 While newer homes have been built recently, much of the housing dates to the 18th, 19th and early 20th centuries, which has created problems such as urban decay and vacant lots.Some neighborhoods, including Northern Liberties and Society Hill, have been rehabilitated through gentrification.According to the Köppen climate classification, Philadelphia falls under the northern periphery of the humid subtropical climate zone (Köppen "Cfa"), whereas according to the Trewartha climate classification, the city has a temperate maritime climate ("Do") limited to the north by the continental climate ("Dc").