Average temperatures over most of the country are below freezing from November through March and are above freezing in April and October. Winter nights can drop to in most years. Summer extremes reach as high as in the southern Gobi region and in Ulaanbaatar. Most of Mongolia is covered by discontinuous permafrost (grading to continuous at high altitudes), which makes construction, road building, and mining difficult. All rivers and freshwater lakes freeze over in the winter, and smaller streams commonly freeze to the bottom. Ulaanbaatar lies at above sea level in the valley of the Tuul River. Located in the relatively well-watered north, it receives an annual average of of precipitation, almost all of which falls in July and in August. Ulaanbaatar has an average annual temperature of and a frost-free period extending on the average from mid-may to late August.