Petersburg is on the middle taiga lowlands along the shores of the Neva Bay of the Gulf of Finland, and islands of the river delta. The largest are Vasilyevsky Island (besides the artificial island between Obvodny canal and Fontanka, and Kotlin in the Neva Bay), Petrogradsky, Dekabristov and Krestovsky. The latter together with Yelagin and Kamenny Island are covered mostly by parks. The Karelian Isthmus, North of the city, is a popular resort area. In the south Saint Petersburg crosses the Baltic-Ladoga Klint and meets the Izhora Plateau. The elevation of Saint Petersburg ranges from the sea level to its highest point of at the Orekhovaya Hill in the Duderhof Heights in the south. Part of the city's territory west of Liteyny Prospekt is no higher than above sea level, and has suffered from numerous floods. Floods in Saint Petersburg are triggered by a long wave in the Baltic Sea, caused by meteorological conditions, winds and shallowness of the Neva Bay. The five most disastrous floods occurred in 1824 ( above sea level, during which over 300 buildings were destroyed), 1924 (), 1777 (), 1955 (), and 1975 (). To prevent floods, the Saint Petersburg Dam has been constructed. Since the 18th century the city's terrain has been raised artificially, at some places by more than , making mergers of several islands, and changing the hydrology of the city. Besides the Neva and its tributaries, other important rivers of the federal subject of Saint Petersburg are Sestra, Okhta and Izhora. The largest lake is Sestroretsky Razliv in the north, followed by Lakhtinsky Razliv, Suzdal Lakes and other smaller lakes. Due to its northerly location at ca. 60° N latitude the day length in Petersburg varies across seasons, ranging from 5 hours 53 minutes to 18 hours 50 minutes. A period from mid-May to mid-July when twilight may last all night is called "the white nights". Saint Petersburg is about from the border with Finland, connected to it via the M10 highway. Under the Köppen climate classification, Saint Petersburg is classified as "Dfb", a humid continental climate. The distinct moderating influence of Baltic Sea cyclones results in warm, humid and short summers and long, moderately cold wet winters. The climate of Saint Petersburg is close to that of Helsinki, although colder in winter and warmer in summer because of its more eastern location.