Iran also known as Persia, officially the Islamic Republic of Iran is a sovereign state in Western Asia. With over 81 million inhabitants, Iran is the world's 18th-most-populous country. Comprising a land area of 1,648,195 km2(636,372 sq mi), it is the second-largest country in the Middle East and the 17th-largest in the world. Iran is bordered to the northwest by Armenia and the Republic of Azerbaijan,[a] to the north by the Caspian Sea, to the northeast by Turkmenistan, to the east by Afghanistan and Pakistan, to the south by the Persian Gulf and the Gulf of Oman, and to the west by Turkey and Iraq. The country's central location in Eurasia and Western Asia, and its proximity to the Strait of Hormuz, give it geostrategic importance. Tehran is the country's capital and largest city, as well as its leading economic and cultural center. Iran's climate is diverse, having 11 climates out of the world's 13, , ranging from arid and semi-arid, to subtropical along the Caspian coast and the northern forests. On the northern edge of the country (the Caspian coastal plain), temperatures rarely fall below freezing and the area remains humid for the rest of the year. Summer temperatures rarely exceed 29 °C (84.2 °F). Annual precipitation is 680 mm (26.8 in) in the eastern part of the plain and more than 1,700 mm (66.9 in) in the western part. Gary Lewis, the United Nations Resident Coordinator for Iran, has said that "Water scarcity poses the most severe human security challenge in Iran today"