The south and southeast of the country are occupied entirely by the great Rub al-Khali desert. Through the desert run largely undefined boundaries with Yemen, Oman, and the United Arab Emirates. In addition to the Rub al-Khali, Saudi Arabia has four major regions. The largest is the Nejd, a central plateau, which rises from c.2,000 ft (610 m) in the east to c.5,000 ft (1,520 m) in the west. Riyadh is located in the Nejd. The Hejaz stretches along the Red Sea from the Gulf of Aqaba south to Asir and is the site of the holy cities of Mecca and Medina. Asir, extending south to the Yemen border, has a fertile coastal plain. Inland mountains in the Asir region rise to more than 9,000 ft (2,743 m). The Eastern Province extends along the Persian Gulf and is the oil region of the country. The oasis of Al-Hasa, located there, is probably the country's largest. Saudi Arabia's climate is generally hot and dry, although nights are cool, and frosts occur in winter. The humidity along the coasts is high.