Philadelphia is the biggest city in the Commonwealth of Pennsylvania and it is only associated city-county, the fifth-most-populous city in the United States, and the essential of the sixth-largest metropolitan area in the country. Climatic conditions fall in the northern periphery of the humid subtropical climate zone. Summers are typically hot and muggy, fall and spring are generally mild, and winter is cold. Snowfall is highly variable, with some winters bringing only light snow and others bringing several major snowstorms, with the normal seasonal snowfall standing at 22.4 in (57 cm); snow in November or April is rare, and a sustained snow cover is rare. Highlights of Philadelphia: Philadelphia is home to many national historical sites that relate to the founding of the United States. Independence National Historical Park is the centre of these historical landmarks being one of the country's 22 UNESCO World Heritage Sites. Independence Hall, where the Declaration of Independence was signed, and the Liberty Bell are the city's most famous attractions. Other historic sites include homes for Edgar Allan Poe, Betsy Ross, and Thaddeus Kosciuszko, early government buildings like the First and Second Banks of the United States, Fort Mifflin, and the Gloria Dei (Old Swedes') Church. Philadelphia alone has 67 National Historic Landmarks, the third most of any city in the country.