Turkmenistan has a cold desert climate that is severely continental. Summers are long (from May through September), hot, and dry, while winters generally are mild and dry, although occasionally cold and damp in the north. Most precipitation falls between January and May; precipitation is slight throughout the country, with annual averages ranging from 300 millimeters (11.8 in) in the Kopet Dag to 80 millimeters (3.15 in) in the northwest. The capital, Ashgabat, close to the Iranian border in south-central Turkmenistan, averages 225 millimeters (8.9 in) of rainfall annually. Average annual temperatures range from 17.1 °C (62.8 °F) in Ashgabat to 12.8 °C (55.0 °F) in Daşoguz, on the Uzbek border in north-central Turkmenistan. The almost constant winds are northerly, northeasterly, or westerly.
Almost 80% of the territory of Turkmenistan lacks a constant source of surface water flow. Its main rivers are located only in the southern and eastern peripheries; a few smaller rivers on the northern slopes of the Kopetdag are diverted entirely to irrigation. The most important river is the Amu Darya, which has a total length of 2,540 km from its farthest tributary, making it the longest river in Central Asia. The Amu Darya flows across northeastern Turkmenistan, thence eastward to form the southern borders of Uzbekistan and Tajikistan. Damming and irrigation uses of the Amu Darya have had severe environmental effects on the Aral Sea, into which the river flows (see Environmental Issues). The river’s average annual flow is 1,940 cubic meters per second. Other major rivers are the Tejen (1,124 km); the Murgab (852 km); and the Atrek (660 km).