Tunisia's climate is hot-summer Mediterranean climate (Köppen climate classification Csa) in the north, where winters are mild with moderate rainfall and summers are hot and dry. Temperatures in July and August can exceed 40 °C (104 °F) when the tropical continental air mass of the desert reaches the whole Tunisia. Winters are mild with temperatures rarely exceeding 20 °C (68 °F) (exception is the south-west of the country). The south of the country is desert. The terrain in the north is mountainous, which, moving south, gives way to a hot, dry central plain. As we go to the south, the climate naturally becomes hotter, drier and sunnier. The southern part has therefore a hot desert climate (Köppen climate classification BWh) with extremely hot summers, warm winters and very low annual rainfall amount. Daytime temperatures consistently turn around 45 °C (113 °F) during summers. However, the warmth of winters is only during daytime because nights can be cold in the desert. A series of salt lakes, known as chottzz or shatts, lie in an east-west line at the northern edge of the Sahara Desert, extending from the Gulf of Gabes into Algeria. The lowest point is Chott el Djerid, at , and the highest is Jebel ech Chambi, at . Annual average rainfall amount is lower than 500 mm (19.68 in) nearly everywhere in Tunisia. Tunisia is therefore a dry, semi-arid country. Areas with a pre-Saharan climate receive below 250 mm (9.84 in) and areas with a typical Saharan climate receive below 100 mm (3.94 in) of annual average precipitation. The southernmost part receives rainfall as low as 50 mm (1,97 in) in areas around El Borma, along the Algerian border.