowlands in the extreme southeast and northwest. In general the Appalachian region divides into three parts, each trending northeast-southwest. They are the Piedmont Plateau, the Ridge and Valley region, and the Allegheny Plateau.
The Piedmont Plateau occupies most of southeastern Pennsylvania. It consists mainly of rolling to moderately hilly terrain, averaging 30 to 50 miles (48 to 80 km) in width. It begins along the fall line, a belt of sloping land extending northeastward from the Delaware border, past Philadelphia, to New Jersey.
The Ridge and Valley region curves in a great are north and west of the Piedmont. The region consists of nearly parallel mountain ridges separated by deep valleys. Included among the ridges are Blue, Jacks, Tuscarora, and South mountains, the latter being the most northerly extension of the Blue Ridge. Chief among the valleys is the Great Valley. It is called the Cumberland Valley southwest of Harrisburg, the Lebanon Valley between Harrisburg and Reading, and the Lehigh Valley between Reading and the Delaware River. Near Stroudsburg, the Delaware River has carved the Delaware Water Gap through Kittatinny Mountain.
The Allegheny Plateau, the state's largest region, covers nearly all the north and the west. Along the plateau's steep eastern edge, called the Allegheny Front, run the Allegheny Mountains. They reach their highest point near the Maryland border at Mount Davis, 3,213 feet (979 m) above sea level, the highest elevation in the state. The rest of the region, including the Pocono Mountains in the northeast, is a rolling to rough tableland deeply cut by rivers. Chief among the deep cuts is Pine Creek Gorge, sometimes called the Grand Canyon of Pennsylvania, near Wellsboro in the north-central part of the state.
The Lowlands. The lowland area in extreme southeastern Pennsylvania is part of the Atlantic Coastal Plain and consists of a narrow strip along the lower Delaware River. The northwestern lowland, which borders Lake Erie, is part of the Central Lowlands region of the United States.Pennsylvania's state tree is the hemlock.
Nearly all of Pennsylvania is drained by three river systems: the Ohio, the Susquehanna, and the Delaware. The Ohio River and the rivers that form it, the Allegheny and the Monongahela, drain the west. Flowing through central Pennsylvania is the Susquehanna with its chief tributaries, the West Branch Susquehanna and the Juniata. In the east is the Delaware, the lower part of which is a broad estuary leading to the Atlantic Ocean. Among the Delaware's main tributaries are the Lehigh and Schuylkill rivers.
Except for small glacial lakes in the Pocono Mountains, there are few natural lakes. The largest bodies of water are reservoirs, among which are Allegheny and Pymatuning reservoirs in the northwest, Raytown Lake in the south, and Lake Wallenpaupack in the northeast.Pennsylvania's state flower is the mountain laurel.
Pennsylvania has a continental climate that varies mainly with topography. Since the winds are predominantly from the west, the Atlantic Ocean has only slight influence on the climate.
Summers are generally warm to hot. The average July temperature in Philadelphia, for example, is 77° F. (25° C.) while the average at Pittsburgh is 72° F. (22° C.). Nearly everywhere daytime temperatures occasionally rise above 90° F. (32° C.).
Winters normally are cold. Average temperatures for January, the coldest month, are 32° F. (0° C.) at Philadelphia and 28° F. (-2° C.) at Pittsburgh.
Precipitation is fairly evenly distributed throughout the state and throughout the year. Annual amounts generally range between 35 and 45 inches (890 and 1,140 mm). Snowfall is heavy in the mountains and along the Lake Erie shore—usually exceeding 54 inches (1,372 mm) a year. The southeastern counties receive only about 20 inches (510 mm).