Much of Botswana is covered with ancient windblown Kalahari sands and the fossilised remains of a former desert. Landlocked, and roughly central to the southern African sub-continent, Botswana extends through nine degrees of latitude. These factors tend toward considerable variation in climate, but a low average rainfall. A very flat country with hilly areas along the Limpopo valley in the east, Botswana is semi-arid with rainfall figures of between 600 mm in the north to 200 mm in the south-west. Temperatures can be quite extreme - small quantities of water may freeze overnight in winter (June, July) while in October and November day temperatures can reach 40?C or more.